WR146 is a WC6+O8 colliding-wind binary (CWB) system with thermal emission from the stellar winds of the two stars, and bright non-thermal emission from the wind-collision region (WCR) where the winds collide. We present high resolution radio observations from 1.4 to 43 GHz that give one of the best quality radio spectra of any CWB to date. Observations at 22 GHz now span 8 years, and reveal the proper motion of the system, allowing comparison of multi-epoch data. VLBI observations show the location of the WCR relative to the stellar components, from which the wind momentum ratio can be shown to be 0.06+/-0.15. The radio spectrum and the spatial distribution of emission are modelled, and we determine the contribution of both stellar winds and the WCR to the observed emission. We show that our current models fail to account for the high frequency spectrum of WR146, and also produce too much emission far from the stagnation point of the wind collision.
A solution is presented for the spectrum of high-energy gamma-ray burst photons confined to a quasi-thermal baryonic photosphere. The solution is valid in the steady-state limit assuming the region under consideration is optically thick to the continuously injected photons. It is shown that for a high luminosity photosphere, the non-thermal electrons resulting from gamma-ray Compton cooling lose their energy by upscattering the soft thermalised radiation. The resulting spectral modifications offer the possibility of diagnosing not only the burst comoving luminosity but also the baryon load of the ejecta. This model leads to a simple physical interpretation of X-ray rich bursts and anomalous low-energy slopes.
I calculate the diffuse flux of electron antineutrinos from all supernovae by imposing compatibility with the SN1987A neutrino data and with the existing constraints on the cosmological rate of core collapse supernovae. For the latter I use observational results on the cosmological star formation rate, taking into account the error on the ratio of the supernova rate and star formation rate. The flux above the SuperKamiokande (SK) energy cut of 19.3 MeV is found to be between 0.02 and 4.3 cm^{-2} s^{-1} (0.03 - 9.0 events/year at SK). Part of this interval is already excluded by the current SK upper limit on the flux of 1.2 cm^{-2} s^{-1}. Still, to place constraints on the cosmological supernova rate, water Cerenkov experiments with the same energy threshold should either improve the bound down to the level of 0.3 cm^{-2} s^{-1} (0.5 events/year at SK). or find evidence of the flux at or above 0.2 cm^{-2} s^{-1} (0.4 events/year at SK). These are realistic goals for the next generation Megaton detectors.
We present UBVRI photometry of 44 type-Ia supernovae (SN Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SN Ia to date, nearly doubling the number of well-observed, nearby SN Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SN Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U-B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to B-band.
We suggest a physical mechanism whereby the acceleration time of cosmic rays by shock waves can be significantly reduced. This creates the possibility of particle acceleration beyond the knee energy at ~10^15eV. The acceleration results from a nonlinear modification of the flow ahead of the shock supported by particles already accelerated to the knee momentum at p ~ p_*. The particles gain energy by bouncing off converging magnetic irregularities frozen into the flow in the shock precursor and not so much by re-crossing the shock itself. The acceleration rate is thus determined by the gradient of the flow velocity and turns out to be formally independent of the particle mean free path (m.f.p.). The velocity gradient is, in turn, set by the knee-particles at p ~ p_* as having the dominant contribution to the CR pressure. Since it is independent of the m.f.p., the acceleration rate of particles above the knee does not decrease with energy, unlike in the linear acceleration regime. The reason for the knee formation at p ~ p_* is that particles with $p > p_*$ are effectively confined to the shock precursor only while they are within limited domains in the momentum space, while other particles fall into ``loss-islands'', similar to the ``loss-cone'' of magnetic traps. This structure of the momentum space is due to the character of the scattering magnetic irregularities. They are formed by a train of shock waves that naturally emerge from unstably growing and steepening magnetosonic waves or as a result of acoustic instability of the CR precursor. These losses steepen the spectrum above the knee, which also prevents the shock width from increasing with the maximum particle energy.
We report successful fitting of a Roche model, with a surface temperature gradient following the von Zeipel gravity darkening law, to observations of Altair made with the Navy Prototype Optical Interferometer. We confirm the claim by Ohishi, Nordgren, & Hutter that Altair displays an asymmetric intensity distribution due to rotation, the first such detection in an isolated star. Instrumental effects due to the high visible flux of this first magnitude star appear to be the limiting factor in the accuracy of this fit, which nevertheless indicates that Altair is rotating at 0.90+/-0.02 of its breakup (angular) velocity. Our results are consistent with the apparent oblateness found by van Belle et al. and show that the true oblateness is significantly larger owing to an inclination of the rotational axis of ~64 degrees to the line of sight. Of particular interest, we conclude that instead of being substantially evolved as indicated by its classification, A7 VI-V, Altair is only barely off the ZAMS and represents a good example of the difficulties rotation can introduce in the interpretation of this part of the HR diagram.
Sub milli-arcsecond astrometry and imaging of the black hole Sgr A* at the Galactic centre may become possible in the near future at infrared and sub-millimetre wavelengths. Motivated by observations of short-term infrared and X-ray variability of Sgr A*, in a previous paper we computed the expected images and light curves, including polarization, associated with an compact emission region orbiting the central black hole. We extend this work, using a more realistic hot-spot model and including the effects of opacity in the underlying accretion flow. We find that at infrared wavelengths the qualitative features identified by our earlier work are present, namely it is possible to extract the black hole mass and spin from spot images and light curves of the observed flux and polarization. At radio wavelengths, disk opacity produces significant departures from the infrared behaviour, but there are still generic signatures of the black hole properties. Detailed comparison of these results with future data can be used to test general relativity and to improve existing models for the accretion flow in the immediate vicinity of the black hole.
Muons decaying in the water volume of a Cherenkov detector of the Pierre Auger Observatory provide a useful calibration point at low energy. Using the digitized waveform continuously recorded by the electronics of each tank, we have devised a simple method to extract the charge spectrum of the Michel electrons, whose typical signal is about 1/8 of a crossing vertical muon. This procedure, moreover, allows continuous monitoring of the detector operation and of its water level. We have checked the procedure with high statistics on a test tank at the Observatory base and applied with success on the whole array.
Extensive statistical-equilibrium calculations on neutral sulfur and zinc were carried out, in order to investigate how the non-LTE effect plays a role in the determination of S and Zn abundances in F, G, and K stars. Having checked on the spectra of representative F-type stars (Polaris, Procyon, and alpha Per) and the Sun that our non-LTE corrections yield a reasonable consistency between the abundances derived from different lines, we tried an extensive non-LTE reanalysis of published equivalent-width data of S I and Zn I lines for metal-poor halo/disk stars. According to our calculations, S I 9212/9228/9237 lines suffer significant negative non-LTE corrections amounting to <~ 0.2--0.3 dex, while LTE is practically valid for S I 8683/8694 lines. Embarrassingly, as far as the very metal-poor regime is concerned, a marked discordance is observed between the [S/Fe] values from these two abundance indicators, in the sense that the former attains a nearly flat plateau (or even a slight downward bending) while the latter shows an ever-increasing trend with a further lowering of metallicity. The reason for this discrepancy is yet to be clarified. Regarding Zn, we almost confirmed the characteristic tendencies of [Zn/Fe] reported from recent LTE studies (i.e., an evident/slight increase of [Zn/Fe] with a decrease of [Fe/H] for very metal-poor/disk stars), since the non-LTE corrections for the Zn I 4722/4810 and 6362 lines (tending to be positive and gradually increasing towards lower [Fe/H]) are quantitatively of less significance (<~ 0.1 dex).
Characterizing the nature and spatial distribution of the lensing objects that produce the previously measured microlensing optical depth toward the Large Magellanic Cloud (LMC) remains an open problem. We present an appraisal of the ability of the SuperMACHO Project, a next-generation microlensing survey directed toward the LMC, to discriminate between various proposed lensing populations. We consider two scenarios: lensing by a uniform foreground screen of objects and self-lensing by LMC stars. We have carried out extensive simulations, based upon data obtained during the first year of the project, to assess the SuperMACHO survey's ability to discriminate between these two scenarios. We find that the event rate itself shows significant sensitivity to the choice of the LMC luminosity function, limiting the conclusions which can be drawn from the absolute rate. If instead we determine the differential event rate across the LMC, we will decrease the impact of these systematic biases and render our conclusions more robust. With this approach the SuperMACHO Project should be able to distinguish between the two categories of lens populations. This will provide important constraints on the nature of the lensing objects and their contributions to the Galactic dark matter halo.
We present an abundance analysis of the heavy elements Zr, Ba, La, Ce, and Nd for Hyades cluster F-K dwarfs based on high resolution, high S/N ratio spectra from Keck/HIRES. The derived abundances show the stellar members to be highly uniform, although some elements show a small residual trend with temperature. The rms scatter for each element for the cluster members is as follows; Zr = 0.055 dex, Ba = 0.049 dex, Ce = 0.025 dex, La = 0.025 dex, Nd = 0.032 dex. This is consistent with the measurement errors, and implies that there is little or no intrinsic scatter among the Hyades members. Several stars thought to be non-members of the cluster based on their kinematics are found to deviate from the cluster mean abundances by about 2sigma. Establishing chemical homogeneity in open clusters is the primary requirement for the viability of chemically tagging Galactic disk stars to common formation sites, in order to unravel the dissipative history of early disk formation.
We investigate self-similar magnetohydrodynamic (MHD) processes in an isothermal self-gravitating fluid with a quasi-spherical symmetry and extend the envelope expansion with core collapse (EECC) solutions of Lou & Shen by incorporating a random magnetic field. Stagnation surfaces of EECC solutions that seperate core collapse and envelope expansion propagate at constant speeds either sub-magnetosonically or super-magnetosonically. Crossing the magnetosonic line twice analytically, there exists an infinite number of discrete magnetized EECC and ECCC solutions. In addition to the EECC shock solution which could change the central accretion rate, the magnetic field can also affect the core accretion rate. As the magnetic parameter $\lambda$ increases, the core accretion rate appropriate for the MHD EWCS becomes larger. Under the frozen-in approximation, magnetic fields in the envelope expansion portion would scale as $B\propto r^{-1}$, while in the core collapse portion they would scale as $B\propto r^{-1/2}$. We discuss several astrophysical applications of EECC similarity solutions to the formation process of proto-planetary nebulae connecting the AGB phase and the planetary nebula, to supernova remnants, collapse of magnetized molecular cloud, to $\textrm{H}\ {\textrm{\small{II}}}$ clouds surrounding massive OB stars and to a certain evolution phase of galaxy clusters.
The quiet Sun photospheric plasma has a variety of magnetic field strengths going from zero to 1800 G. The empirical characterization of these field strengths requires a probability density function (PDF), i.e., a function P(B) describing the fraction of quiet Sun occupied by each field strength B. We show how to combine magnetic field strength measurements based on the Zeeman effect and the Hanle effect to estimate an unbiased P(B). The application of the method to real observations renders a set of possible PDFs, which outline the general characteristics of the quiet Sun magnetic fields. Their most probable field strength differs from zero. The magnetic energy density is a significant fraction of the kinetic energy of the granular motions at the base of the photosphere (larger than 15% or larger than 2 10^{3} erg cm^{-3}). The unsigned flux density (or mean magnetic field strength) has to be between 130 G and 190 G. A significant part of the unsigned flux (between 10% and 50%) and of the magnetic energy (between 45% and 85%) are provided by the field strengths larger than 500 G which, however, occupy only a small fraction of the surface (between 1% and 10%). The fraction of kG fields in the quiet Sun is even smaller, but they are important for a number of reasons. The kG fields still trace a significant fraction of the total magnetic energy, they reach the high photosphere, and they appear in unpolarized light images. The quiet Sun photosphere has far more unsigned magnetic flux and magnetic energy than the active regions and the network all together.
We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3). These objects are very faint in the optical, making their spectroscopic identification extremely challenging. Instead, our near-IR spectroscopic observations have been successful in identifying the redshift of two of them (z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When combined with the X-ray properties, our results clearly indicate that all these objects host obscured QSOs (4e44 < L(2-10keV) < 1.5e45 erg/s, 2e22 < N_H < 4e23 cm-2) at high redshift. The only object with unresolved morphology in the K band shows broad Halpha emission, but not broad Hbeta, implying a type 1.9 AGN classification. The other two objects are resolved and dominated by the host galaxy light in the K band, and appear relatively quiescent: one of them has a LINER-like emission line spectrum and the other presents only a single, weak emission line which we tentatively identify with Halpha. The galaxy luminosities for the latter two objects are an order of magnitude brighter than typical local L* galaxies and the derived stellar masses are well in excess of 10^11 Msun. For these objects we estimate black hole masses higher than 10^9 Msun and we infer that they are radiating at Eddington ratios L/L_Edd < 0.1. We discuss the implications of these findings for the coevolution of galaxies and black hole growth. Our results provide further support that X-ray sources with high X/O ratios and very red colors tend to host obscured QSO in very massive galaxies at high redshift.
While kinematical modelling of the low PN velocity dispersions observed in the outer regions of elliptical galaxies suggest a lack of dark matter around these galaxies, we report on an analysis of a suite of $N$-body simulations (with gas) of major mergers of spiral galaxies embedded in dark matter halos, and find that the outer velocity dispersions are as low as observed for the PNe. The inconsistency between our dynamical modelling and previous kinematical modelling is caused by very radial stellar orbits and projection effects when viewing face-on oblate ellipticals. Our simulations (weakly) suggest the youth of PNe around ellipticals, and we propose that the universality of the PN luminosity function may be explained if the bright PNe in ellipticals are formed after the regular accretion of very low mass gas-rich galaxies.
We present radio continuum observations of the planetary nebula (PN) IRAS 17347-3139 (one of the only two known to harbour water maser emission), made to derive its spectral index and the turnover frequency of the emission. The spectrum of the source rises in the whole frequency range sampled, from 2.4 to 24.9 GHz, although the spectral index seems to decrease at the highest frequencies (0.79+-0.04 between 4.3 and 8.9 GHz, and 0.64+-0.06 between 16.1 and 24.9 GHz). This suggests a turnover frequency around 20 GHz (which is unusual among PNe, whose radio emission usually becomes optically thin at frequencies < 10 GHz), and a relatively high emission measure (1.5 x 10^9 cm^{-6} pc). The radio continuum emission has increased by a factor of ~1.26 at 8.4 GHz in 13 years, which can be explained as expansion of the ionized region by a factor of ~1.12 in radius with a dynamical age of ~120 yr and at an expansion velocity of ~5-40 km/s. These radio continuum characteristics, together with the presence of water maser emission and a strong optical extinction suggest that IRAS 17347-3139 is one of the youngest PNe known, with a relatively massive progenitor star.
We present high resolution VLT UVES observations of the active K dwarf GJ 117. 6^Li enhancement has been shown for energetic solar events, one chromospherically active binary, and several dwarf halo stars. Our analysis reveals the detection of 6^Li on this source with 6^Li/7^Li = 0.030+/-0.010. We found no significant contribution from other lines, including Ti I, in the Li profile of GJ 117 and a template star of similar spectral type and metallicity. We discuss the possibility for 6^Li production by spallation and find it to be consistent with the activity levels of the object.
We present an analytical model for the hydrodynamic outflow from the disk of
a starburst galaxy. The model is used to calculate the cosmic ray propagation
and the radio intensity distribution in the nuclear starbust region of NGC 253.
We find that the cosmic ray energy production rate of the central 600 pc of NGC
253 is about $3\cdot 10^{41}$ erg s$^{-1}$, that is about 15 percent of the
total mechanical supernova power.
For this inner region we estimate a terminal outflow velocity of 900 km
s$^{-1}$ and a mass loss rate of $\dot{M}=2\div 4M_\odot$ per year.
We present the first results of a survey to obtain Integral Field Spectroscopy of merging galaxies along the Toomre Sequence. In the present work, we concentrate on the star cluster complexes in the Antennae galaxies (NGC 4038/39) in the overlap region as well as the nuclear region of NGC 4038. Using optical spectroscopy we derive the extinction age, metallicity, velocity, velocity dispersion of the gas and star formation rate for each of the eight complexes detected. We supplement this study with archival HST-WFPC2 U, B, V, H$\alpha$, and I band imaging. Correcting the observed colours of the star clusters within the complexes for extinction, measured through our optical spectra, we compare the clusters with simple stellar population models, with which we find an excellent agreement. In five of the complexes we detect strong Wolf-Rayet emission features, indicating young ages (3-5 Myr). The ionized gas surrounding the complexes is expanding at speeds of 20-40 km/s. This slow expansion can be understood as a bubble, caused by the stellar winds and supernovae within the complexes, expanding into the remnant of the progenitor giant molecular cloud. We also find that the complexes themselves are grouped, at about the largest scale of which young star clusters are correlated, representing the largest coherent star forming region. We show that the area normalized star formation rates of these complexes clearly place them in the regime of star forming regions in starburst galaxies, thereby justifying the label of localized starbursts. Finally, we estimate the stability of the complexes, and find that they will probably loose a large fraction of their mass to the surrounding environment, although the central regions may merge into a single large star cluster.
The universe media is considered as a non-perfect fluid with bulk viscosity and described by a more general equation of state. We assume the bulk viscosity is a linear combination of the two terms: one is constant, and the other is proportional to the scalar expansion $\theta=3\dot{a}/a$. The equation of state is described as $p=(\gamma-1)\rho+p_0$, where $p_0$ is a parameter. This model can be used to explain the dark energy dominated universe. Different choices of the parameters may lead to three kinds of fates of the cosmological evolution: no future singularity, big rip, or Type III singularity of Ref. [S. Nojiri, S.D. Odintsov, and S. Tsujikawa, Phys. Rev. D \textbf{71}, 063004 (2005)].
This thesis examines some of the applications of scaling relations in understanding non linear structure formation.
This review forms the Weak Lensing part of the Saas-Fee Advanced Course on Gravitational Lensing. It describes the basicsm applications and results of weak lensing. Contents: (1) Introduction (2) The principles of weak gravitational lensing (3) Observational issues and challenges (4) Clusters of galaxies: Introduction, and strong lensing (5) Mass reconstructions from weak lensing (6) Cosmic shear -- lensing by the LSS (7) Large-scale structure lensing: results (8) The mass of, and associated with galaxies (9) Additional issues in cosmic shear (10) Concluding remarks.
Results from the KASCADE air shower experiment investigating the origin of cosmic rays in the energy region from 10^13 to 10^17 eV are presented. Attention is drawn on the investigation of interactions in the atmosphere and the energy spectrum and mass composition of cosmic rays.
A new code and methodology are introduced for solving the general relativistic magnetohydrodynamic (GRMHD) equations in fixed background spacetimes using time-explicit, finite-volume discretization. The code has options for solving the GRMHD equations using traditional artificial-viscosity (AV) or non-oscillatory central difference (NOCD) methods, or a new extended AV (eAV) scheme using artificial-viscosity together with a dual energy-flux-conserving formulation. The dual energy approach allows for accurate modeling of highly relativistic flows at boost factors well beyond what has been achieved to date by standard artificial viscosity methods. It provides the benefit of Godunov methods in capturing high Lorentz boosted flows but without complicated Riemann solvers, and the advantages of traditional artificial viscosity methods in their speed and flexibility. Additionally, the GRMHD equations are solved on an unstructured grid that supports local adaptive mesh refinement using a fully threaded oct-tree (in three dimensions) network to traverse the grid hierarchy across levels and immediate neighbors. A number of tests are presented to demonstrate robustness of the numerical algorithms and adaptive mesh framework over a wide spectrum of problems, boosts, and astrophysical applications, including relativistic shock tubes, shock collisions, magnetosonic shocks, Alfven wave propagation, blast waves, magnetized Bondi flow, and the magneto-rotational instability in Kerr black hole spacetimes.
We present new astrometric and spectroscopic data to confirm two new M/L dwarf systems, G124-62 and LHS5166, and discuss the nature of a third system (LP261-75). Age and thus mass determinations of the L dwarf companions are discussed based on various activity-age relationships of the M dwarf primaries. This publication will update the list of widely separated substellar companions to nearby stars.
CHICOS (California HIgh school Cosmic ray ObServatory) is presently an array
of more than 140 detectors distributed over a large area (~400 km^2) of
southern California, and will consist of 180 detectors at 90 locations in the
near future. These sites, located at area schools, are equipped with
computerized data acquisition and automatic nightly data transfer (via
internet) to our Caltech lab. The installed sites make up the largest currently
operating ground array for ultra-high energy cosmic ray research in the
northern hemisphere. The goal of CHICOS is to provide data related to the flux
and distribution of arrival directions for ultra-high energy cosmic rays.
We have performed detailed Monte-Carlo calculations to determine the density
and arrival-time distribution of charged particles in extensive air showers for
the CHICOS array. Calculations were performed for proton primaries with
energies 10^18 to 10^21 eV and zenith angles out to 50 degrees. We have
developed novel parameterizations for both distributions as functions of
distance from the shower axis, primary energy, and incident zenith angle. These
parameterizations are used in aperture calculations and reconstruction of
shower data, enabling preliminary analysis of ultra-high energy shower data
from CHICOS.
The first well-localized short-duration gamma ray bursts (GRBs), GRB 050509b, GRB 050709 and GRB 050724, could have been the narrowly beamed initial spike of a burst/hyper flare of soft gamma ray repeaters (SGRs) in host galaxies at cosmological distances. Such bursts are expected if SGRs are young hyperstars, i.e. neutron stars where a considerable fraction of their neutrons have converted to hyperons and/or strange quark matter.
The highly anisotropic distribution and apparent alignment of the Galactic satellites in polar great planes begs the question how common such distributions are. The satellite system of M31 is the only nearby system for which we currently have sufficiently accurate distances to study the three-dimensional satellite distribution. We present the spatial distribution of the 15 presently known M31 companions in a coordinate system centered on M31 and aligned with its disk. Through a detailed statistical analysis we show that the full satellite sample describes a plane that is inclined by -56 deg with respect to the poles of M31 and that has an r.m.s. height of 100 kpc. With 87.4% the statistical significance of this plane is low and it is unlikely to have a physical meaning. The great stellar stream found near Andromeda is inclined to this plane by 7 deg. There is little evidence for a Holmberg effect. If we confine our analysis to early-type dwarfs, we find a best-fit polar plane within 5 deg to 7 deg from the pole of M31. This polar great plane has a statistical significance of 99.7% and includes all dSphs (except for And II), M32, NGC 147, and PegDIG. The r.m.s. distance of these galaxies from the polar plane is 16 kpc. The nearby spiral M33 has a distance of only about 3 kpc from this plane, which points toward the M81 group. We discuss the anisotropic distribution of M31's early-type companions in the framework of three scenarios, namely as remnants of the break-up of a larger progenitor, as tracer of a prolate dark matter halo, and as tracer of collapse along large-scale filaments. (Abridged)
We report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Underground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days, giving spectrum-weighted exposures of 34 kg-d for germanium and 12 kg-d for silicon targets after cuts, averaged over recoil energies 10-100 keV for a WIMP mass of 60 GeV. A blind analysis was conducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceeding expected backgrounds was observed. When combined with our previous results from Soudan, the 90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6 x 10^{-43} cm^2 from Ge, and 3 x 10^{-42} cm^2 from Si, for a WIMP mass of 60 GeV. The combined limit from Ge (Si) is a factor of 2.5 (10) lower than our previous results, and constrains predictions of supersymmetric models.
Weak lensing surveys that can potentially place strong constraints on dark energy parameters can only do so if the source redshift means and error distributions are very well known. We investigate prospects for controlling errors in these quantities by exploiting their influence on the power spectra of the galaxies. Although, from the galaxy power spectra alone, sufficiently precise and simultaneous determination of redshift biases and variances is not possible, a strong consistency test is. Given the redshift error rms, galaxy power spectra can be used to determine the mean redshift of a group of galaxies to subpercent accuracy. Although galaxy power spectra cannot be used to determine the redshift error rms, they can be used to determine this rms divided by the Hubble parameter, a quantity that may be even more valuable for interpretation of cosmic shear data than the rms itself. We also show that galaxy power spectra, due to the baryonic acoustic oscillations, can potentially lead to constraints on dark energy that are competitive with those due to the cosmic shear power spectra from the same survey.
Recent observations have revealed that damped Ly$\alpha$ clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift (z ~ 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85%, of the cloud, creating conditions for star formation in a timescale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ~4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85% of the gas initially in the clump is converted into stars, we obtain a star formation rate of $\sim 0.25 M_\odot {yr}^{-1} {kpc}^{-2}$. This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean-free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution).
We study the kinematics of the gaseous cosmic web at high redshift with Lyman alpha forest absorption in multiple QSO sightlines. Using a simple analytic model and a cosmological hydrodynamic simulation we constrain the underlying three-dimensional distribution of velocities from the observed line-of-sight distribution of velocity shear across the plane of the sky. The distribution is found to be in good agreement with the intergalactic medium (IGM) undergoing large scale motions dominated by the Hubble flow. Modeling the Lyman alpha clouds analytically and with a hydrodynamics simulation, the average expansion velocity of the gaseous structures causing the Lyman alpha forest in the lower redshift (z = 2) sample appears about 20 percent lower than the local Hubble expansion velocity. We interpret this as tentative evidence for some clouds undergoing gravitational collapse. However, the distribution of velocities is highly skewed, and the majority of clouds at redshifts from 2 to 3.8 expand typically about 5 - 20 percent faster than the Hubble flow. This behavior is explained if most absorbers in the column density range typically detectable are expanding filaments that stretch and drain into more massive nodes. We find no evidence for the observed distribution of velocity shear being significantly influenced by processes other than Hubble expansion and gravitational instability, like galactic winds. To avoid overly disturbing the IGM, winds may be old and/or limp by the time we observe them in the Lyman alpha forest, or they may occupy only an insignificant volume fraction of the IGM. (abridged)
We used fully cosmological, high resolution N-body+SPH simulations to follow the formation of disk galaxies with a rotational velocity between 140 and 280 Km/sec in a Lambda CDM universe. The simulations include gas cooling, star formation (SF), the effects of a uniform UV background and a physically motivated description of feedback from supernovae (SN). Feedback parameters have been chosen to match the star formation rate and interstellar medium (ISM) properties of local galaxies. In cosmological simulations galaxies formed rotationally supported disks with realistic exponential scale lengths and fall on the I-band and baryonic Tully Fisher relations. The combination of UV background and SN feedback drastically reduced the number of visible satellites orbiting inside a Milky Way sized halo, bringing it in fair agreement with observations. Feedback delays SF in small galaxies and more massive ones contain older stellar populations. Here we focus on the SF and feedback implementations. We also briefly discuss how high mass and force resolution and a realistic description of SF and feedback are important ingredients to match the observed properties of galaxies.
We analyze radial velocities for a sample of 31 Class I and flat spectrum protostars in Taurus-Auriga, rho Ophiuchi and Serpens for evidence of the global dynamical state of extremely young stellar populations buried within parental molecular clouds. Comparing the radial velocity of each protostar to that of the local CO gas, we are able to constrain the one dimensional radial velocity dispersion of Class I and flat spectrum objects to ~ 2.5 km/sec or below. This upper limit to the protostellar velocity dispersion is consistent with the velocity dispersions of surrounding CO gas which we measure to be ~ 1.4 km/sec, suggesting that the motions of protostars and local CO gas are dynamically linked and dominated by the gravitational potential of the molecular cloud. However, the upper limit on the protostellar velocity dispersion could still allow for slightly inflated motions of protostars relative to the local molecular gas. Four of the protostars analyzed appear to have velocities more than 3 sigma (7.5 km/sec) away from the central local CO gas velocity while showing spectroscopic indicators of youth and accretion such as H_2 emission, HI Br Gamma emission, or K band continuum veiling. These radial velocity outliers may represent protostellar spectroscopic binaries or ejected cluster members.
The central parsec of the Galaxy contains dozens of massive stars with a cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among these stellar winds produce the hot plasma that pervades the central part of the galaxy. We argue that these stellar wind shocks also efficiently accelerate electrons and protons to relativistic energies. The relativistic electrons inverse Compton scatter the ambient ultraviolet and far infrared radiation field, producing high energy gamma-rays with a roughly constant luminosity from \~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the Galactic Center. Our model predicts a GLAST counterpart to the HESS source with a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron radiation from the same relativistic electrons should produce detectable emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3} ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field strength in units of mG. The observed level of diffuse thermal X-ray emission in the central parsec requires B < 300 micro-G in our models. Future detection of the diffuse synchrotron background in the central parsec can directly constrain the magnetic field strength, providing an important boundary condition for models of accretion onto Sgr A*.
We perform local numerical experiments to investigate the nonlinear stability of thin, radially-stratified disks. We demonstrate the presence of radial convective instability when the disk is nearly in uniform rotation, and show that the net angular momentum transport is slightly inwards, consistent with previous investigations of vertical convection. We then show that a convectively-unstable equilibrium is stabilized by differential rotation. Convective instability is determined by the Richardson number Ri = N_r^2/(q\Omega)^2, where N_r is the radial Brunt-Vaisala frequency and q\Omega is the shear rate. Classical convective instability in a nonshearing medium (Ri -> -infinity) is suppressed when Ri > -1, i.e. when the shear rate becomes greater than the growth rate. Disks with a nearly-Keplerian rotation profile and radial gradients on the order of the disk radius have Ri > -0.01 and are therefore stable to local nonaxisymmetric disturbances. One implication of our results is that the ``baroclinic'' instability recently claimed by Klahr & Bodenheimer is either global or nonexistent. We estimate that our simulations would detect any genuine growth rate > 0.0025\Omega.
We have studied 23 long-lived G dwarfs that belong to the thin disk and thick disk stellar populations. Abundances have been derived for 24 elements: O, Na, Mg, Al, Si, Ca, Ti, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, and Eu. We find that the behavior of [alpha/Fe] and [Eu/Fe] vs. [Fe/H] are quite different for the two populations. As has long been known, the thin disk O, Mg, Si, Ca, and Ti ratios are enhanced relative to iron at the lowest metallicities, and decline toward solar values as [Fe/H] rises above -1.0. For the thick disk, the decline in [alpha/Fe] and [Eu/Fe] does not begin at [Fe/H] = -1.0, but at -0.4. Other elements share this behavior, including Sc, Co, and Zn, suggesting that at least in the chemical enrichment history of the thick disk, these elements were manufactured in similar-mass stars. Combining our results for the oldest and longest-lived stars with prior work, we find clear signs for an independent origin for the Galactic thick disk. (Abridged)
The fact that we apparently live in an accelerating universe places limitations on where humans might visit. If the current energy density of the universe is dominated by a cosmological constant, a rocket could reach a galaxy observed today at a redshift of 1.7 on a one-way journey or merely 0.65 on a round trip. Unfortunately these maximal trips are impractical as they require an infinite proper time to traverse. However, calculating the rocket trajectory in detail shows that a rocketeer could nearly reach such galaxies within a lifetime (a long lifetime admittedly -- about 100 years). For less negative values of $w$ the maximal redshift increases becoming infinite for $w\geq -1/3$.
The Cryogenic Dark Matter Search (CDMS) is an experiment to detect weakly interacting massive particles (WIMPs) based on their interactions with Ge and Si nuclei. We report the results of an analysis of data from the first two runs of CDMS at the Soudan Underground Laboratory in terms of spin-dependent WIMP-nucleon interactions on 73Ge and 29Si. These data exclude new regions of spin-dependent WIMP-nucleon interaction parameter space, including regions relevant to spin-dependent interpretations of the annual modulation signal reported by the DAMA/NaI experiment.
We report on the abundances of 13 elements in the planetary host HD 13189, a massive giant star. Abundances are found to be sub-solar, with [Fe/H] = -0.58 +/- 0.04$; HD 13189 is one of the most metal-poor planetary hosts yet discovered. Abundance ratios relative to Fe show no peculiarities with respect to random field stars. A census of metallicities of the seven currently known planet-harboring giants results in a distribution that is more metal-poor than the well-known metal-rich distribution of main sequence (MS) planetary hosts. This finding is discussed in terms of accretion of H-depleted material, one of the possible mechanisms responsible for the high-metallicity distribution of MS stars with planets. We estimate the mass of the HD 13189 progenitor to be 3.5 M_sun but cannot constrain this value to better than 2-6 M_sun. A stellar mass of 3.5 M_sun implies a planetary mass of m sin i = 14.0 +/- 0.8 M_J, placing the companion at the planet/brown dwarf boundary. Given its physical characteristics, the HD 13189 system is potentially unique among planetary systems, and its continued investigation should provide invaluable data to extrasolar planetary research.
Oxygen abundances have been derived from the near-IR, high-excitation Lambda 7774 O I triplet in high-resolution, high signal-to-noise spectra of 45 Hyades dwarfs using standard one dimensional, plane-parallel LTE models. Effective temperatures of the stellar sample range from 4319-6301 K, and the derived relative O abundances as a function of T_eff evince a trichotomous morphology. At T_eff > 6100 K, there is evidence of an increase in the O abundances with increasing T_eff, consistent with non-LTE (NLTE) predictions. At intermediate T_eff (5450 < T_eff < 6100 K), the O abundances are flat, and star-to-star values are in good agreement, having a mean value of [O/H] = +0.25 +/- 0.02; however, systematic errors at the ~0.10 dex level might exist. The O abundances for stars with T_eff < 5450 K show a striking increase with decreasing T_eff, in stark contrast to expectations and canonical NLTE calculations. The cool Hyades triplet results are compared to those recently reported for dwarfs in the Pleiades cluster and the UMa moving group; qualitative differences between the trends observed in these stellar aggregates point to a possible age-related diminution of triplet abundance trends in cool open cluster dwarfs. Correlations with age-related phenomena, i.e., chromospheric activity and photospheric spots, faculae, and/or plages, are investigated. No correlation with Ca II H+K chromospheric activity indicators is observed. Multi-component LTE ``toy'' models have been constructed in order to simulate photospheric temperature inhomogeneities that could arise from the presence of starspots, and we demonstrate that photospheric spots are a plausible source of the triplet trends among the cool dwarfs.
In this paper we use the recently released Type Ia Supernova (SNIa) data to constrain the interactions between the neutrinos and the dark energy scalar fields. In the analysis we take the dark energy scalars to be either Quintessence-like or Phantom-like. Our results show the data mildly favor a model where the neutrinos couple to a phantom-like dark energy scalar, which implies the equation of state of the coupled system behaves like Quintom scenario in the sense of parameter degeneracy. We find future SNAP observations alone are potentially promising to measure the couplings between neutrino and dark energy.
Recent observations of gamma-ray bursts (GRBs) are providing prompt few-arcminute gamma-ray localizations, rapid few-arcsecond X-ray positions, and rapid and extensive follow-up in the X-ray, UV, optical, and radio bands. Thirteen of these bursts include extraordinary optical upper limits at very early epochs after the burst, in marked contrast to the bright optical flashes previously believed to be the norm. Although host extinction can explain the properties of some bursts, and the natural range of burst energies and distances can explain some others, comparison of our optical, X-ray, and gamma-ray data sets reveals that these considerations alone cannot explain the full diversity of the burst population. Instead, one or more mechanisms must act to suppress the optical flash and provide a significantly enhanced efficiency of the prompt gamma-ray emission for some bursts. One possibility is that a fraction of the burst population is powered by Poynting flux-dominated outflows, resulting in a very inefficient transfer of magnetic energy to the ambient medium; if so, then the diversity of the GRB population would extend to the very heart of their highly-relativistic engines.
We analyze the mass accretion histories (MAHs) and density profiles of cluster-size halos in a flat LCDM cosmology.To fit the MAHs of all systems we need to generalize the MAH fit found in previous systematic studies of predominantly galactic halos.We find that the concentration of the density distribution is tightly correlated with the halo MAH and its formation redshift. During the early period of fast mass growth the concentration remains roughly constant and low (~3-4), while during the slow accretion phase it increases with decreasing redshift as 1/(1+z).We consider fits of three widely discussed analytic density profiles.We find that there is no unique best fit for all clusters.At the same time, if a cluster is best fit by a particular analytic profile at z=0, the same is usually true at earlier epochs out to z ~ 1-2.The local logarithmic slope of the density profiles at 3% of the virial radius ranges from -1.2 to -2.0.In addition, the logarithmic slope becomes shallower with decreasing radius without reaching an asymptotic value down to the smallest resolved scale (<1% of the virial radius).During the early MAH period of rapid mass growth the density profiles can be well described by a single power law rho(r) ~ r^-a with a ~ 1.5-2.The relatively shallow power law slopes result in low concentrations at these stages of evolution, as the scale radius where the density profiles reaches the slope of -2 is at large radii.This indicates that the inner power law like density distribution of halos is built up during the periods of rapid mass accretion and active merging, while the outer steeper profile is formed when the mass accretion slows down.
We present here the first detailed study of a giant radio galaxy of the Fat-Double type. The lobes of the double radio galaxy SGRS J0515-8100 have transverse widths that are 1.3 times their extent from the center, their surface brightness is the lowest among known giant radio sources and the lobes have relatively steep radio spectra. We infer that these wide lobes were created as a result of a highly variable and intermittent jet whose axis direction also varied significantly: the Fat-Double lobes in this giant radio source are a result of the ejection and deposition of synchrotron plasma over a wide range of angles over time rather than the expansion of relic lobes. Additionally, the optical host shows evidence for an ongoing galaxy-galaxy interaction. SGRS J0515-8100 supports the hypothesis that interactions with companions might perturb the inner accretion disk that produces and sustains the jets at the centers of active galactic nuclei. As a result, it appears unnecessary to invoke black-hole coalescence to explain such morphologies, implying that the corresponding event rates predicted for gravitational wave detectors may be overestimates.
Using the luminosity function and apparent GRB rate inferred from the spectral peak energy-luminosity relation, we discuss the absolute GRB formation rate taking into account both the jet-luminosity evolution and jet opening angle evolution effects. In the case of no jet opening angle evolution, the jet-corrected luminosity of high-redshift GRB is much larger than the typical value of low-z (z~1) bursts. On the other hand, if the jet-luminosity does not evolve with time, the jet opening angle for high-z bursts should be much smaller than that of low-z GRBs. Therefore, it is preferable that both evolution effects are taken into account. We also estimate the local GRB event rate in a galaxy of 10^{-7}-10^{-5}/yr.
In many models of dusty objects in space the grains are assumed to be
composite or fluffy. However, the computation of the optical properties of such
particles is still a very difficult problem. We analyze how the increase of
grain porosity influences basic features of cosmic dust -- interstellar
extinction, dust temperature, infrared bands and millimeter opacity. Porous
grains can reproduce the flat extinction across the $3 - 8 \mkm$ wavelength
range measured for several lines of sight by {\it ISO} and {\it Spitzer}.
Porous grains are generally cooler than compact grains. At the same time, the
temperature of very porous grains becomes slightly larger in the case of the
EMT-Mie calculations in comparison with the results found from the
layered-sphere model. The layered-sphere model predicts a broadening of
infrared bands and a shift of the peak position to larger wavelengths as
porosity grows. In the case of the EMT-Mie model variations of the feature
profile are less significant. It is also shown that the millimeter mass
absorption coefficients grow as porosity increases with a faster growth
occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for
very porous particles the coefficients given by two models can differ by a
factor of about 3.
It is found that an increase of porosity leads to an increase of extinction
cross sections at some wavelengths and a decrease at others depending on the
grain model. However, this behaviour is sufficient to reproduce the extinction
curve in the direction of the star $\sigma$ Sco using current solar abundances.
In the case of the star $\zeta$ Oph our model requires larger amounts of carbon
and iron in the dust-phase than is available.
We describe a modified SIMD architecture suitable for single-chip integration of a large number of processing elements, such as 1,000 or more. Important differences from traditional SIMD designs are: a) The size of the memory per processing elements is kept small. b) The processors are organized into groups, each with a small buffer memory. Reduction operation over the groups is done in hardware. The first change allows us to integrate a very large number of processing elements into a single chip. The second change allows us to achieve a close-to-peak performance for many scientific applications like particle-based simulations and dense-matrix operations.
The effectiveness of the thermal coupling of ions and electrons in the context of optically thin, hot accretion flows is investigated. In the limit of complete coupling, we focus on the one-temperature accretion flows. Based on a global analysis, the results are compared with two-temperature accretion flow models and with the observations of black hole sources. Many features are quite similar. That is, hot one-temperature solutions are found to exist for mass flow rates less than a critical value; i.e., $\dot{M}\la 10\alpha^2\dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}= L_{\rm Edd}/c^2$ is the Eddington accretion rate. At low mass flow rates, $\dot{M}\la 10^{-3}\alpha^2 \dot{M}_{\rm Edd}$, the solution is in the advection-dominated accretion flow (ADAF) regime. But at higher rates, radiative cooling is effective and is mainly balanced by advective {\em heating}, placing the solution in the regime of luminous hot accretion flow (LHAF). To test the viability of the one-temperature models, we have fitted the spectra of the two black hole sources, Sgr A* and XTE J1118+480, which have been examined successfully with two-temperature models. It is found that the one-temperature models do not provide acceptable fits to the multi-wavelength spectra of Sgr A* nor to XTE J1118+480 as a result of the higher temperatures characteristic of the one-temperature models. It is concluded that the thermal coupling of ions and electrons cannot be fully effective and that a two-temperature description is required in hot accretion flow solutions.
We report the detection of redshifted iron K-alpha absorption lines in the Chandra LETG spectrum of the narrow-line quasar, PG 1211+143. The absorption lines are observed at 4.22 keV and 4.93 keV in the quasar spectrum, corresponding to 4.56 keV and 5.33 keV in the rest frame of PG 1211+143. From Monte Carlo simulations, the chance probability of both lines being false detections is low at 1.36e-04. Highly redshifted ionized iron K-alpha (Fe XXV or Fe XXVI) is the most plausible identification for the lines at their observed energies. If identified with H-like iron K-alpha at 6.97 keV, then the relativistic velocity shifts required are 0.40c and 0.26c. The extreme velocities can be explained by pure gravitational redshift if the matter exists in a stable orbit within 6 gravitational radii of the black hole. This would require a Kerr metric for the black hole. Alternatively the absorption may be the result of matter infalling directly onto the black hole, with a maximum observed velocity of 0.38c at 6Rg in the Schwarzschild metric. This matter may originate in a failed outflow or jet, which does not escape the gravitational potential of the black hole.
We present evidence that the 8 $\mu$m (dust) and 24 $\mu$m luminosities of star-forming galaxies are both strongly correlated with their 1.4 GHz and H$\alpha$ luminosities over a range in luminosity of two-to-three orders of magnitude. At the bright end, the correlations are found to be essentially linear over a luminosity range of about two orders of magnitude (corresponding to star-formation rates of several-tenths to several tens of solar masses per year). However, at the faint end, there appears to be a slope change for dwarf galaxies, possibly due to the lower dust-to-gas ratios and lower metallicities of the dwarfs. The correlations suggest that PAH features and mid-IR continuum emissions are good measures of the star formation rates of galaxies, and we present calibrations of star-formation rates based on existing radio and H$\alpha$ relations. Our findings are based on a sample of star-forming galaxies selected from the main field of the Spitzer First Look Survey with the aid of spectroscopic data from the Sloan Digital Sky Survey and VLA 1.4 GHz data.
Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaux and declines in the present day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features can be misinterpreted as the form of the intrinsic stellar initial mass function (IMF) if the star formation rate is assumed to be constant or slowly varying with time. This effect applies to all regions that have formed stars for longer than the age of the most massive stars, including OB associations, star complexes, and especially galactic field stars. Related problems may apply to embedded clusters. Evidence is summarized for temporal SFR variations from parsec scales to entire galaxies, all of which should contribute to inferred IMF distortions. We give examples of various star formation histories to demonstrate the types of false IMF structures that might be seen. These include short-duration bursts, stochastic histories with log-normal amplitude distributions, and oscillating histories with various periods and phases. The inferred IMF should appear steeper than the intrinsic IMF over mass ranges where the stellar lifetimes correspond to times of decreasing SFRs; shallow portions of the inferred IMF correspond to times of increasing SFRs. If field regions are populated by dispersed clusters and defined by their low current SFRs, then they should have steeper inferred IMFs than the clusters. The SFRs required to give the steep field IMFs in the LMC and SMC are determined. Structure observed in several determinations of the Milky Way field star IMF can be accounted for by a stochastic and bursty star formation history.
We examine pulsational stability of low $m$ $r$ modes in SPB stars by calculating fully nonadiabatic oscillations of uniformly rotating stars, where $m$ is an integer representing the azimuthal wave number around the rotation axis. $R$ modes are rotationally induced, non-axisymmetric, oscillation modes, whose oscillation frequency strongly depends on the rotation frequency $\Omega$ of the star. They are conveniently classified by using two integer indices $m$ and $l^\prime\ge |m|$ that define the asymptotic oscillation frequency $2m\Omega/[l^\prime(l^\prime+1)]$ in the limit of $\Omega\to 0$. We find low $m$, high radial order, odd $r$ modes with $l^\prime=m$ in SPB stars are excited by the same iron opacity bump mechanism that excites low frequency $g$ modes of the variables, when the rotation frequency $\Omega$ is sufficiently high. No even $r$ modes with low $m$ are found to be pulsationally unstable. Since the surface pattern of the temperature perturbation of odd modes is antisymmetric about the equator of the star, observed photometric amplitudes caused by the unstable odd $r$ modes with $l^\prime=m$ are strongly dependent on the inclination angle between the axis of rotation and the line of sight. Applying the wave-meanflow interaction formalism to nonadiabatic $r$ modes in rapidly rotating SPB models, we find that because of the $r\phi$ component of the Reynolds stress and the radial transport of the eddy fluctuation of density in the rotating star, the surface rotation is accelerated by the forcing due to the low $l^\prime=m$ unstable $r$ modes.
We give an update of the results of a campaign to obtain orbital solutions of subdwarf B stars from the Edinburgh-Cape survey (Stobie et al. 1997). To date we have obtained blue spectra of 40 subdwarf B stars from the Edinburgh-Cape catalogue using the grating spectrograph at the 1.9m Radcliffe telescope at the South African Astronomical Observatory. We find that 17 out of these 40 are certain binaries with a few other objects showing radial velocity variations of small amplitude. The binary fraction found in our sample, after correcting for our binary detection efficiency, is 48%. We have secured the orbital parameters for 4 of the 17 systems and narrowed down the orbits of another 7 to a small range of periods.
Galactic warps represent an old unresolved problem, since the discovery, at the end of the fifties, of the HI warp of the Milky Way. In this paper, we propose a new scenario explaining a large fraction of the observed optical warps. Based on N-body simulations, we show that realistic galactic disks, where the dark matter is essentially distributed in a disk, are subject to bending instabilities. S, U-shaped, as well as asymmetric warps are spontaneously generated and in some cases are long-lived. While this scenario presents the advantage of explaining the three observed types of warps, it also brings new constraints on the dark matter distribution in spiral galaxies. Finally, it gives us a unified picture of galaxies where galactic asymmetries, like bars, spirals and warps result from gravitational instabilities.
A geodynamo-model based on an \alpha-effect which has been computed under conditions suitable for the geodynamo is constructed. For a highly restricted class of radial \alpha-profiles the linear \alpha2-model exhibits oscillating solutions on a timescale given by the turbulent diffusion time. The basic properties of the periodic solutions are presented and the influence of the inner core size on the characteristics of the critical range that allows for oscillating solutions is shown. Reversals are interpreted as half of such an oscillation. They are rather seldom events because they can only occur if the \alpha-profile exists long enough within the small critical range that allows for periodic solutions. Due to strong fluctuations on the convective timescale the probability of such a reversal is very small. Finally, a simple non-linear mean-field model with reasonable input parameters based on simulations of Giesecke et al. (2005) demonstrates the plausibility of the presented theory with a long-time series of a (geo-)dynamo reversal sequence.
Recently, Shen et al. (2005) studied the contributions of curvature effect of fireball to the spectral lag, and the resulting lags are very closed to the observed one. However some factors, such as the Doppler effect and the time dilation effect between the rest frame and the observer frame, are not taken into consideration in their model. As they pointed out that, the observed large lags ($\sim 0.1s$) require extreme physical parameter values, e.g., $\Gamma<50$, or $\alpha>-0.5$. However, Qin (2002, 2003, 2004) had derived in detail the observed photon count rate based on the model of highly symmetric expanding fireballs, where the two effects above were taken into account, and no terms are omitted in his corresponding derivation. Motivated by this, we investigate the same issue based on Qin model, and come to the different conclusions, such as Qin model don't requires such extreme physical parameters above to explain those observed large lags, even for the larger lags ($\sim10s$) (see Norris et al. 2005), it only requires wider local pulse ($\Delta t_{\theta,FWHM} = 2\times10^{5}s$). The lag's dependences on the parameters of Qin model are examined and find that lag $\propto \Gamma^{-\epsilon}$ with $\epsilon>$ 2; lag is proportional to the local pulse width and then the FWHM of the observed light curves; a large lag requires such spectral indices: large $\alpha_0$, small $\beta_0$, and a larger lag would be produced when they vary with time in the rest frame; lag $\propto R_c$ (where $R_c$ is the radius of fireball); lag $\propto$ E (energy); lag is proportional to the viewing angle when $\theta_{max} < 0.6\Gamma^{-1}$. Our studies show that merely the Doppler effect could produce the observed lags.
We measure the luminosity function of morphologically selected E/S0 galaxies from z=0.5 to z=1.0 using deep high resolution Advanced Camera for Surveys (ACS) imaging data. Our data extend 2 magnitudes deeper than the Deep Groth Strip Survey (DGSS). At 0.5<z<0.75, we find M_B^*-5\log h_{0.7}=-21.1+/-0.3 and \alpha=-0.53+/-0.2, and at 0.75<z<1.0, we find M_B^*-5\log h_{0.7}=-21.4+/-0.2. Our morphologically selected luminosity functions are similar in both shape and number density to other morphologically selected luminosity functions (e.g., DGSS), but we find significant differences to the luminosity functions of samples selected using morphological proxies like colour or SED. The difference is due to incompleteness from blue E/S0 galaxies, which make up to ~30% of the sample and contamination from early-type spirals. Most of the blue E/S0 galaxies have similar structural properties to the red E/S0s and could passively evolve to form giant red ellipticals at z=0. However, the bluest, (U-V)_0<1.2, have much smaller Sersic parameters and would evolve into much fainter galaxies. These may be the progenitors of dwarf ellipticals. We demonstrate the need for both morphological and colour information to constrain the evolution of E/S0 galaxies.
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of the Galactic HII region RCW 79. The millimetre continuum data reveal the presence of massive fragments located in a dust emission ring surrounding the ionized gas. The two most massive fragments are diametrically opposite each other in the ring. The near-IR data, centred on the compact HII region located at the south-eastern border of RCW 79, show the presence of an IR-bright cluster containing massive stars along with young stellar objects with near-IR excesses. A bright near- and mid-IR source is detected towards maser emissions, 1.2 pc north-east of the compact HII region centre. Additional information, extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of the bright IR sources observed towards RCW 79. Twelve luminous Class I sources are identified towards the most massive millimetre fragments. All these facts strongly indicate that the massive-star formation observed at the border of the HII region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.
The diffuse continuum emission from the Galactic plane in the energy range 18-1000 keV has been studied using 16 Ms of data from the SPI instrument on INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to achieve a good separation of point sources from the various diffuse components. Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number of sky distribution models we obtained spectra resolved in Galactic longitude. We can identify spectral components of a diffuse continuum of power law shape with index about 1.7, a positron annihilation component with a continuum from positronium and the line at 511 keV, and a second, roughly power-law component from detected point sources. Our analysis confirms the concentration of positron annihilation emission in the inner region (|l|<10), the disk (10<|l|<30) being at least a factor 7 weaker in this emission. The power-law component in contrast drops by only a factor 2, showing a quite different longitude distribution and spatial origin. Detectable sources constitute about 90% of the total Galactic emission between 20 and 60 keV, but have a steeper spectrum than the diffuse emission, their contribution to the total emission dropping rapidly to a small fraction at higher energies. In the SPI energy range the flux is lower than found by OSSE, probably due to the more complete accounting for sources by SPI. The power-law emission is difficult to explain as of interstellar origin, inverse Compton giving at most 10%, and instead a population of unresolved point sources is proposed as a possible origin, AXPs with their spectra hardening above 100 keV being plausible candidates. We present a broadband spectrum of the emission.
We suggest that the current acceleration of the universe may be explained by the vacuum energy of a hidden sector which is stuck in a state of equilibrium between phases. The phases are associated to a late-time first-order phase transition, where phase coexistence originates at a temperature $T_c \sim 10^{-3}eV$ and lasts indefinitely. During phase coexistence, the energy density has an effective cosmological constant component with the observed magnitude. This scenario does not require supercooling and arises naturally in realistic models.
Through a simple physical argument we show that the slant optical depth through the atmosphere of a "hot Jupiter" planet is 35-90 times greater than the normal optical depth. This not unexpected result has direct consequences for the method of transmission spectroscopy for characterizing the atmospheres of transiting giant planets. The atmospheres of these planets likely contain minor condensates and hazes which at normal viewing geometry have negligible optical depth, but at slant viewing geometry have appreciable optical depth that can obscure absorption features of gaseous atmospheric species. We identify several possible condensates. We predict that this is a general masking mechanism for all planets, not just for HD 209458b, and will lead to weaker than expected or undetected absorption features. Constraints on an atmosphere from transmission spectroscopy are not the same as constraints on an atmosphere at normal viewing geometry.
We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/d\tau \propto \tau^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationally bound and were disrupted near the times they formed by the energy and momentum input from young stars to the interstellar matter of the protoclusters. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived.
The formation and evolution of disk-dominated galaxies is difficult to explain, yet these objects exist. We therefore embarked on a study aimed at a better understanding of these enigmatic objects. We used data from the SDSS DR1 in order to identify edge-on galaxies with disks in a uniform, reproducible, automated fashion. We identified 3169 edge-on disk galaxies, which we subdivided into disk galaxies with bulge, intermediate types, and simple disk galaxies without any obvious bulge component. We subdivided these types further into subclasses: Sa(f), Sb(f), Sc(f), Scd(f), Sd(f), Irr(f), where the (f) indicates that these galaxies are seen edge-on. A number of incompleteness effects affect our catalog, but it contains almost a factor of four more bulgeless galaxies with prominent simple disks (flat galaxies) within the area covered here than previous optical catalogs, which were based on the visual selection from photographic plates. We find that approximately 15% of the edge-on disk galaxies in our catalog are flat galaxies, demonstrating that these galaxies are fairly common, especially among intermediate-mass star-forming galaxies. Bulgeless disks account for roughly one third of our galaxies when also puffy disks and edge-on irregulars are included.(Abridged)
We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT, of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe star. Remarkably, evidence of surface abundance spots is detected in the spectra of HD 72106A. The magnetic fields of these objects display intensities of order 1 kG at the photospheric level, are ordered on global scales, and appear in approximately 10% of the stars studied. Based on these properties, the detected stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and 2.4 solar masses) represent additional contradictions to the hypothesis of Hubrig et al. (2000), who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. Finally, we fail to confirm claims by Hubrig et al. (2004) of magnetic fields in the Herbig Ae star HD 139614.
We report follow-up observations of two gravitational lens candidates identified in the Sloan Digital Sky Survey (SDSS) dataset. We have confirmed that SDSS J102111.02+491330.4 is a previously unknown gravitationally lensed quasar. This lens system exhibits two images of a $z = 1.72$ quasar, with an image separation of $1{\farcs}14 \pm 0.04$. Optical and near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Observations of SDSS J112012.12+671116.0 indicate that it is more likely a binary quasar than a gravitational lens. This system has two quasars at a redshift of $z = 1.49$, with an angular separation of $1{\farcs}49 \pm 0.02$. However, the two quasars have markedly different SEDs and no lens galaxy is apparent in optical and near-IR images of this system. We also present a list of 31 SDSS lens candidates which follow-up observations have confirmed are \textit{not} gravitational lenses.
The integrated luminosity and average energy of the neutrino emission spectrum are essential diagnostics of core-collapse supernovae. The SN 1987A electron antineutrino observations by the Kamiokande-II and IMB detectors are only roughly consistent with each other and theory. Using new measurements of the star formation rate history, we represent the Super-Kamiokande upper bound on the electron antineutrino flux from all past supernovae as an excluded region in neutrino emission parameter space. A gadolinium-enhanced Super-Kamiokande should be able to jointly measure these parameters, and a future megaton-scale detector would enable precision studies.
We present a spectral analysis of the e+e- annihilation emission from the Galactic Centre region based on the first year of measurements made with the spectrometer SPI of the INTEGRAL mission. We have found that the annihilation spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus an ortho-Ps continuum. The broad line is detected with a flux of (0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in agreement with the expected broadening of 511 keV photons emitted in the annihilation of Ps that are formed by the charge exchange process of slowing down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3 s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum flux yields a fraction of Ps of (96.7+/-2.2)%. To derive in what phase of the interstellar medium positrons annihilate, we have fitted annihilation models calculated for each phase to the data. We have found that 49(+2,-23)% of the annihilation emission comes from the warm neutral phase and 51(+3,-2)% from the warm ionized phase. While we may not exclude that less than 23% of the emission might come from cold gas, we have constrained the fraction of annihilation emission from molecular clouds and hot gas to be less than 8% and 0.5%, respectively. We have compared our knowledge of the interstellar medium in the bulge and the propagation of positrons with our results and found that they are in good agreement if the sources are diffusively distributed and if the initial kinetic energy of positrons is lower than a few MeV. Despite its large filling factor, the lack of annihilation emission from the hot gas is due to its low density, which allows positrons to escape this phase.
We present the results of a systematic analysis of the world sample of optical/near-infrared afterglow light curves observed in the pre-Swift era by the end of 2004. After selecting the best observed 16 afterglows with well-sampled light curves that can be described by a Beuermann equation, we explore the parameter space of the light curve parameters and physical quantities related to them. In addition, we search for correlations between these parameters and the corresponding gamma-ray data, and we use our data set to look for a fine structure in the light curves.
(abridged) Three-dimensional hydrodynamic simulations with an Eulerian PPM code are presented for the time-dependent evolution of accretion tori around nonrotating and rotating stellar-mass black holes (BHs), using a pseudo-Newtonian gravitational potential to approximate the effects of general relativity. The initial configurations are assumed to be remnants of binary neutron star (NS) or NS+BH mergers and consist of a 4 solar mass BH with varied spin, girded by a torus with a mass between 0.01 and 0.2 solar masses. The evolution of tori without and with physical shear viscosity is simulated, using a realistic equation of state and following the energy loss and lepton number change due to neutrino emission by a neutrino-trapping scheme. The time-dependent efficiency of converting rest-mass energy to neutrinos is found to reach 10 percent, the efficiency of converting neutrino energy into a pair-photon fireball by neutrino annihilation can reach several percent. The rate of the latter process declines with time much less steeply than the total neutrino luminosity, because the ongoing protonization of the torus ensures a rather stable product of neutrino and antineutrino luminosities. The neutrino emission increases steeply with higher viscosity, larger torus mass, and larger BH spin in corotation with the torus. For rotation rates as expected for post-merger BHs (a > 0.5) and reasonable values of the alpha viscosity (alpha ~ 0.1), the considered tori release sufficient energy in neutrinos to account for the energetics of the well-localized short gamma-ray bursts recently detected by Hete and Swift, if collimation is invoked as predicted by hydrodynamic jet simulations.
We introduce a class of models in which statistical isotropy is broken spontaneously in the CMB by a non-linear response to long-wavelength fluctuations in a mediating field. These fluctuations appear as a gradient locally and pick out a single preferred direction. The non-linear response imprints this direction in a range of multipole moments. We consider two manifestations of isotropy breaking: additive contributions and multiplicative modulation of the intrinsic anisotropy. Since WMAP exhibits an alignment of power deficits, an additive contribution is less likely to produce the observed alignments than the usual isotropic fluctuations, a fact which we illustrate with an explicit cosmological model of long-wavelength quintessence fluctuations. This problem applies to other models involving foregrounds or background anisotropy that seek to restore power to the CMB. Additive models that account directly for the observed power exacerbate the low power of the intrinsic fluctuations. Multiplicative models can overcome these difficulties. We construct a proof of principle model that significantly improves the likelihood and generates stronger alignments than WMAP in 30-45% of realizations.
Recently, the Spitzer Space Telescope discovered L1014-IRS, a mid-infrared source with protostellar colors, toward the heretofore "starless" core L1014. We present deep near-infrared observations that show a scattered light nebula extending from L1014-IRS. This nebula resembles those typically associated with protostars and young stellar objects, tracing envelope cavities presumably evacuated by an outflow. The northern lobe of the nebula has an opening angle of ~100 degrees, while the southern lobe is barely detected. Its morphology suggests that the bipolar cavity and inferred protostellar disk is not inclined more than 30 degrees from an edge-on orientation. The nebula extends at least 8" from the source at Ks, strongly suggesting that L1014-IRS is embedded within L1014 at a distance of 200 pc rather than in a more distant cloud associated with the Perseus arm at 2.6 kpc. In this case, the apparently low luminosity of L1014-IRS, 0.090 Lsun, is consistent with it having a substellar mass. However, if L1014-IRS is obscured by a circumstellar disk, its luminosity and inferred mass may be greater. Using near-infrared colors of background stars, we investigate characteristics of the L1014 molecular cloud core. We determine a mass of 5.7 Msun for regions of the core with Av > 2 magnitudes. A comparison of the radial extinction profile of L1014 with other cores suggests that L1014 may be among the most centrally condensed cores known, perhaps indicative of the earliest stages of brown dwarf or star formation processes.
Detection of gamma-ray bursts (GRBs) from redshifts z > 7 would open a new window into the earliest epoch of cosmic star formation. We construct separate star formation histories at high redshifts for normal (Pop I and II) stars, and for predominantly massive (Pop III) stars. Based on these separate histories, we predict the GRB redshift distribution to be observed by the {\it Swift} mission. Regardless of whether Pop III progenitors are able to trigger GRBs, we find that a fraction 10% of all bursts detected by {\it Swift} will originate at z > 5. This baseline contribution is due to Pop I/II star formation which must have extended out to high redshifts in rare massive galaxies that were enriched by heavy elements earlier than the typical galaxies. In addition, we consider the possible contribution of Pop III progenitors to the observable GRB rate. Pop III stars are viable progenitors for long-duration GRBs which are triggered by the collapsar mechanism, as long as they can lose their outer envelope through mass transfer to a companion star in a close binary. We find that the likelihood of Pop III binaries to satisfy the conditions required by the collapsar mechanism could be enhanced significantly relative to Pop I/II binaries. If Pop III binaries are common, {\it Swift} will be first observatory to probe Pop III star formation at redshifts z > 7.
We present the spectral evolution, light curve, and corresponding interpretation for the "normal-bright" Type Ia Supernova 2005cg discovered by ROTSE-IIIc. The host is a low-luminosity (M_r = -16.75), blue galaxy with strong indications of active star formation and an environment similar to that expected for SNe Ia at high redshifts. Early-time (t ~ -10 days) optical spectra obtained with the HET reveal an asymmetric, triangular-shaped Si II absorption feature at about 6100 \AA with a sharp transition to the continuum at a blue shift of about 24,000 km s^-1. By 4 days before maximum, the Si II absorption feature becomes symmetric with smoothly curved sides. Similar Si II profile evolution has previously been observed in other supernovae, and is predicted by some explosion models, but its significance has not been fully recognized. Although the spectra predicted by pure deflagration and delayed detonation models are similar near maximum light, they predict qualitatively different chemical abundances in the outer layers and thus give qualitatively different spectra at the earliest phases. The Si line observed in SN 2005cg at early times requires the presence of burning products at high velocities and the triangular shape is likely to be formed in an extended region of slowly declining Si abundance that characterizes delayed detonation models. The spectra show a high-velocity Ca II IR feature that coincides in velocity space with the Si II cutoff. This supports the interpretation that the Ca II is formed when the outer layers of the SN ejecta sweep up about 5 x 10^-3 M_sun of material within the progenitor system. (Abridged)
A fine balance between dark and baryonic mass is observed in spiral galaxies. As the contribution of the baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for \LCDM galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.
Goertz and Morfill (1983) propose that spokes on Saturn's rings form under radially moving plasma clouds produced by meteoroid impacts. We demonstrate that the speed at which a plasma cloud can move relative to the ring material is bounded from above by the difference between the Keplerian and corotation velocities. The radial orientation of new spokes requires radial speeds that are at least an order of magnitude larger than this upper limit, thus the model advanced by Goertz and Morfill fails to make radial spokes.
We explore low angular momentum accretion flows onto black holes formed after the collapse of massive stellar cores. In particular, we consider the state of the gas falling quasi-spherically onto stellar-mass black holes in the hypercritical regime, where the accretion rates are in the range 0.001< Mdot < 0.5 solar masses per second and neutrinos dominate the cooling. Previous studies have assumed that in order to have a black hole switch to a luminous state, the condition l >> R_g c needs to be fulfilled. We argue that flows in hyperaccreting, stellar mass disks around black holes are likely to transition to a highly radiative state when their angular momentum is just above the threshold for disk formation, l ~ 2 R_g c. In a range R_g c < l < 2R_g c, a dwarf disk forms in which gas spirals fast into the black hole without any help from horizontal viscous stresses due to general relativistic effects. For high rotation rates l > 2 R_g c, the luminosity is supplied by large, hot equatorial bubbles around the black hole. The highest neutrino luminosities are obtained for l ~2 R_g c, and this value of angular momentum also produces the most energetic neutrinos, and thus also the highest energy deposition rates. Given the preferential range of l explored in this work, we argue that, as long as l > 2R_g c, low angular momentum cores may in fact be better suited for producing neutrino--driven explosions in core collapse supernovae.
We present HST/ACS observations of the most distant radio galaxy known, TN
J0924-2201 at z=5.2. This radio galaxy has 6 spectroscopically confirmed Lya
emitting companion galaxies, and appears to lie within an overdense region. The
radio galaxy is marginally resolved in i_775 and z_850 showing continuum
emission aligned with the radio axis, similar to what is observed for lower
redshift radio galaxies. Both the half-light radius and the UV star formation
rate are comparable to the typical values found for Lyman break galaxies at
z~4-5. The Lya emitters are sub-L* galaxies, with deduced star formation rates
of 1-10 Msun/yr. One of the Lya emitters is only detected in Lya. Based on the
star formation rate of ~3 Msun/yr calculated from Lya, the lack of continuum
emission could be explained if the galaxy is younger than ~2 Myr and is
producing its first stars.
Observations in V_606, i_775, and z_850 were used to identify additional
Lyman break galaxies associated with this structure. In addition to the radio
galaxy, there are 22 V-break (z~5) galaxies with z_850<26.5 (5sigma), two of
which are also in the spectroscopic sample. We compare the surface density of
2/arcmin^2 to that of similarly selected V-dropouts extracted from GOODS and
the UDF Parallel fields. We find evidence for an overdensity to very high
confidence (>99%), based on a counts-in-cells analysis applied to the control
field. The excess is suggestive of the V-break objects being associated with a
forming cluster around the radio galaxy.
We study the production of main sequence mergers of tidally-synchronized primordial short-period binaries. The principal ingredients of our calculation are the angular momentum loss rates inferred from the spindown of open cluster stars and the distribution of binary properties in young open clusters. We compare our results with the expected number of systems that experience mass transfer in post-main sequence phases of evolution and compute the uncertainties in the theoretical predictions. We estimate that main-sequence mergers can account for the observed number of single blue stragglers in M67. Applied to the blue straggler population, this implies that such mergers are responsible for about one quarter of the population of halo blue metal poor stars, and at least one third of the blue stragglers in open clusters for systems older than 1 Gyr. The observed trends as a function of age are consistent with a saturated angular momentum loss rate for rapidly rotating tidally synchronized systems. The predicted number of blue stragglers from main sequence mergers alone is comparable to the number observed in globular clusters, indicating that the net effect of dynamical interactions in dense stellar environments is to reduce rather than increase the blue straggler population. A population of subturnoff mergers of order 3-4% of the upper main sequence population is also predicted for stars older than 4 Gyr, which is roughly comparable to the small population of highly Li-depleted halo dwarfs. Other observational tests are discussed.
We present a comprehensive series of $N$-body as well as $N$-body + SPH
simulations to study the secular evolution of the structure of disk galaxies.
Our simulations are organized in a hierarchy of increasing complexity, ranging
from rigid-halo collisionless simulations to fully live simulations with gas
and star formation. Comparisons between the different types of simulations
allow us to isolate the role of various physical mechanisms. We focus on the
evolution of systems expected in a LCDM universe.
Our goal is to examine which structural properties of disk galaxies may
result from secular evolution rather than from direct hierarchical assembly. In
the vertical direction, we find that various mechanisms can lead to heating.
The strongest heating occurs during the vertical buckling instability of a bar.
Among the consequences of this instability is the formation of peanut-shaped
bulges which produce clear kinematic signatures when observed face-on. We find
that bars are robust structures that are not destroyed by buckling. They can be
destroyed instead by a central mass concentration but we find that this mass
needs to be a large fraction of the total mass of the disk. We then study the
evolution of stellar surface density profiles showing how angular momentum
redistribution leads to increasing central densities and disk scale lengths and
to profile breaks at large radii. The breaks in these simulations are in
excellent agreement with observed breaks, even when the evolution is purely
collisionless. Disk scale-lengths increase even when the total disk angular
momentum is conserved; thus mapping halo angular momenta to scale-lengths is
non-trivial. [Abridged]
Using HST/STIS, we have detected far-ultraviolet nuclear activity in the giant elliptical galaxy NGC 1399, the central and brightest galaxy in the Fornax I cluster. The source reached a maximum observed far-UV luminosity of \~1.2 x 10e39 ergs/s in January 1999. It was detectable in earlier HST archival images in 1996 (B band) but not in 1991 (V band) or 1993 (UV). It faded by a factor of ~4x by mid-2000. The source is almost certainly associated with the low luminosity AGN responsible for the radio emission in NGC 1399. The properties of the outburst are remarkably similar to the UV-bright nuclear transient discovered earlier in NGC 4552 by Renzini et al. (1995). The source is much fainter than expected from its Bondi accretion rate (estimated from Chandra high resolution X-ray images), even in the context of "radiatively inefficient accretion flow" models, and its variability also appears inconsistent with such models. High spatial resolution UV monitoring is a valuable means to study activity in nearby LLAGNs.
Gas-rich dwarf and disk galaxies overlap in numerous physical quantities that make their classification subjective. We report the discovery of a separation between dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be because of their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low and high mass galaxies or produced by hierarchical mergers where disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics where the strong rotation in disks produces an axial symmetric object that undergoes highly efficient star formation in contrast to the lower rotation, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.
Peculiar velocities thoughout the region of the local supercluster are reconstructed by two different orbit-retracing methods. The requirement of the optimal correlation between the radial components of reconstructed velocities and the observed peculiar velocities derived from our extensive new catalog of distances puts stringent constraints on the values of the cosmological parameters. Our constraints intersect those from studies of microwave background fluctuations and statistical properties of galaxy clustering: the ensemble of constraints are consistent with Omega_m=0.22\pm 0.02. While motions throughout the Local Supercluster provide a measure of the mean ratio of mass to light, there can be large local fluctuations. Our reconstruction of the infall velocities in the immediate vicinity of the Virgo Cluster shows that there is a mass-to-light anomaly of a factor of 3 to 6 between groups in the general field environment and the heavily populated Virgo Cluster.
We report that neutral hydrogen (HI) gas clouds, resembling High Velocity Clouds (HVCs) observed in the Milky Way (MW), appear in MW sized disk galaxies formed in high resolution Cold Dark Matter (CDM) cosmological simulations which self-consistently include gas-dynamics, radiative cooling, star formation, supernova feedback, and metal enrichment. Two disk galaxies found in cosmological simulations are analyzed, and HI column density and velocity distributions in full-sky aitoff projections are constructed. The simulations demonstrate that CDM is able to create galaxies with sufficient quantities of HVCs to explain the HVCs observed within the MW, and that they are found within a galactocentric radius of 150 kpc. We also find that one of the galaxies has an outer gas ring in polar orbit, i.e. a polar gas ring, with radius 30 kpc, which appears as a large structure of HVCs in the aitoff projection. Such large structures may share an origin similar to large HVCs observed in the MW, such as Complex C.
We present our diagnosis of the role massive stars play in the formation of low- and intermediate-mass stars in OB associations. In the bright-rimmed and comet-shaped clouds in both the Ori OB1 and Lac OB1 associations, there is compelling evidence of low- and intermediate-mass star formation dominated by the triggering process by the massive stars in the regions. The expanding ionization fronts from the O stars appear to have compressed nearby molecular clouds which are engraved and shaped into bright-rimmed or comet-shaped clouds. Implosive pressure on the surface layers then causes a cloud to collapse and prompts subsequent formation of stars, including low- and intermediate-mass stars. The triggering process may propagate through one cloud after another, and the majority of the stellar population in an entire OB association with a scale of tens of parsec may be formed in the sequence.
We report photopolarimetric observations of a new eclipsing polar (AM Herculis system) discovered in the Calan-Tololo survey. The photometry and polarimetry are modulated on a period of ~101 minutes. Circular polarization variations are seen from ~-8 to +12 per cent and from ~0 to ~5 per cent in the red and blue parts of the optical spectrum, respectively. Two linearly polarized pulses are detected at orbital phases coinciding with the reversals in the circular polarization. This is consistent with a magnetic field strength of ~20 MG for the white dwarf primary, where accretion takes place at two regions. Both accretion regions are self-occulted by the white dwarf during parts of the orbit. We estimate some of the system's parameters from its eclipses, which we further refine by modelling the polarimetric observations.
Recent studies of the substellar population in the Taurus cloud have revealed a deficit of brown dwarfs (BD) compared to the Trapezium cluster population (Briceno et al 1998; Luhman 2000; Luhman et al 2003a; Luhman 2004). However, these works have concentrated on the highest stellar density regions of the Taurus cloud. We have performed a large scale optical survey of this region, covering a total area of 30 deg^2, and encompassing the densest part of the cloud as well as their surroundings, down to a mass detection limits of 15 Jupiter Masses (MJ). In this paper, we present the optical spectroscopic follow-up observations of 97 photometrically selected potential new low-mass Taurus members, of which 27 are strong late-M (SpT < M4V) candidates. These observations reveal 5 new very low mass (VLM) Taurus members and 12 new BDs. Combining our observations with previously published results, we derive an updated substellar to stellar ratio in Taurus of Rss =0.23 +/- 0.05. This ratio now appears consistent with the value previously derived in the Trapezium cluster under similar assumptions of 0.26 +/- 0.04. We find strong indication that the relative numbers of BDs with respect to stars is decreased by a factor 2 in the central regions of the aggregates with respect to the more distributed population. Our findings are best explained in the context of the embryo-ejection model where brown dwarfs originate from dynamical interactions in small N unstable multiple systems.
The growth and virialization of spherical top-hat fluctuations, in coupled Dark Energy models, causes segregation between Dark Matter (DM) and baryons, as the gravitational infall into the potential well proceeds more slowly for the baryons than for DM. As a consequence, after attaining their turn-around and before full virialization, halos have outer layers rich of baryons. Accordingly, a natural ambiguity exists on the definition of the virial density contrast. In fact, when the outer baryon layers infall onto the DM-richer core, they carry with them DM materials outside the original fluctuation; hence, no time exists when all materials originally belonging to the fluctuation -and only them- have virialized. Baryon-DM segregation can have various astrophysical consequences on different length-scales. The smallest halos may loose up to 50% of the original baryonic contents and become hardly visible. Subhalos in cluster-size halos may loose much baryonic materials, which could then be observed as intra-cluster light. Isolated halos, in general, can be expected to have a baryon component richer than the cosmological proportions, due to the cosmic enrichement of baryons lost in small halo incounters.
We have used DOLORES at the TNG to obtain B,V time series photometry of NGC
2419, one of the most distant and bright clusters in the Galactic halo. These
data will be used to study its variable star population in order to check
whether the cluster could be the relic of an extragalactic system accreted by
the Milky Way.
Using the Image Subtraction technique (Alard 2000) we have identified about
300 candidate variables, many of which are in the cluster central regions.
Several of the variables appear to be RR Lyrae stars, but we detected
variability also around the tip of the red giant branch, and in other regions
of the colour-magnitude diagram. To improve the light curve sampling and to
resolve variables in the cluster inner regions, the TNG data were combined with
HST archive data. Preliminary results are presented on the light curves from
the combined data set.
The present status of our knowledge about the dark matter and dark energy is reviewed. Particular emphasis is put on the. Bounds on the content of cold and hot dark matter from cosmological observations are discussed in some detail. I also review current bounds on the physical properties of dark energy, mainly its equation of state and effective speed of sound.
We present updated results of spectroscopic follow-up observations of a sample of 45 M dwarf candidates identified by Phan-Bao et al. (2003) based on the DENIS photometry and proper motion measurements. Forty one of these are nearby late-M dwarfs (d < 30 pc) with spectral types ranging from M5.0 to M8.5 computed from the spectral indices. One contaminant is probably an F-G main sequence star reddened by intervening dust and three stars that were not observed have previous classifications as M dwarfs in the literature. In this paper, we identify three M7.5, five M8.0, one M8.5 dwarf and confirm two new M8.0 dwarf members of the 25 pc volume.
The first string of the neoteric high energy neutrino telescope IceCube successfully began operating in January 2005. It is anticipated that upon completion the new detector will vastly increase the sensitivity and extend the reach of AMANDA to higher energies. A discussion of the IceCube's discovery potential for extra-terrestrial neutrinos, together with the prospects of new physics derived from the ongoing AMANDA research will be the focus of this paper. Preliminary results of the first antarctic high energy neutrino telescope AMANDA searching in the muon neutrino channel for localized and diffuse excess of extra-terrestrial neutrinos will be reviewed using data collected between 2000 and 2003. Neutrino flux limits obtained with the all-flavor dedicated UHE and cascade analyses will be described. A first neutrino spectrum above one TeV in agreement with atmospheric neutrino flux expectations and no extra-terrestrial contribution will be presented, followed by a discussion of a limit for neutralino CDM candidates annihilating in the center of the Sun.
We address the question of how well the density profile of galaxy clusters can be determined by combining strong lensing and velocity dispersion data, as recently done by Sand et al. We use cosmological dark matter simulations of clusters to test the reliability of the method, producing mock catalogues of tangential and radial gravitational arcs and simulating the radial velocity dispersion profile of the cluster brightest central galaxy. The density profiles of the simulated clusters closely follow the NFW form, but we find that the recovered values of the inner slope are systematically underestimated, by about 0.4 in the mean, if the lens is assumed to be axially symmetric, as Sand et al. did. However, if the ellipticity and orientation of the iso-contours of the cluster lensing potential are taken into account, then the inner slopes can be recovered quite accurately for a significant subset of the clusters whose central surface density profiles appear the most regular. For the remaining clusters, where the lensing potential is strongly perturbed by active merging or by substructure, the correct determination of the inner slope requires a more accurate model for the lens. When the halo profile is modelled by a generalised NFW profile, we find that the inferred scale radius and characteristic density, unlike the inner slope, are generally poorly constrained, since there is a strong degeneracy between these two parameters.
Origin of ultra high energy cosmic rays is an unsolved problem. Several proposals such as Z-burst, decay of super massive matter, susy particles as a primary, neutrino as a primary in extra dimension models exist in the literature which try to address this issue. Many of these proposals solve the problem of propagation of cosmic rays over cosmological distances by introducing new physics. However these do not explain the origin of such high energy cosmic rays. The possible astrophysics sites, such as active galactic nuclei, are highly constrained. Here we determine whether these cosmic rays originated from the decay of some exotic objects, such as primordial black holes (PBHs), present in the early universe. In contrast to the usual Top Down scenario we do not assume that this exotic object necessarily has to decay in our astrophysical neighbourhood since we assume a beyond the standard model scenario, where the propagation problem is absent. We consider the standard 4-dimension PBHs as well as the brane world PBHs. We find that in both cases it is unable to produce the observed ultra high energy cosmic ray flux.
The properties of the outer crust of non-accreting cold neutron stars are studied by using modern nuclear data and theoretical mass tables updating in particular the classic work of Baym, Pethick and Sutherland. Experimental data from the atomic mass table from Audi, Wapstra, and Thibault of 2003 is used and a thorough comparison of many modern theoretical nuclear models, relativistic and non-relativistic ones, is performed for the first time. In addition, the influences of pairing and deformation are investigated. State-of-the-art theoretical nuclear mass tables are compared in order to check their differences concerning the neutron dripline, magic neutron numbers, the equation of state, and the sequence of neutron-rich nuclei up to the dripline in the outer crust of non-accreting cold neutron stars.
We present a detailed and uniform study of oxygen abundances in 155 solar type stars, 96 of which are planet hosts and 59 of which form part of a volume-limited comparison sample with no known planets. EW measurements were carried out for the [O I] 6300 \AA line and the O I triplet, and spectral synthesis was performed for several OH lines. NLTE corrections were calculated and applied to the LTE abundance results derived from the O I 7771-5 \AA\ triplet. Abundances from [O I], the O I triplet and near-UV OH were obtained in 103, 87 and 77 dwarfs, respectively. We present the first detailed and uniform comparison of these three oxygen indicators in a large sample of solar-type stars. There is good agreement between the [O/H] ratios from forbidden and OH lines, while the NLTE triplet shows a systematically lower abundance. We found that discrepancies between OH, [O I] and the O I triplet do not exceed 0.2 dex in most cases. We have studied abundance trends in planet host and comparison sample stars, and no obvious anomalies related to the presence of planets have been detected. All three indicators show that, on average, [O/Fe] decreases with [Fe/H] in the metallicity range -0.8<[Fe/H]<0.5. The planet host stars present an average oxygen overabundance of 0.1-0.2dex with respect to the comparison sample.
We present MERLIN observations of OH maser and radio continuum emission from the Ultra Luminous IR Galaxy Markarian 231. The 1665- and 1667-MHz transitions have a combined velocity extent of 720 km/s and show a similar position-velocity structure including a gradient of 1.7 km/s/pc from NW to SE along the 420-pc major axis, steeper in the inner few tens of pc. The maser distribution is modelled as a torus rotating about an axis inclined at ~45deg. We estimate the enclosed mass density to be 320(90) Msun in a flattened distribution, including a central unresolved mass of </=8E+06 Msun. All the maser emission is projected against a region with a radio continuum brightness temperature >/=1E+05 K, giving a maser gain of </=2.2. The 1667:1665-MHz line ratio is close to the LTE ratio of 1.8 consistent with radiatively pumped, unsaturated masers. The size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273 where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification as a Seyfert 1. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.
Recently, many dark energy models whose the equation-of-state parameter can cross the phantom divide $w_{de}=-1$ have been proposed. In a previous paper [Class. Quant. Grav. 22, 3189 (2005); hep-th/0501160], we suggest such a model named hessence dark energy model, in which a non-canonical complex scalar field plays the role of dark energy. In this work, the cosmological evolution of the hessence dark energy is investigated. We consider two cases: one is the hessnece field with an exponential potential, and the other is with a (inverse) power law potentials. We separately investigate the dynamical system with four different interaction forms between hessence and background perfect fluid. It is found that the big rip always does not appear in the hessence model even in the most general cases beyond above particular potentials and interaction forms.
We present a new model of the heliospheric interface - the region of the solar wind interaction with the local interstellar medium. This new model performs a multi-component treatment of charged particles in the heliosphere. All charged particles are divided into several co-moving types. The coldest type, with parameters typical of original solar wind protons, is considered in the framework of fluid approximation. The hot pickup proton components created from interstellar H atoms and heliospheric ENAs by charge exchange, electron impact ionization and photoionization are treated kinetically. The charged components are considered self-consistently with interstellar H atoms, which are described kinetically as well. To solve the kinetic equation for H atoms we use the Monte Carlo method with splitting of trajectories, which allows us 1) to reduce statistical uncertainties allowing correct interpretation of observational data, 2) to separate all H atoms in the heliosphere into several populations depending on the place of their birth and on the type of parent protons.
In this document we collect the 18 contributions of the IceCube Collaboration to the 29th International Cosmic Ray Conference (ICRC 2005), Pune, India, Aug. 2005
The first extrasolar planets have been detected by the measurement of the wobble of the parent star. This wobble leads to the periodic modulation of three observables: the radial velocity, the position on the sky and the time of arrival of periodic signals. We show that the same wobble, and therefore the same modulation of the three observables, can be due to the presence of a more distant binary stellar companion. Thus, the observation of the wobble does not, by itself, constitute a proof of a planet detection. In particular, astrometric confirmation of a wobble does not necessarily provide a sufficient proof of the existence of a planet candidate detected by radial velocity. Additional conditions, which we discuss here, must be fulfilled. We investigate the observed wobble for the planet candidates already detected and we find that, for each case, a wobble due to a binary stellar companion can be excluded. But for apparent Saturn-like planets in wide orbits, there may be an ambiguity in future detections, especially in spaceborne astrometric missions. We conclude that, in some cases, a definitive proof for the presence of a planet requires further observations such as direct imaging.
Using an eigenspectrum decomposition technique, we separate the host galaxy from the broad line active galactic nucleus (AGN) in a set of 4666 spectra from the Sloan Digital Sky Survey (SDSS), from redshifts near zero up to about 0.75. The decomposition technique uses separate sets of galaxy and quasar eigenspectra to efficiently and reliably separate the AGN and host spectroscopic components. The technique accurately reproduces the host galaxy spectrum, its contributing fraction, and its classification. We show how the accuracy of the decomposition depends upon S/N, host galaxy fraction, and the galaxy class. Based on the eigencoefficients, the sample of SDSS broad-line AGN host galaxies spans a wide range of spectral types, but the distribution differs significantly from inactive galaxies. In particular, post-starburst activity appears to be much more common among AGN host galaxies. The luminosities of the hosts are much higher than expected for normal early-type galaxies, and their colors become increasingly bluer than early-type galaxies with increasing host luminosity. Most of the AGNs with detected hosts are emitting at between 1% and 10% of their estimated Eddington luminosities, but the sensitivity of the technique usually does not extend to the Eddington limit. There are mild correlations among the AGN and host galaxy eigencoefficients, possibly indicating a link between recent star formation and the onset of AGN activity. The catalog of spectral reconstruction parameters is available as an electronic table.
We compute the emission of gravitational radiation from pulsating white dwarfs. This is done by using an up-to-date stellar evolutionary code coupled with a state-of-the-art pulsational code. The emission of gravitational waves is computed for a standard 0.6 solar masses white dwarf with a liquid carbon-oxygen core and a hydrogen-rich envelope, for a massive DA white dwarf with a partially crystallized core for which various l=2 modes have been observed (BPM 37093) and for PG 1159-035, the prototype of the GW Vir class of variable stars, for which several quadrupole modes have been observed as well. We find that these stars do not radiate sizeable amounts of gravitational waves through their observed pulsation g-modes, in line with previous studies. We also explore the possibility of detecting gravitational waves radiated by the f-mode and the p-modes. We find that in this case the gravitational wave signal is very large and, hence, the modes decay very rapidly. We also discuss the possible implications of our calculations for the detection of gravitational waves from pulsating white dwarfs within the framework of future space-borne interferometers like LISA.
We investigate the star formation activity in galaxy pairs in chemical hydrodynamical simulations consistent with a Lambda-CDM scenario. A statistical analysis of the effects of galaxy interactions on the star formation activity as a function of orbital parameters shows that close encounters (r < 30 kpc/h) can be effectively correlated with an enhancement of star formation activity with respect to galaxies without a close companion. Our results suggest that the stability properties of systems are also relevant in this process. We found that the passive star forming galaxies pairs tend to have deeper potential wells, older stellar populations, and less leftover gas than active star forming ones. In order to assess the effects that projection and interlopers could introduce in observational samples, we have also constructed and analysed projected simulated catalogs of galaxy pairs. In good agreement with observations, our results show a threshold (rp < 25 kpc/h) for interactions to enhance the star formation activity with respect to galaxies without a close companion. Finally, analysing the environmental effect, we detect the expected SFR-local density relation for both pairs and isolated galaxy samples, although the density dependence is stronger for galaxies in pairs suggesting a relevant role for interactions in driving this relation.
We present the design and first test results of a new FADC-based data
acquisition (DAQ) system for the Grande array of the KASCADE-Grande experiment.
The original KASCADE experiment at the Forschungszentrum Karlsruhe, Germany,
has been extended by 37 detector stations of the former EAS-TOP experiment
(Grande array)to provide sensitivity to energies of primary particles from the
cosmos of up to $10^{18}$ eV. The new FADC-based DAQ system will improve the
quality of the data taken by the Grande array by digitizing the scintillator
signals with a 250 MHz sampling rate. events per second.
Two Grande stations have been equipped with the FADC-based data acquisition
system and first data are shown.
In preparation for the COROT space mission, we determined the fundamental parameters (spectral type, temperature, gravity, vsini) of the Be stars observable by COROT in its seismology fields (64 Be stars). We applied a careful and detailed modeling of the stellar spectra, taking into account the veiling caused by the envelope, as well as the gravitational darkening and stellar flattening due to rapid rotation. Evolutionary tracks for fast rotators were used to derive stellar masses and ages. The derived parameters will be used to select Be stars as secondary targets (i.e. observed for 5 consecutive months) and short-run targets of the COROT mission. Furthermore, we note that the main part of our stellar sample is falling in the second half of the main sequence life time, and that in most cases the luminosity class of Be stars is inaccurate in characterizing their evolutionary status.
We report on the latest version of our GNU-Linux port of the ATLAS - SYNTHE - WIDTH suite of codes for the stellar atmosphere modeling. The latest version (8.1 at the time of the workshop) of the Intel Fortran Compiler allowed for a significantly better backward compatibility with the VMS version of the code, thus allowing us to remove almost all the modifications we initially introduced in order to compile the code under IFC. We now provide ported versions both of ATLAS 9 (the ODF version of ATLAS) and ATLAS 12 (the opacity sampling version). A comprehensive website has been created to host the ported codes, along with a growing body of documentation and resources. Also, three mailing lists have been created at the university of Ljubljana in order to cover general usage, code development and ATLAS-related announcements.
The observed near infrared background excess over light from known galaxies is commonly ascribed to redshifted radiation from early, very massive, PopIII stars. We show here that this interpretation must be discarded as it largely overpredicts the number of J-dropouts and Ly\alpha emitters in ultra deep field searches. Independently of the detailed physics of Ly\alpha line emission, J-dropouts limit the background excess fraction due to PopIII sources to be (at best) < 1/24. As alternative explanations can either be rejected (e.g. miniquasars, decaying neutrinos) or appear unlikely (zodiacal light), whereas the reality of the excess is supported by the interpretation of the angular fluctuations, the origin of this component remains very puzzling. We briefly discuss possible hints to solve the problem.
Observations of hot stars belonging to the young cluster LMC-NGC2004 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. 25 Be stars were discovered; the proportion of Be stars compared to B-type stars is found to be of the same order in the LMC and in the Galaxy fields. 23 hot stars were discovered as spectroscopic binaries (SB1 and SB2), 5 of these are found to be eclipsing systems from the MACHO database, with periods of a few days. About 75% of the spectra in our sample are polluted by hydrogen (Halpha and Hgamma), [SII] and [NII] nebular lines. These lines are typical of HII regions. They could be associated with patchy nebulosities with a bi-modal distribution in radial velocity, with higher values (+335 kms^{-1}) preferentially seen inside the southern part of the known bubble LMC4 observed in HI at 21 cm.
With the Spitzer Space Telescope Infrared Spectrograph (IRS) we have observed seven powerful FR2 radio galaxies and seven quasars. Both samples, the galaxies and the quasars, are comparable in isotropic 178 MHz luminosity (10^26.5 W/Hz < P 178MHz < 10^29.5 W/Hz) and in redshift range (0.05 < z < 1.5). We find for both samples similar distributions in the luminosity ratios of high- to low-excitation lines ([NeV] 14.3 mu / [NeII] 12.8 mu) and of high-excitation line to radio power ([NeV] 24.3 \mu / P 178MHz). This solves the long debate about the apparent difference of quasars and radio galaxies in favor of the orientation-dependent unified schemes. Furthermore, the luminosity ratio [OIII] 500.7 nm / [OIV] 25.9 mu of most galaxies is by a factor of ten lower than that of the quasars. This suggests that the optical emission from the central NLR is essentially absorbed (A_V} > 3) in the powerful FR2 galaxies and that the [OIII] 500.7 nm luminosity does not serve as isotropic tracer for testing the unified schemes.
The near-infrared shape of the big blue bump component in quasar spectra has been essentially unknown. It usually cannot be observed directly, due to the strong hot dust emission which dominates quasar spectra longward of ~1micron. However this is quite an important part of the spectrum theoretically. At least bare disk models provide quite a robust prediction for the overall continuum shape in the near-infrared. Self-gravity should become important in the outer, near-infrared emitting regions of the putative disk, possibly leaving a signature of disk truncation in the near-infrared. We propose here that this important part of the spectrum can be revealed for the first time by observing polarized flux from normal quasars. At least in some polarized quasars, the emission lines are all unpolarized and so the polarized flux should originate interior to the broad line region, and therefore also interior to the dust emitting region. This can then be used to eliminate the dust emission. We present the results of near-infrared polarimetry for such three quasars (Ton202, 4C37.43, B2 1208+32). The data for Ton202 have the highest S/N, and the near-infrared polarized flux in this case is measured to have quite a blue shape, nu^+0.42+-0.29 in F_nu, intriguingly consistent with the simple multi-temperature black body, bare disk prediction of nu^+1/3. All these data, although still with quite low S/N for the other two objects, demonstrate the unique potential of the technique with future better data. We also present similar data for other quasars and radio galaxies, and briefly discuss the nature of the polarization.
X-ray observations have revealed that many microquasars and low-mass X-ray binaries (LMXBs) exhibit narrow absorption features identified with resonant absorption from Fe XXV and Fe XXVI and other abundant ions. In many well studied systems there is evidence for blue-shifts, indicating outflowing plasmas. We succesfully model the changes in both the X-ray continuum and the Fe absorption features during dips from all the bright dipping LMXBs observed by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371 and XB 1916-053) as resulting primarily from an increase in column density and a decrease in the ionization state of a highly-ionized absorber in a similar way as was done for 4U 1323-62. This implies that the complex spectral changes in the X-ray continua observed from the dip sources as a class can be most simply explained primarily by changes in the highly ionized absorbers present in these systems. There is no need to invoke unusual abundances or partial covering of extended emission regions. Outside of the dips, the absorption line properties do not vary strongly with orbital phase. This implies that the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk. Since dipping sources are simply normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs.
We present new Spitzer IRS observations of HD164270 (WC9, WR103). A quantitative analysis of the UV, optical, near- and mid-IR spectrum of HD164270 is presented, allowing for line blanketing and wind clumping, revealing T* ~ 48kK, log L/L_sun ~ 4.9, dM/dt ~ 1e-5 M_sun/yr for a volume filling factor of f \~ 0.1. Our models predict that He is partially recombined in the outer stellar wind, such that recent radio-derived mass-loss rates of WC9 stars have been underestimated. We obtain C/He ~ 0.2 and O/He ~ 0.01 by number from optical diagnostics. Mid-IR fine structure lines of [NeII] 12.8 and [SIII] 18.7micron are observed, with [NeIII] and [SIV] absent. From these we obtain Ne/He ~ Ne^+/He = 2.2e-3 by number, 7 times higher than the Solar value (as recently derived by Asplund et al.), and S/He ~ S^2+/He = 5.1e-5 by number. From a comparison with similar results for other WC subtypes we conclude that WC9 stars are as chemically advanced as earlier subtypes. We consider why late WC stars are exclusively observed in high metallicity environments. In addition, we compare the UV/optical/mid-IR spectroscopic morphology of HD164270 with the Planetary Nebula central star BD+30 3639 ([WC9]). Their UV and optical signatures are remarkably similar, such that our quantitative comparisons confirm similarities in stellar temperature, wind densities and chemistry first proposed by Smith & Aller, in spite of completely different evolutionary histories, with HD164270 presently a factor of ten more massive than BD+30 3639. At mid-IR wavelengths, the dust from the dense young, nebula of BD+30 3639 completely dominates its appearance, in contrast with HD164270.
We summarize the results of our ongoing investigation of the chemical abundances in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and in the Canis Major Overdensity (CMa). 12 RGB stars were analyzed in the Sgr dSph, plus 5 in the associated globular Terzan 7, together with three CMa candidate members. Detailed abundances have been derived for up to 23 elements from Oxygen to Europium.
When analysing HII regions, a possible source of systematic error on empirically derived physical quantities is the limited size of the slit used for the observations. A grid of photoionization models was built through the Aangaba code varying the ionizing radiation spectrum emitted by a stellar cluster, as well as the gas abundance. The calculated line surface brightness was then used to simulate slit observations and to derive empirical parameters using the usual methods described in the literature. Depending on the fraction of the object covered by the slit, the parameters can be different from those obtained from observations of the whole object, an effect that is mainly dependent on the age of the ionizing stellar cluster. The low-ionization forbidden lines are more sensitive to the size of the area covered by the slit than the high-ionization forbidden lines or recombination lines. Regarding the temperature indicator T[OIII], the slit effects are small since this temperature is derived from [OIII] lines. On the other hand, for the abundance indicator R23, which depends also on the [OII] line, the slit effect is slightly higher. Therefore, the systematic error due to slit observations on the O abundance is low, being usually less than 10%, except for HII regions powered by stellar clusters with a relative low number of ionizing photons between 13.6 and 54.4 eV, which create a smaller O++ emitting volume. In this case, the systematic error on the empirical O abundance deduced from slit observations is more than 10% when the covered area is less than 50%.
We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC.
We report the first time-resolved photometric and spectroscopic optical observations of the X-ray source RX J2133.7+5107, identified in the ROSAT survey. A clear persistent optical light pulsation is discovered with fast photometry at a period of P_{omega} =(570.823 +/-0.013) s which we associate with the spin period of an accreting white dwarf. Radial velocity curves of the strong emission lines show modulation with a period of P_{Omega} =(7.193 +/- 0.016) hr, identified as the orbital period. These observations establish that the source is a member of the intermediate polar class (IPs) of magnetic cataclysmic variables. With only 4 IPs with longer orbital periods, RX J2133.7+5107 is among the widest systems. It is a unique IP with an orbital period in the middle of the so-called (6-10)hr IP gap and it shows a significant degree of asynchronism with a ratio P_{omega}/P_{Omega} of 0.02. When attributed to the motion of the white dwarf, the emission lines orbital modulation yields a mass function of f_m = (1.05 +/- 0.21) 10^{-2} Msun which, for a probable inclination i < 45 deg and a white dwarf mass M_{wd} = (0.6-1.0) Msun, corresponds to a secondary mass M_{s} > (0.27-0.37) Msun.
We show, theoretically and via MHD simulations, how a short burst of
reconnection localized in three dimensions on a one-dimensional current sheet
creates a pair of reconnected flux tubes. We focus on the post-reconnection
evolution of these flux tubes, studying their velocities and shapes. We find
that slow-mode shocks propagate along these reconnected flux tubes, releasing
magnetic energy as in steady-state Petschek reconnection. The geometry of these
three-dimensional shocks, however, differs dramatically from the classical
two-dimensional geometry. They propagate along the flux tube legs in four
isolated fronts, whereas in the two-dimensional Petschek model, they form a
continuous, stationary pair of V-shaped fronts.
We find that the cross sections of these reconnected flux tubes appear as
teardrop shaped bundles of flux propagating away from the reconnection site.
Based on this, we argue that the descending coronal voids seen by Yohkoh SXT,
LASCO, and TRACE are reconnected flux tubes descending from a flare site in the
high corona, for example after a coronal mass ejection. In this model, these
flux tubes would then settle into equilibrium in the low corona, forming an
arcade of post-flare coronal loops.
We present simultaneous X-ray, far-ultraviolet, and near-ultraviolet spectra of the Seyfert 1 galaxy NGC 7469 obtained with the Chandra X-Ray Observatory, the Far Ultraviolet Spectroscopic Explorer, and the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Previous non-simultaneous observations of this galaxy found two distinct UV absorption components, at -560 and -1900 km/s, with the former as the likely counterpart of the X-ray absorber. We confirm these two absorption components in our new UV observations, in which we detect prominent O VI, Ly alpha, N V, and C IV absorption. In our Chandra spectrum we detect O VIII emission, but no significant O VIII or O VII absorption. We also detect a prominent Fe K alpha emission line in the Chandra spectrum, as well as absorption due to hydrogen-like and helium-like neon, magnesium, and silicon at velocities consistent with the -560 km/s UV absorber. The FUSE data reveal that the H I and C IV column densities in this UV- and X-ray- absorbing component have increased over time, as the UV continuum flux decreased. We use measured H I, N V, C IV, and O VI column densities to model the photoionization state of both absorbers self-consistently. We confirm the general physical picture of the outflow in which the low velocity component is a highly ionized, high density absorber with a total column density of 10^20 cm^-2, located near the broad emission line region, although due to measurable columns of N V and C IV, we assign it a somewhat smaller ionization parameter than found previously, U~1. The high velocity UV component is of lower density, log N=18.6, and likely resides farther from the central engine as we find its ionization parameter to be U=0.08.
Recent observations have revealed a class of unusually HI-rich early-type galaxies. By combining observations of their morphology, stellar populations and neutral hydrogen we aim to understand how these galaxies fit into the hierarchical formation paradigm. Here we present the result of our radio and optical observations of a test case galaxy, the E/S0 IC 4200.
We present Spitzer/MIPS 24 micron observations of 50 supernova host galaxies at 0.1<z<1.7 in the Great Observatories Origins Deep Survey (GOODS) fields. We also discuss the detection of SN host galaxies in SCUBA/850 micron observations of GOODS-N and Spitzer/Infrared Spectrograph (IRS) 16 micron observations of GOODS-S. About 60% of the host galaxies of both Type Ia and core-collapse supernovae are detected at 24 microns, a detection rate which is a factor of 1.5 higher than the field galaxy population. Among the 24 micron detected hosts, 80% have far-infrared luminosities that are comparable to or greater than the optical luminosity indicating the presence of substantial amounts of dust in the hosts. The median bolometric luminosity of the Type Ia SN hosts is \~10^10.5 L_sun, very similar to that of core-collapse SN hosts. Using the high resolution Hubble/ACS data, we have studied the variation of rest-frame optical/ultraviolet colors within the 24 micron detected galaxies at z<1 to understand the origin of the dust emission. The 24 micron detected galaxies have average colors which are redder by ~0.1 mag than the 24 micron undetected hosts while the latter show greater scatter in internal colors. This suggests that a smooth distribution of dust is responsible for the observed mid- and far-infrared emission. 70% of supernovae that have been detected in the GOODS fields are located within the half-light radius of the hosts where dust obscuration effects are significant. Although the dust emission that we detect cannot be translated into a line of sight A_V, we suggest that the factor of 2-3 larger scatter in the peak B-V colors that is seen in the high-z Type Ia supernova sample relative to the low-z supernovae might be partially due to the dust that we detect in the hosts.
The carrier of the dust-associated photoluminescence process causing the extended red emission (ERE) in many dusty interstellar environments remains unidentified. Several competing models are more or less able to match the observed broad, unstructured ERE band. We now constrain the character of the ERE carrier further by determining the wavelengths of the radiation that initiates the ERE. Using the imaging capabilities of the Hubble Space Telescope, we have resolved the width of narrow ERE filaments appearing on the surfaces of externally illuminated molecular clouds in the bright reflection nebula NGC 7023 and compared them with the depth of penetration of radiation of known wavelengths into the same cloud surfaces. We identify photons with wavelengths shortward of 118 nm as the source of ERE initiation, not to be confused with ERE excitation, however. There are strong indications from the well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic cirrus that the photon flux with wavelengths shortward of 118 nm is too small to actually excite the observed ERE, even with 100% quantum efficiency. We conclude, therefore, that ERE excitation results from a two-step process. While none of the previously proposed ERE models can match these new constraints, we note that under interstellar conditions most polycyclic aromatic hydrocarbon (PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH di-cations is consistent with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations deserve further study as potential carriers of the ERE. (abridged)
Understanding the mechanism that drives accretion has been the primary
challenge in accretion disk theory. Turbulence provides a natural means of
dissipation and the removal of angular momentum, but firmly establishing its
presence in disks proved for many years to be difficult. The realization in the
1990s that a weak magnetic field will destabilize a disk and result in a
vigorous turbulent transport of angular momentum has revolutionized the field.
Much of accretion disk research now focuses on understanding the implications
of this mechanism for astrophysical observations. At the same time, the success
of this mechanism depends upon a sufficient ionization level in the disk for
the flow to be well-coupled to the magnetic field. Many disks, such as disks
around young stars and disks in binary systems that are in quiescence, are too
cold to be sufficiently ionized, and so efforts to establish the presence of
turbulence in these disks continues.
This dissertation focuses on several possible mechanisms for the turbulent
transport of angular momentum in weakly-ionized accretion disks: gravitational
instability, radial convection and vortices driving compressive motions. It
appears that none of these mechanisms are very robust in driving accretion. A
discussion is given, based on these results, as to the most promising
directions to take in the search for a turbulent transport mechanism that does
not require magnetic fields. Also discussed are the implications of assuming
that no turbulent transport mechanism exists for weakly-ionized disks.
How and when did the first generation of stars form at the end of the cosmic dark ages? Quite generically, within variants of the cold dark matter model of cosmological structure formation, the first sources of light are expected to form in ~ 10^{6} M_sun dark matter potential wells at redshifts z > 20. I discuss the physical processes that govern the formation of the first stars. These so-called Population~III stars are predicted to be predominantly very massive, and to have contributed significantly to the early reionization of the intergalactic medium. Such an early reionization epoch is inferred from the recent measurement of the Thomson optical depth by the {\it WMAP} satellite. I address the importance of heavy elements in bringing about the transition from an early star formation mode dominated by massive stars, to the familiar mode dominated by low mass stars, at later times, and present possible observational probes. This transition could have been gradual, giving rise to an intermediate-mass population of still virtually metal-free stars (``Population II.5''). These stars could have given rise to the peculiar class of black-hole forming supernovae inferred from the abundance pattern of extremely iron-poor stars.
The population synthesis of cataclysmic variables below the period is investigated. A grid of detailed binary evolutionary sequences has been calculated and included in the simulations to take account of additional angular momentum losses beyond that associated with gravitational radiation and mass loss, due to nova outbursts, from the system. As a specific example, we consider the effect of a circumbinary disk to gain insight into the ingredients necessary to reproduce the observed orbital period distribution. The resulting distributions show that the period minimum lies at about 80 minutes with the number of systems monotonically increasing with increasing orbital period to a maximum near 90 minutes. There is no evidence for an accumulation of systems at the period minimum which is a common feature of simulations in which only gravitational radiation losses are considered. The period distribution is found to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The steepness of the lower edge of the period gap can be reproduced, for example, by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic variable binary systems from orbital periods longer than 2.75 hrs. The good agreement with the cumulated distribution function of observed systems within the framework of our model indicates that the angular momentum loss by a circumbinary disk or a mechanism which mimics its features coupled with a weighting factor to account for selection effects in the discovery of such systems and a flow of systems from above the period gap to below the period gap are important ingredients for understanding the overall period distribution of cataclysmic variable binary systems.
As a result of deep hard X-ray observations by Chandra and XMM-Newton a significant fraction of the cosmic X-ray background (CXRB) has been resolved into individual sources. These objects are almost all active galactic nuclei (AGN) and optical followup observations find that they are mostly obscured Type 2 AGN, have Seyfert-like X-ray luminosities (i.e., L_X ~ 10^{43-44} ergs s^{-1}), and peak in redshift at z~0.7. Since this redshift is similar to the peak in the cosmic star-formation rate, this paper proposes that the obscuring material required for AGN unification is regulated by star-formation within the host galaxy. We test this idea by computing CXRB synthesis models with a ratio of Type 2/Type 1 AGN that is a function of both z$ and 2-10 keV X-ray luminosity, L_X. The evolutionary models are constrained by comparing with the observed Type 1 AGN fractions from the recent work by Barger et al. The model which simultaneously best accounts for Barger's data, the CXRB spectrum and the X-ray number counts has a local, low-L_X Type 2/Type 1 ratio of 4, and predicts a Type 2 AGN fraction which evolves as (1+z)^{0.3}. Models with no redshift evolution yielded much poorer fits to the Barger Type 1 AGN fractions. This evolution predicts a Type 2/Type 1 ratio of 1-2 for log L_X > 44, and thus the deep X-ray surveys are missing about half the obscured AGN with these luminosities. These objects are likely to be Compton thick. Overall, these calculations show that the current data strongly supports a change to the AGN unification scenario where the obscuration is connected with star formation in the host galaxy rather than a molecular torus alone. The evolution of the obscuration implies a close relationship between star formation and AGN fueling, most likely due to minor mergers or interactions.
We derive an analytic model for nonlinear "photon bubble" wave trains driven
by buoyancy forces in magnetized, radiation pressure-dominated atmospheres.
Continuous, periodic wave solutions exist when radiative diffusion is slow
compared to the dynamical timescale of the atmosphere. We identify these waves
with the saturation of a linear instability discovered by Arons - therefore,
these wave trains should develop spontaneously. The buoyancy-driven waves are
physically distinct from photon bubbles in the presence of rapid diffusion,
which evolve into trains of gas pressure-dominated shocks as they become
nonlinear.
Like the gas pressure-driven shock trains, buoyancy-driven photon bubbles can
exhibit very large density contrasts, which greatly enhance the flow of
radiation through the atmosphere. However, steady-state solutions for
buoyancy-driven photon bubbles exist only when an extra source of radiation is
added to the energy equation, in the form of a flux divergence. We argue that
this term is required to compensate for the radiation flux lost via the
bubbles, which increases with height. We speculate that an atmosphere subject
to buoyancy-driven photon bubbles, but lacking this compensating energy source,
would lose pressure support and collapse on a timescale much shorter than the
radiative diffusion time in the equivalent homogeneous atmosphere.
(Abridged) Recent X-ray variability studies on a large sample of Active Galactic Nuclei (AGN) have suggested that the fractional rms variability amplitude, $\rms$, seems to correlate with black hole mass, $\mbh$. The fact that the rms amplitude can be easily obtained from the observed X-ray light curves has motivated the theoretical modeling of the $\rms^2$-$\mbh$ relationship. Inspired by the similarities shared by the timing properties of AGN and X-ray binaries, previous studies have explored model power spectral densities characterized by broken power laws. With the advent of RXTE, however, it has been shown that the band-limited noise spectra of X-ray binaries are not broken power laws but can often be described in terms of a small set of broad Lorentzians. In this paper, motivated by the latest timing results from X-ray binaries, we consider that AGN broad-band noise spectra consist of a small number of Lorentzian components. This formalism allows, for the first time, to fully account for sampling effects in theoretical models of X-ray variability in an analytic manner. We demonstrate the excellent agreement between our analytical results and Monte Carlo simulations designed to account for the distortions suffered by the underlying power spectral density due to sampling effects. We show that, neglecting sampling effects when deriving the fractional rms from the model power spectrum density can lead to underestimating it by a factor of a few with respect to its true value. We discuss the implications of our results for the derivation of AGN masses from the $\rms^2$-$\mbh$ correlation.
Cataclysmic Variables (CVs) should be present in large numbers in Globular Clusters (GCs). Numerous low-luminosity X-ray sources identified over the past few years as candidate CVs in GCs support this notion. Yet, very few "cataclysms," the characteristic feature of this class of objects in the field, have been observed in GCs. We address this discrepancy here, within the framework of the standard Disk Instability Model for CV outbursts. We argue that the paucity of outbursts in GCs is probably not a direct consequence of the donors' low metallicities. We present diagnostics based on outburst properties allowing tests of the hypothesis that rare cataclysms are entirely due to lower mass transfer rates in GCs relative to the field, and we argue against this explanation. Instead, we propose that a combination of low mass transfer rates (>~ 10^14-15 g/s) and moderately strong white dwarf magnetic moments (>~ 10^30 G cm^3) stabilize CV disks in GCs and thus prevent most of them from experiencing frequent outbursts. If it is so, rare cataclysms in GCs would signal important evolutionary differences between field and cluster CVs.
We study the photometric and spectral properties of 39320 early-type galaxies within the SDSS, as a function of both local environment and redshift. The distance to the nearest cluster of galaxies and the distance to the 10th nearest luminous neighbor were used to define two extremes in environment. The properties of early-type galaxies are weakly but significantly different in these two extremes: objects in dense environments are less than ~1 Gyr older and alpha-enhanced by ~0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that, in dense regions, the stars in early-type galaxies formed at slightly earlier times, and on a slightly shorter timescale, than in less dense regions. In addition, objects at lower redshifts are older, but have the same metallicities and alpha-enhancements as their counterparts of the same sigma at higher redshifts, as expected if the low redshift sample is a passively-aged version of the sample at higher redshifts. These significant conclusions are derived from several pieces of evidence discussed in this paper, including the FP of early-types, the observed evolution of absorption linestrengths (measured from a large ensemble of composite spectra which we provide in tabular form) over small lookback times, and comparison with the predictions of single burst (non-solar abundance) stellar population synthesis models (which also indicate that age, metallicity and alpha-enhancement all increase with velocity dispersion).
Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 micron images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z=1.325+/-0.002 with an apparent Far-Infrared luminosity of 3.2(+/-0.7) x 10^13 Lsun. Unlike other z>1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z=1.034, although the amplification is likely modest (~10). We argue that the contribution to the observed optical/Near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio Spectral Energy Distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high redshift dusty starburst galaxy in great detail.
Classical T Tauri stars are encircled by accretion discs most of the time unresolved by conventional imaging observation. However, numerical simulations show that unresolved aperture linear polarimetry can be used to extract information about the geometry of the immediate circumstellar medium that scatter the starlight. Monin, Menard & Duchene (1998) previously suggested that polarimetry can be used to trace the relative orientation of discs in young binary systems in order to shed light on the stellar and planet formation process. In this paper, we report on new VLT/FORS1 optical linear polarisation measurements of 23 southern binaries spanning a range of separation from 0.8'' to 10''. In each field, the polarisation of the central binary is extracted, as well as the polarisation of nearby stars in order to estimate the local interstellar polarisation. We find that, in general, the linear polarisation vectors of individual components in binary systems tend to be parallel to each other. The amplitude of their polarisations are also correlated. These findings are in agreement with our previous work and extend the trend to smaller separations. They are also similar to other studies, e.g., Donar et al. 1999; Jensen et al. 2000, 2004; Wolf et al. 2001. However, we also find a few systems showing large differences in polarisation level, possibly indicating different inclinations to the line-of-sight for their discs.
We propose an alternative theoretical approach showing how the existence of an extra dimension in RS model can estimate the correction in the Schwarzschild radius of black holes, and consequently its measurability in terms of the variation of quasar luminosity, which can be caused by a imprint of an extra dimension endowing the geometry of a brane-world scenario in an AdS_5 bulk. This paper is intended to investigate the variation of luminosity due to accretion of gas in black holes (BHs) in the center of quasars, besides also investigating the variation of luminosity in supermassive BHs by brane-world effects, using RS model.
Using the filled bolometer array SHARC-II on the Caltech Submillimeter Observatory (CSO), we have obtained deep (rms ~ 15mJy/beam), high-quality 350 micron maps of five of the most luminous high-z radio galaxies known. In all cases the central radio galaxy is detected at the >3sigma level, and in some cases the high resolution of SHARC-II (FWHM ~ 9") allows us to confirm the spatially extended nature of the dust emission. In PKS1138-262 (z=2.156), 8C1909+722 (z=3.538) and 4C41.17 (z=3.792), additional sources -- first discovered by SCUBA at 850 micron and believed to be dusty, merging systems associated with the central radio galaxy -- are detected at 350 micron. Furthermore, in PKS1138 and 4C41.17 additional SHARC-II sources are seen which were not detected at 850 micron, although the reality of these sources will have to be confirmed by independent submm observations. Thus, our observations seem to support the notion of extended star formation taking place in radio galaxies at high redshifts, and that these systems sit at the centers of overdense regions harbouring a complexity of dusty and vigorously star forming systems. At the redshift of the radio galaxies, the 350 micron observations sample very close to the rest-frame dust peak (typically at ~100 micron), and they therefore contribute a particularly important point to the spectral energy distributions of these sources, which we use this in conjunction with existing (sub)millimeter data to derive FIR luminosities, dust temperatures and spectral emissivities of the central radio galaxies.
We examine the nucleosynthesis products that are produced in the outflow from rapidly accreting disks. We find that the type of element synthesis varies dramatically with the degree of neutrino trapping in the disk and therefore the accretion rate of the disk. Disks with relatively high accretion rates such as 10 M_solar/s can produce very neutron rich nuclei that are found in the r process. Disks with more moderate accretion rates can produce copious amounts of Nickel as well as the light elements such as Lithium and Boron. Disks with lower accretion rates such as 0.1 M_solar/s produce large amounts of Nickel as well as some unusual nuclei such as Ti-49, Sc-45, Zn-64, and Mo-92. This wide array of potential nucleosynthesis products is due to the varying influence of electron neutrinos and antineutrinos emitted from the disk on the neutron-to-proton ratio in the outflow. We use a parameterization for the outflow and discuss our results in terms of entropy and outflow acceleration.
Recent wide-field photometric surveys which target a specific field for long durations are ideal for studying both long and short period stellar variability. Here we report on 75 variable stars detected during observations of a field in Pegasus using the WASP0 instrument, 73 of which are new discoveries. The variables detected include 16 delta Scuti stars, 34 eclipsing binaries, 3 BY Draconis stars, and 4 RR Lyraes. We estimate that the fraction of stars in the field brighter than V ~ 13.5 exhibiting variable behaviour with an amplitude greater than 0.6% rms is ~ 0.4%. These results are compared with other wide-field stellar variability surveys and implications for detecting transits due to extra-solar planets are discussed.
The thermonuclear runaway that culminates in the explosion of a Chandrasekhar mass white dwarf as a Type Ia supernova begins centuries before the star actually explodes. Here, using a 3D anelastic code, we examine numerically the convective flow during the last minute of that runaway, a time that is crucial in determining just where and how often the supernova ignites. We find that the overall convective flow is dipolar, with the higher temperature fluctuations in an outbound flow preferentially on one side of the star. Taken at face value, this suggests an asymmetric ignition that may well persist in the geometry of the final explosion. However, we also find that even a moderate amount of rotation tends to fracture this dipole flow, making ignition over a broader region more likely. Though our calculations lack the resolution to study the flow at astrophysically relevant Rayleigh numbers, we also speculate that the observed dipolar flow will become less organized as the viscosity becomes very small. Motion within the dipole flow shows evidence of turbulence, suggesting that only geometrically large fluctuations (~1 km) will persist to ignite the runaway. We also examine the probability density function for the temperature fluctuations, finding evidence for a Gaussian, rather than exponential distribution, which suggests that ignition sparks may be strongly spatially clustered.
The Wide Field Imager Lyman-Alpha Search (WFILAS) is a search for Lya
emitting galaxies at z~5.7. Deep images from the Wide Field Imager (WFI) on the
ESO/MPI 2.2m telescope have been used to detect 7 bright Lya emitting
candidates in three fields covering 0.74 sq. degree on the sky. For this we
used three narrowband (FWHM ~70A), one encompassing intermediate band (FWHM
\~220A) and broadband B and R filters. One has thus far been spectroscopically
confirmed as a Lya emitting galaxy at z=5.721 using FORS2 at the VLT. This
galaxy shows a bright, well resolved asymmetric line profile, which is
characteristic of Lya emitting galaxies.
In one of our three fields, the Chandra Deep Field South (CDFS), we find an
overdensity of Lya emitters in agreement with other surveys that have targeted
this region. A statistically complete sample of our candidates probes the
bright-end of the luminosity function, confirming earlier results from other
smaller, deeper surveys.
We analyse Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) data of a double-peaked burst from the low mass X-ray binary (LMXB) 4U 1636-536 that shows no evidence for photospheric radius expansion (PRE). We find that the X-ray emitting area on the star increases with time as the burst progresses, even though the photosphere does not expand. We argue that this is a strong indication of thermonuclear flame spreading on the stellar surface during such bursts. We propose a model for such double-peaked bursts, based on thermonuclear flame spreading, that can qualitatively explain their essential features, as well as the rarity of these bursts.
We report on a study of the evolution of burst oscillation properties during the rising phase of X-ray bursts from 4U 1636-536 observed with the proportional counter array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE). We present evidence for significant harmonic structure of burst oscillation pulses during the early rising phases of bursts. This is the first such detection in burst rise oscillations, and is very important for constraining neutron star structure parameters and the equation of state models of matter at the core of a neutron star. The detection of harmonic content only during the initial portions of the burst rise is consistent with the theoretical expectation that with time the thermonuclear burning region becomes larger, and hence the fundamental and harmonic amplitudes both diminish. We also find, for the first time from this source, strong evidence of oscillation frequency increase during the burst rise. The timing behavior of harmonic content, amplitude, and frequency of burst rise oscillations may be important in understanding the spreading of thermonuclear flames under the extreme physical conditions on neutron star surfaces.
Observations suggest that the structural parameters of disk galaxies have not changed greatly since redshift 1. We examine whether these observations are consistent with a cosmology in which structures form hierarchically. We use SPH/N-body galaxy-scale simulations to simulate the formation and evolution of Milky-Way-like disk galaxies by fragmentation, followed by hierarchical merging. The simulated galaxies have a thick disk, that forms in a period of chaotic merging at high redshift, during which a large amount of alpha-elements are produced, and a thin disk, that forms later and has a higher metallicity. Our simulated disks settle down quickly and do not evolve much since redshift z~1, mostly because no major mergers take place between z=1 and z=0. During this period, the disk radius increases (inside-out growth) while its thickness remains constant. These results are consistent with observations of disk galaxies at low and high redshift.
We present the results of a photometric survey for variable stars in the central region of the nearby globular cluster NGC 6397.Time series photometry was obtained for 30 variable objects. The sample includes 12 new objects, of which 6 show periodic lightcurves and 2 are eclipsing binaries of unknown period. Six variables possess certain and three possess likely X-ray counterparts detected with the Chandra observatory. Among them four are cataclysmic variables and one is a foreground eclipsing binary. The cataclysmic variable CV2 exhibited a likely dwarf nova type outburst in May 2003. The cataclysmic variable CV3 was observed at 18.5<V<20.0 during 5 observing runs, but went into a low state in May 2003 when it reached V>22. We have found that thelight curve of the optical companion to the millisecond pulsar PSRJ1740-5340 exhibits noticeable changes of its amplitude on a time scale of a few months. A shallow eclipse with Delta_V=0.03 mag was detected in one of the cluster turnoff stars suggesting the presence of a large planet or brown dwarf in orbit.
We propose a new two-stage model for acceleration of electrons in solar flares. In the first stage, electrons are accelerated stochastically in a post-reconnection turbulent downflow. The second stage is the reprocessing of a subset of these electrons as they pass through a weakly compressive fast shock above the apex of the closed flare loop on their way to the chromosphere. We call this the "shock reprocessing" model. The model reproduces the energy dependent arrival time delays observed for both the pulsed and smooth components of impulsive solar flare x-rays with physically reasonable parameters for the downflow region. The model also predicts an emission site above the loop-top, as seen in the Masuda flare. The loop-top source distinguishes the shock reprocessing model from previous models. The model makes testable predictions for the energy dependence of footpoint pulse strengths and the location and spectrum of the loop-top emission, and can account for the observed soft-hard-soft trend in the spectral evolution of footpoint emission. Our model highlights the concept that reconnection is an acceleration environment rather than a single process. Which combination of processes operate may depend on the initial conditions that determine, for example, whether the reconnection downflow is turbulent. The shock reprocessing model comprises one such combination.
We present diffraction-limited images in the 11.3 micron PAH band and in the continuum at 11.9 microns of the S287B star forming region obtained with the newly commissioned mid-infrared imager/spectrometer VISIR at the VLT. In both filters five point-like sources are detected, of which at least one has a spectral energy distribution reminiscent of a Lada Class I source. We also report on the discovery of a new Herbig Ae star in this region. It is particularly striking that the brightest mid-infrared source in this region has not been detected at shorter wavelengths; it is a good candidate to be the driving source of the bipolar molecular outflow and optical reflection nebulosity previously reported in S287B.
We investigate the stellar composition of bulges and elliptical galaxies as predicted by the CDM paradigm using semi-analytical modelling. We argue that spheroid stars are built up of two main components, {\it merger} and {\it quiescent}, according to the origin of the stars. The merger component is formed during major mergers by gas driven to the centre, while the quiescent component is formed in gaseous discs and added later to the spheroid during major mergers. Galaxies more massive than $M_C=3 \times 10^{10}$ M$_{\odot}$ have on average only a 15% merger component in their spheroids, while smaller galaxies can have up to 30%. Additionally, the fraction of stars in bulges for galaxies more massive than $M_C$ is larger than 50%. We argue here that the surface mass density of galaxies is connected to the fraction of the merger component in spheroids, in such a way that a higher fraction results in a higher surface mass density. Using this assumption, we are able to reproduce the scatter in the observed size distribution of elliptical galaxies and the environmental dependence of the surface mass density. The scatter in the size distribution comes as a consequence of the scatter in the time since the last major merger experienced by an elliptical galaxy. The environmental dependence is a combination of earlier mergers in high density environments and continued occurrence of major mergers for massive galaxies. Furthermore we propose, based on the size distribution, a new method to determine the epoch of the last major merger of elliptical galaxies.
We model the 10 micron absorption spectra of nonspherical particles composed of amorphous silicate. We consider two classes of particles, compact ones and fractal aggregates composed of homogeneous spheres. For the compact particles we consider Gaussian random spheres with various degrees of non-sphericity. For the fractal aggregates we compute the absorption spectra for various fractal dimensions. The 10 micron spectra are computed for ensembles of these particles in random orientation using the well-known Discrete Dipole Approximation. We compare our results to spectra obtained when using volume equivalent homogeneous spheres and to those computed using a porous sphere approximation. We conclude that, in general, nonspherical particles show a spectral signature that is similar to that of homogeneous spheres with a smaller material volume. This effect is overestimated when approximating the particles by porous spheres with the same volume filling fraction. For aggregates with fractal dimensions typically predicted for cosmic dust, we show that the spectral signature characteristic of very small homogeneous spheres (with a volume equivalent radius r_V<0.5 micron) can be detected even in very large particles. We conclude that particle sizes are underestimated when using homogeneous spheres to model the emission spectra of astronomical sources. In contrast, the particle sizes are severely overestimated when using equivalent porous spheres to fit observations of 10 micron silicate emission.
We present the first results of our variability survey for the globular cluster NGC 362. We found numerous variable stars in the field including 23 RR Lyr stars, 4 eclipsing binaries and 5 other pulsating stars. Four of the RR Lyr ae stars are located behind the cluster, possibly in the farthest extensions of the SMC.
We have developed a simple, static model designed to place a very solid lower limit on the star formation rate (SFR) expected in a dwarf disk galaxy, which leads to the prediction of a previously undocumented relation between HI mass, M_HI, and SFR. Over the mass range 10^8--10^10 M_sun, a wide variety of galaxies are observed to follow such a relation -- SFR ~ M_HI^1.4 -- with the same slope and similar scatter to our prediction. Within the model, this relation is a manifestation of self-regulating star formation (SF), in which the ISM is kept warm and stable by a UV interstellar radiation field (ISRF) that is maintained by constant regeneration of O--B stars. Regardless of the actual mode of star formation, it seems that the majority of dwarfs are presently forming stars in the same way.
We discuss various models and scenaria proposed to explain the nature of the V838 Mon type eruptions. To this class of eruptive objects we include: M31 RV (erupted in 1988), V4332 Sgr (erupted in 1994) and V838 Mon (erupted in 2002). We concentrate on three models: (i) thermonuclear runaway on an accreting white dwarf (nova-like event); (ii) He-shell flash in a post asymptotic giant branch star (born-again AGB); and (iii) merger of stars. We show that models (i) and (ii) cannot account for the majority of the observed properties of the objects. Most significantly, in both nuclear burning type models the object is expected to heat up before declining and fade as a very hot compact star. In the observed eruptions the objects declined as very cool giants or supergiants. We show that the stellar merger model can account for all the observed properties and conclude that presently this is the most promising model to explain the eruptions of the V838 Mon type.
Observational data on the early galactic abundances of the light elements lithium, beryllium and boron are combined with data related to the reionization of the intergalactic medium (IGM) in a search of processes happening in the early universe. Early massive metal-free stars (Pop III), largely held responsible for the reionization of the IGM, are proposed to have been also at the origin of the lithium-6 plateau, through their winds. In this sense the evolution of the Li-6/Be-9 ratio appears to be a key parameter for the history of nucleosynthesis, as a monitor of the early formation of metals and their subsequent injection in high energy particles.
Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be unevolved objects, due to their blue colors, compact appearance and large gas fractions. Many of these objects show an ongoing intense burst of star formation or have experienced it in the recent past. By means of 2-D hydrodynamical simulations, coupled with detailed chemical yields originating from SNeII, SNeIa, and intermediate-mass stars, we study the dynamical and chemical evolution of model galaxies with structural parameters similar to NGC1569, a prototypical starburst galaxy. A burst of star formation with short duration is not able to account for the chemical and morphological properties of this galaxy. The best way to reproduce the chemical composition of this object is by assuming long-lasting episodes of star formation and a more recent burst, separated from the previous episodes by a short quiescent period. The last burst of star formation, in most of the explored cases, does not affect the chemical composition of the galaxy, since the enriched gas produced by young stars is in a too hot phase to be detectable with the optical spectroscopy. Models assuming the infall of a big cloud towards the center of the galaxy reproduce the chemical composition of the NGC1569, but the pressure exercised by the cloud hampers the expansion of the galactic wind, at variance with what observed in NGC1569.
We present a study of the HI distribution and the dynamics of the nearby spiral galaxy NGC5055 based on observations with the Westerbork Synthesis Radio Telescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to 3.5 R_25 and shows a pronounced warp, starting at the end of the bright optical disk (R_25= 11.6 kpc). The warp is very extended and symmetric. Its large-scale symmetry and the long rotation period of its outer parts (~ 1.5 Gyr at 40 kpc) suggest a long-lived phenomenon. The rotation curve has a steep rise in the central parts up to the maximum velocity (v_{max}= 206 km/s). Beyond the bright stellar disk (R_25) it shows a decline of about 25 km/s and then it remains flat out to the last measured point. The standard analysis with luminous and dark matter components shows the dynamical importance of the disk. The best fit to the rotation curve is obtained with a ``maximum disk''. Less satisfactory fits with lighter disks help to set a firm lower limit of 1.4 to the mass-to-light ratio in F band of the disk. Such a ``minimum disk'' is contributing about 60% of the observed maximum rotational velocity. NGC5055 shows remarkable overall regularity and symmetry. A mild lopsidedness is noticeable, however, both in the distribution and in the kinematics of the gas. The tilted ring analysis of the velocity field has led us to the adoption of different values for the kinematical centre and for the systemic velocity for the inner and the outer parts of the system. This has produced a remarkable result: the kinematical and geometrical asymmetries have disappeared, both at the same time. These results point at two different dynamical regimes: an inner region, dominated by the stellar disk, and an outer one, dominated by a dark matter halo offset with respect to the disk.
We present X-ray properties of NGC 300 point sources, extracted from 66 ksec of XMM-Newton data taken in 2000 December and 2001 January. A total of 163 sources were detected in the energy range of 0.3-6 kev. We report on the global properties of the sources detected inside the D25 optical disk, such as the hardness ratio and X-ray fluxes, and on the properties of their optical counterparts found in B, V, and R images from the 2.2m MPG/ESO telescope. Furthermore, we cross-correlate the X-ray sources with SIMBAD, the USNO-A2.0 catalog, and radio catalogues.
We perform an analytical study and a Monte Carlo (MC) analysis of the main features for microlensing events in pixel lensing observations towards M31. Our main aim is to investigate the lens nature and location of the 14 candidate events found by the MEGA collaboration. Assuming a reference model for the mass distribution in M31 and the standard model for our galaxy, we estimate the MACHO-to-self lensing probability and the event time duration towards M31. Reproducing the MEGA observing conditions, as a result we get the MC event number density distribution as a function of the event full-width half-maximum duration $t_{1/2}$ and the magnitude at maximum $R_{\mathrm {max}}$. For a MACHO mass of $0.5 M_{\odot}$ we find typical values of $t_{1/2} \simeq 20$ day and $R_{\mathrm {max}} \simeq 22$, for both MACHO-lensing and self-lensing events occurring beyond about 10 arcminutes from the M31 center. A comparison of the observed features ($t_{1/2}$ and $R_{\mathrm {max}}$) with our MC results shows that for a MACHO mass $>0.1 M_{\odot}$ the four innermost MEGA events are most likely self-lensing events, whereas the six outermost events must be genuine MACHO-lensing events.
We discuss particle acceleration by strong and weak MHD turbulence in
magnetic pressure (low-beta) and gaseous pressure (high-beta) dominated
plasmas. We consider the acceleration by large scale compressions in both slow
and fast particle diffusion limits. We compare the results with the
acceleration rate that arises from resonance scattering and Transit-Time
Damping (TTD).
We establish that fast modes accelerate particles more efficiently than slow
modes. We find that particle acceleration by pitch-angle scattering and TTD
dominates acceleration by slow or fast modes when the spatial diffusion rate is
small. When the rate of spatial diffusion of particles is high, we establish a
substantial enhancement of the efficiency of particle acceleration by slow
modes in weak turbulence. We show that highly supersonic turbulence is an
efficient agent for particle acceleration. We find that even incompressible
turbulence can accelerate particles on the scales comparable with the particle
mean free path.
We present results from an XMM-Newton observation of the head-tail radio galaxy IC 310 located in the southwest region of the Perseus cluster. The spectrum is well-fitted by an absorbed power-law model with a photon index of $2.50 \pm 0.02$ with no significant absorption excess. The X-ray image shows a point-like emission at IC 310 without any signs of a structure correlated with the radio halo tail. The temperature of the intracluster medium surrounding IC 310 declines as a function of distance from the cluster center, from $ kT \sim 6$ keV in the northeast corner of the field of view to about 3 keV in the southwest region. Although we do not find any sharp edges in the surface brightness profile, a brightness excess over a smooth $\beta$ model by about 20% is seen. The temperature also rises by about 10% in the same region. This indicates that the IC 310 region is a subcluster probably infalling into the Perseus cluster, and the gas in front of IC 310 towards the Perseus cluster is likely to be compressed by the large-scale motion, which supports the view that the IC 310 system is undergoing a merger.
We present the status of an ongoing study to built a a high resolution near infrared Echelle spectrograph (NAHUAL) for the 10.4-m-Gran Telescopio Canarias (GTC) which will be especially optimised for planet searches by means of high precision radial velocity measurements. We show that infrared radial velocity programs are particularly suitable to search for planets very low mass stars and brown dwarfs, as well as active stars. The goal of NAHUAL is to reach an accuracy of the radial velocity measurement of a few m/s, which would allow the detection of planets with a few earth-masses orbiting low-mass stars and brown dwarfs. It is planed that NAHUAL covers simultaneously the full wavelength range in the J, H, and K-band, and will also serve as a general purpose high resolution near infrared spectrograph of the GTC. The planed instrument will have a resolution of R=50,000 with a 0.175 arcsec slit, and an AO-system. An absorption cell will serve as a simultaneous wavelength reference.
The first large sample of Lyman Break Galaxies (LBGs) at z~1 is selected from GALEX Far-UV and Near-UV images of the Chandra Deep Field South. This multi-wavelength study is based on the wealth of data available in this field: SPITZER-MIPS to estimate total IR luminosities and dust attenuations, redshifts from COMBO-17, EIS and GOODS to check the validity of dropout identifications and GOODS to derive the morphology of about 40 of our LBGs. The advantages of this "nearby" LBG sample are that images reach fainter fluxes and surface brightnesses, allowing a deeper detection at all wavelengths and a better morphological analysis. Moreover, the 24-mic fluxes (i.e. 12-mic rest-frame) permit a good estimation of dust attenuations and total IR luminosities. The main results are that the vast majority of our LBGs are also Luminous Infrared Galaxies (LIRGs). The morphology of 75% of our LBGs is consistent with a disk. Previous estimates of dust attenuations based on the ultraviolet slope are likely to be too large by a factor of about 2, which implies that estimated star formation rates were too large by the same amount. This sample in the z~1 universe provides us with a high quality reference sample of LBGs.
An extensive photometric monitoring of KH 15D, an enigmatic variable in the young star cluster NGC 2264, has been conducted. Simultaneous and accurate near-infrared (JHKs-bands) photometry is presented between 2003 December and 2005 March covering most of the variable phase. The infrared variability is characterized by large-amplitude and long-lasting eclipse, as observed at optical. The period of variability is 48.3 +/- 0.2 days, the maximum photometric amplitude of variability is ~4.2 mag, and the eclipse duration is \~0.5 in phase units. These are consistent with the most recent period, amplitude, and duration at optical. The blueing of the J-H color (~0.16 mag) during the eclipse, which has been suggested before, is unambiguously confirmed; a similar blueing at H-Ks is less clear but is probably present at a similar level. The overall shape of the JHKs light curves is very similar to the optical one, including a fair time-symmetry and a less stable flux during the eclipse with a slight hump near the zero phase. Most of these variability features of KH 15D observed at near-infrared wavelengths can be explained with the recent model employing an eclipse by the inclined, precessing disk and an outer scattering region around a pre-main-sequence binary.
Long baseline interferometry is now able to resolve the pulsational change of the angular diameter of a significant number of Cepheids in the solar neighborhood. This allows the application of a new version of the Baade-Wesselink (BW) method to measure their distance, for which we do not need to estimate the star's temperature. Using angular diameter measurements from the VLT Interferometer, we derived the distances to four nearby Cepheids. For three additional stars, we obtained average values of their angular diameters. Based on these new measurements and already existing data, we derived calibrations of the Period-Luminosity and Period-Radius relations. We also obtained reliable surface brightness-color relations, that can be employed for the infrared surface brightness version of the BW method.
We study the photometric parameters of the bulges of galaxies of different Hubble types including ellipticals, lenticulars, early and late type spirals and early type dwarf galaxies. Analyzing the distributions of various photometric parameters, and two- and three-dimensional correlations between them, we find that there is a difference in the correlations exhibited by bright (M_K < -22) and faint bulges, irrespective of their Hubble type. Importantly, the bright bulges, which include typically E/S0 galaxies and bulges of early type spirals, are tightly distributed around a common photometric plane (PP), while their fainter counter parts, mainly bulges of late type spirals and dwarf galaxies show significant deviation from the planar distribution. We show that the specific entropy, determined from the bulge structural parameters, systematically increases as we move from late to early Hubble types. We interpret this as evidence for hierarchical merging and passive evolution scenarios for bright and faint bulges respectively.
A gauge invariant and hence physically meaningful definition of magnetic helicity density for random fields is proposed, using the Gauss linking formula, as the density of correlated field line linkages. This definition is applied to the random small scale field in weakly inhomogeneous turbulence, whose correlation length is small compared with the scale on which the turbulence varies. For inhomogeneous systems, with or without boundaries, our technique then allows one to discuss the local magnetic helicity density evolution, in a gauge independent fashion, which was not possible earlier. This evolution equation is governed by local sources (owing to the mean field) and by the divergence of a magnetic helicity flux density. The role of magnetic helicity fluxes in alleviating catastrophic quenching of mean field dynamos is discussed.
We describe our method to find the primordial binary population in the Sco OB2 association. We present the results of our ADONIS and VLT/NACO near-infrared adaptive optics binarity surveys of A and late-B stars in Sco OB2. We combine these results with literature data on visual, spectroscopic, and astrometric binary stars. With our observations we remove part of the selection effects present in the combined dataset. Using simulated observations the remaining biases can be removed in order to derive the true binary population. Detailed N-body simulations, including stellar and binary evolution are required to derive constraints on the binary population which was present just after the natal gas has been removed from Sco OB2, i.e., the primordial binary population.
In this note we stress the necessity of a careful check of both the theoretical arguments and the numerical experiments used by Lopez-Caniego et al. (2005) to support the superior performances of the biparametric scale adaptive filter (BSAF) with respect to the classic matched filter (MF) in the detection of sources on a random Gaussian background.
We consider the modification of extragalactic cosmic ray spectrum caused by cosmic ray interactions with infrared background photons which are present in the extragalactic space together with relic photons. It is assumed that cosmic ray spectrum at superhigh energies has extragalactic origin and is proton dominated.
Four years of AMANDA-II data have been searched for neutrinos from point sources. No statistically significant excess of events has been detected, neither integrated in the years 2000 to 2003, nor in the searches for occasional signals. An interesting coincidence of neutrinos with gamma-ray flares emerges when inspecting the time of the events detected from the direction of the Blazar 1ES1959+650. The exceptional character of the gamma-ray observation provides a strong motivation for consolidating similar search strategies with AMANDA and its successor IceCube, as well as for multidisciplinary investigations of this and other gamma-ray sources. We report the outcomes of the most recent survey of the northern sky to search for neutrino point sources with AMANDA-II. We also discuss possible viable collaborations between the gamma-ray and the high energy neutrino observatories.
The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. Objective global fitting of our updated double-layered envelope models to recent observations of the pulsating DB star CBS 114, and to existing observations of the slightly cooler star GD 358, lead to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG 1159 stars.
We comment on the necessity of a unified approximative scheme within relativistic cosmology which would allow us to classify different cosmological models in a systematic way. We also report on recent progresses in formulating a cosmological post-Newtonian approximation and the problems related to such a scheme.
More than 600 high resolution spectra of stars of spectral type F and later have been obtained in order to search for signatures of differential rotation in line profiles. In 147 stars, the rotation law could be measured; 28 of them are found to be differentially rotating. Comparison to rotation laws in stars of spectral type A reveals that differential rotation sets in at the convection boundary in the HR-diagram; no star significantly hotter than the convection boundary exhibits signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at vsini~100 km/s show extraordinarily strong absolute shear at short rotation periods around one day. It is suggested that this is due to their small convection zone depth and is connected to a narrow range in surface velocity. Detection frequencies of differential rotation are analyzed in stars with varying temperature and rotation velocity. Measurable differential rotation is more frequent in late-type stars and slow rotators. The strength of absolute shear and differential rotation are examined as functions of stellar effective temperature and rotation period. The largest shear is found at rotation periods between two and three days. In slower rotators, the strongest shear at a given rotation rate is approximately given by DOmega_max ~ P^{-1}. In faster rotators, alpha_max and DOmega_max are diminishing less rapidly. A comparison with differential rotation measurements in stars of later spectral type shows that F-stars exhibit stronger shear than cooler stars do, the upper boundary in absolute shear DOmega with temperature is consistent with the temperature scaling law found in Doppler Imaging measurements.
We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We present an improved spectral reduction and a new orbital solution with two additional epochs. Our improved analysis confirms our spectral type of M8 (+/- 1) and mass of 0.090+/-0.003 solar masses for AB Dor C. Comparing the results for AB Dor C with other young, low-mass objects with dynamical masses we find a general trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for low mass stars and brown dwarfs. Given our precision, there is a ~99% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.
We present measurements at optical wavelengths of the spectral reflectance, the rotational light curve, and the solar phase curve of 2003 EL61,. With apparent visual magnitude 17.5 at 51 AU from the sun, this newly discovered member of the classical Kuiper Belt is now the third brightest KBO after Pluto and 2005 FY9. Our observations reveal an unambiguous, double-peaked rotational light curve with period 3.9154 +/- 0.0002 hours and peak to peak amplitude 0.28 +/- 0.04 mag. This is the fastest rotation period reliably determined for any body in the solar system larger than 100 km. Assuming the body has relaxed over time to the shape taken by a homogenous fluid body, our observations tightly constrain the shape and density. Given the mass we recently determined for 2003 EL61 from the orbit of a small satellite, we also constrain the size and albedo. We find a total length of 1960 to 2500 km, a mean density of 2600 to 3340 kg m-3, and a visual albedo greater than 0.6. We also measure a neutral reflectance at visible wavelengths and a linear phase curve with slope varying from 0.09 mag deg-1 in the B band to 0.13 mag deg-1 in the I band. The absolute V-band magnitude is 0.259 +/- 0.028.
We study the chemical and kinematic properties of the first galaxies which formed at a high redshift, using high resolution cosmological numerical simulations, and compared them with the recent observational results for the Sculptor dwarf spheroidal galaxy by Tolstoy et al., who found two distinct stellar populations: the lower metallicity stars are more spatially extended and possess a higher velocity dispersion than the higher metallicity stars. Our calculations reproduce these observations as the result of a steep metallicity gradient, within a single populations, induced by dissipative collapse of the gas component. We also predict strong [N/O] enhancements in the lowest metallicity stars in dwarf spheroidals, due to the preferential retention of ejected gas from intermediate mass stars, compared to Type II supernovae.
The space telescope MOST is now providing us with extremely accurate low frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this paper that these data, when combined with ground based measurements of the high frequency p-mode spectrum, can be reproduced with stellar models that include the effects of turbulence in their outer layers. Without turbulence, the l=0 modes of our models deviate from either the ground based or the space data by about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including turbulence in the models and we can exactly match 12 out of 13 MOST frequencies that we identified as l=0 modes in addition to 13 out of 21 ground based frequencies within their observational 2 sigma tolerances. The better agreement between model frequencies and observed ones depends for the most part on the turbulent kinetic energy which was taken from a 3D convection simulation for the Sun.
Evidence from the WMAP polarization data indicates that the Universe may have been reionized at very high redshift. It is often suggested that the ionizing UV flux originates from an early population of massive or very massive stars. Depending on their mass, such stars can explode either as type II supernovae or pair-instability supernovae, or may entirely collapse into a black hole. The resulting neutrino emission can be quite different in each case. We consider here the relic neutrino background produced by an early burst of Population III stars coupled with a normal mode of star formation at lower redshift. The computation is performed in the framework of hierarchical structure formation and is based on cosmic star formation histories constrained to reproduce the observed star formation rate at redshift z \la 6, the observed chemical abundances in damped Lyman alpha absorbers and in the intergalactic medium, and to allow for an early reionization of the Universe at z \sim 10-20. We find that although the high redshift burst of Population III stars does lead to an appreciable flux of neutrinos at relatively low energy (E_\nu \approx 1 MeV), the observable neutrino flux is dominated by the normal mode of star formation. We also find that predicted fluxes are at the present level of the SuperK limit. As a consequence, the supernova relic neutrino background has a direct impact on models of chemical evolution and/or supernova dynamics.
We use the fourth data release of the Sloan Digital Sky Survey to investigate the orientations of 4327 satellite galaxies with respect to their hosts. The orientation of the satellites is inconsistent with a random distribution at the 99.96% confidence level, and the satellites show a preference for radial alignment toward their hosts. Further, on scales < 50 kpc the major axes of the host galaxies and their satellites are preferentially aligned with each other. Phrased in the terminology of weak lensing, the images of the satellites have a mean shear of gamma_T = -0.030 +/- 0.007, averaged over scales 10 kpc < r < 70 kpc. In a galaxy-galaxy lensing study where lenses and sources are separated solely on the basis of apparent magnitude, we estimate that on scales < 250 kpc satellite galaxies account for between 10% and 15% of the objects that are identified as sources. In such studies, the radial alignment of the satellites will cause a reduction of the galaxy-galaxy lensing shear by of order 25% to 40%. Hence, the radial alignment of satellite galaxies toward their hosts is a potentially important effect for precision studies of galaxy-galaxy lensing, and argues strongly in favor of the use of accurate photometric redshifts in order to identify lenses and sources in future studies.
kappa Pegasi is a well-known, nearby triple star system. It consists of a ``wide'' pair with semi-major axis 235 milli-arcseconds, one component of which is a single-line spectroscopic binary (semi-major axis 2.5 milli-arcseconds). Using high-precision differential astrometry and radial velocity observations, the masses for all three components are determined and the relative inclinations between the wide and narrow pairs' orbits is found to be 43.8 +/- 3.0 degrees, just over the threshold for the three body Kozai resonance. The system distance is determined to 34.60 +/- 0.21 parsec, and is consistent with trigonometric parallax measurements.
We explore the use of Integral Field Spectroscopy (IFS) to observe extrasolar planet transits. Although this technique should find its full potential in space observations (e.g. JWST, TPF), we have tested its basics with ground based time series observations of HD209458b obtained with WHT+INTEGRAL during a transit. For this analysis we used 5550 spectra, obtained in 150 exposures during a period of >7 hours. We found that IFS offers 3 fundamental advantages with respect to previously used methods. (i) It improves the effective S/N in photon limited observations by distributing the light coming from the star into the 2 dimensions of the detector. (ii) This type of IFS data allows to 'auto-calibrate' instrumental and background effects. (iii) Since the star image characteristics as well as its photometric properties are extracted from the same data-cube, it is possible to decorrelate photometric instabilities induced by PSF variations. These data have also allowed us to explore the accuracy limits of ground based 'relative' spectrophotometry. This was done using a photometric index that probes the NaD lines, for which we obtained a nominal 1-sigma error of ~1.0x10^-4. This result, based on observations of only 1 transit, indicates that this type of ground observation can constrain the characterization of the transmission spectrum of extrasolar planets. The present observations are compatible with no extra NaD depression during the transit. Though this result seems to be inconsistent with the recently reported HST-STIS findings we point out its limited statistical meaning: the results disagree within 1-sigma, but agree within 2-sigma. We also give some recommenda-tions to instrument developers in order to enhance the efficiency of the method.
We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2-455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of Pdot<2e-10 s/s, which implies that the spin-down luminosity is Edot<3e33 erg/s. The X-ray luminosity of the pulsar is L_X = 3(+10,-2)e33 (D/5 kpc)^2 erg/s, and the spectrum can be described by a kT = 0.61+/-0.02 keV blackbody with a radius of R_bb = 0.27+/-0.03 (D/5 kpc}) km. Deep infrared observations reveal no counterpart with K<18.5, which rules out binary companions with M>1 Msun. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2-455216 only 1.6' from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses M_i=35 Msun and >50 post-main-sequence stars that indicate the cluster is 4+/-1 Myr old. Therefore, the progenitor to this pulsar had an initial mass M_i>40 Msun. This is the most secure result among a handful of observational limits to the masses of neutron stars.
We present the first detection of interstellar acetone [(CH3)2CO] toward the high mass star forming region Orion-KL and the first detection of vibrationally excited (CH3)2CO in the ISM. Using the BIMA Array, 28 emission features that can be assigned to 54 acetone transitions were detected. Furthermore, 37 of these transitions have not been previously observed in the ISM. The observations also show that the acetone emission is concentrated toward the hot core region of Orion-KL, contrary to the distribution of other large oxygen bearing molecules. From our rotational-temperature diagram we find a beam averaged (CH3)2CO column density of (2.0(0.3)-8.0(1.2))x10^16 cm^-2 and a rotational temperature of 176(48)-194(66) K.
This paper describes the results of a study on the general lines, main construction criteria, crucial points, parameters and required preliminary R&D activities for the construction of a LAr (liquid argon) imaging detector with active mass in the 10-100 kTon range. Such detectors are crucial for supernova detection, proton decay, LBL neutrino physics and other astroparticle physics applications.
Microquasars occasionally exhibit massive jet ejections which are distinct from the continuous or quasi-continuous weak jet ejections. Because those massive jet ejections are rare and short events, they have hardly been observed in X-ray so far. In this paper, the first X-ray observation of a massive jet ejection from the microquasar SS 433 with the Rossi X-ray Timing Explorer (RXTE) is reported. SS 433 undergoing a massive ejection event shows a variety of new phenomena including a QPO-like feature near 0.1 Hz, rapid time variability, and shot-like activities. The shot-like activity may be caused by the formation of a small plasma bullet. A massive jet may be consist of thousands of those plasma bullets ejected from the binary system. The size, mass, internal energy, and kinetic energy of the bullets and the massive jet are estimated.
Precision radial velocity data for HD 208487 has been re-analyzed using a new Bayesian multi-planet Kepler periodogram. The periodgram employs a parallel tempering Markov chain Monte Carlo algorithm with a novel statistical control system. We confirm the previously reported orbit (Tinney et al. 2005) of 130 days. In addition, we conclude there is strong evidence for a second planet with a period of 998 -62 +57 days, an eccentricity of 0.19 -0.18 +0.05, and an M sin i = 0.46 -0.13 +0.05 of Jupiter's mass.
The issue of fragmentation in self-gravitating gaseous accretion discs has implications both for the formation of stars in discs in the nuclei of active galaxies, and for the formation of gaseous planets or brown dwarfs in circumstellar discs. It is now well established that fragmentation occurs if the disc is cooled on a timescale smaller than the local dynamical timescale, while for longer cooling times the disc reaches a quasi-steady state in thermal equilibrium, with the cooling rate balanced by the heating due to gravitational stresses. We investigate here how the fragmentation boundary depends on the assumed equation of state. We find that the cooling time required for fragmentation increases as the specific heat ratio, gamma, decreases, exceeding the local dynamical timescale for gamma = 7/5. This result can be easily interpreted as a consequence of there being a maximum stress (in units of the local disc pressure) that can be sustained by a self-gravitating disc in quasi-equilibrium. Fragmentation occurs if the cooling time is such that the stress required to reach thermal equilibrium exceeds this value, independent of gamma. This result suggest that a quasi-steady, self-gravitating disc can never produce a stress that results in the viscous alpha parameter exceeding ~ 0.06.
The Galactic diffuse emission is potentially able to reveal much about the sources and propagation of cosmic rays (CR), their spectra and intensities in distant locations. It can possibly unveil WIMP dark matter (DM) through its annihilation signatures. The extragalactic background may provide vital information about the early stages of the universe, neutralino annihilation, and unresolved sources (blazars?) and their cosmological evolution. The gamma-ray instrument EGRET on the CGRO contributed much to the exploration of the Galactic diffuse emission. The new NASA Gamma-ray Large Area Space Telescope (GLAST) is scheduled for launch in 2007; study of the diffuse gamma-ray emission is one of the priority goals. We describe current understanding of the diffuse emission and its potential for future discoveries.
Recent measurements of distant type Ia supernovae (SNIa) as well as other observations indicate that our universe is in accelerating phase of expansion. In principle there are two alternative explanation for such an acceleration. While in the first approach an unknown form of energy violating the strong energy condition is postulated, in second one some modification of FRW dynamics is postulated. The both approaches are in well agreement with present day observations which is the manifestation of the degeneracy problem appearing in observational cosmology. We use the Akaike (AIC) and Bayesian (BIC) information criteria of model selection to overcome this degeneracy and to determine a model with such a set of parameters which gives the most preferred fit to the SNIa data. We consider five representative evolutional scenarios in each of groups. Among dark energy proposal the $\Lambda$CDM model, CDM model with phantom field, CDM model with topological defect, model with Chaplygin gas, and the model with the linear dynamical equation of state parameter. As an alternative prototype scenarios we consider: brane world Dvali Gabadadze Porrati scenario, brane models in Randall-Sundrum scenario, Cardassian models with dust matter and radiation, bouncing model with the cosmological constant and metric-affine gravity (MAG) inspired cosmological models. Applying the model selection criteria we show that both AIC and BIC indicates that additional contribution arises from nonstandard FRW dynamics are not necessary to explain SNIa. Adopting the model selection information criteria we show that the AIC indicates the flat phantom model while BIC indicates both flat phantom and flat $\Lambda$CDM models.
We integrate for the first time the hydrodynamic Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a $^{1}S_{0}$-paired neutron superfluid in a rotating spherical shell, using a pseudospectral collocation algorithm coupled with a time-split fractional scheme. Numerical instabilities are smoothed by spectral filtering. Three numerical experiments are conducted, with the following results. (i) When the inner and outer spheres are put into steady differential rotation, the viscous torque exerted on the spheres oscillates quasiperiodically and persistently (after an initial transient). The fractional oscillation amplitude ($\sim 10^{-2}$) increases with the angular shear and decreases with the gap width. (ii) When the outer sphere is accelerated impulsively after an interval of steady differential rotation, the torque increases suddenly, relaxes exponentially, then oscillates persistently as in (i). The relaxation time-scale is determined principally by the angular velocity jump, whereas the oscillation amplitude is determined principally by the gap width. (iii) When the mutual friction force changes suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear array of quantized Feynman-Onsager vortices is destabilized by a counterflow to form a reconnecting vortex tangle, the relaxation time-scale is reduced by a factor of $\sim 3$ compared to (ii), and the system reaches a stationary state where the torque oscillates with fractional amplitude $\sim 10^{-3}$ about a constant mean value. Preliminary scalings are computed for observable quantities like angular velocity and acceleration as functions of Reynolds number, angular shear, and gap width. The results are applied to the timing irregularities (e.g., glitches and timing noise) observed in radio pulsars.
(Abridged) We use techniques from nonparametric function estimation theory to extract the density profiles, and their derivatives, from a set of N-body halos, and compare these with a variety of parametric models. We consider halos generated from isolated spherical collapses and LCDM simulations of gravitational clustering. The models include an NFW-like model with arbitrary inner power-law slope gamma, Sersic's r^{1/n} model (traditionally applied to projected light-profiles), and the density model of Prugniel & Simien (PS) that was designed to match the deprojected form of the Sersic model and is applied here for the first time to dark matter halos. The Sersic and PS models perform the best, with an rms scatter four times smaller than that obtained with the NFW-like model in the case of the cold collapses. The location of the (10^{12} M_sun) galaxy-sized, N-body halos in the <mu>_e-log(R_e) and log(rho_e)-log(R_e) diagrams coincides with that of brightest cluster galaxies, consistent with the new relation log(rho_e) = 1.15 - 2.5log(R_e) defined by luminous elliptical galaxies. We provide analytical expressions for the slopes of the empirical models, and compare these with data from real galaxies. Depending on the Sersic parameters of the dark matter halo, one can expect an extrapolated, inner (0.01 - 1 kpc), logarithmic profile slope ranging from -0.2 to -1.5, with a typical value at 0.1 kpc around -0.7. We also present the radial behavior of rho(r)/sigma(r)^3 for the Sersic and PS models, finding they are well approximated by a power-law with slope slightly shallower than -2 near r=r_{-2}.
We develop a new formalism to study nonlinear evolution in the growth of large-scale structure, by following the dynamics of gravitational clustering as it builds up in time. This approach is conveniently represented by Feynman diagrams constructed in terms of three objects: the initial conditions (e.g. perturbation spectrum), the vertex (describing non-linearities) and the propagator (describing linear evolution). We show that loop corrections to the linear power spectrum organize themselves into two classes of diagrams: one corresponding to mode-coupling effects, the other to a renormalization of the propagator. Resummation of the latter gives rise to a quantity that measures the memory of perturbations to initial conditions as a function of scale. As a result of this, we show that a well-defined (renormalized) perturbation theory follows, in the sense that each term in the remaining mode-coupling series dominates at some characteristic scale and is subdominant otherwise. This is unlike standard perturbation theory, where different loop corrections can become of the same magnitude in the nonlinear regime. In companion papers we compare the resummation of the propagator with numerical simulations, and apply these results to the calculation of the nonlinear power spectrum. Remarkably, the expressions in renormalized perturbation theory can be written in a way that closely resembles the halo model.
We study the nonlinear propagator, a key ingredient in renormalized perturbation theory (RPT) that allows a well-controlled extension of perturbation theory into the nonlinear regime. We show that it can be thought as measuring the memory of density and velocity fields to their initial conditions. This provides a clean definition of the validity of linear theory, which is shown to be much more restricted than usually recognized in the literature. We calculate the nonlinear propagator in RPT and compare to measurements in numerical simulations, showing remarkable agreement well into the nonlinear regime. We also show that N-body simulations require a rather large volume to recover the correct propagator, due to the missing large-scale modes. Our results for the nonlinear propagator provide an essential element to compute the nonlinear power spectrum in RPT.
A new 1.8-m wide-field alt-az survey telescope was installed at Mt John University Observatory in New Zealand in October 2004. The telescope will be dedicated to the MOA (Microlensing Observations in Astrophysics) project. The instrument is equipped with a large 10-chip mosaic CCD camera with 80 Mpixels covering about 2 square degrees of sky. It is mounted at the f/3 prime focus. The telescope will be used for finding and following microlensing events in the galactic bulge and elsewhere, with an emphasis on the analysis of microlensing light curves for the detection of extrasolar planets. The MOA project is a Japan-New Zealand collaboration, with the participation of Nagoya University and four universities in New Zealand.
We study the dependence of column densities derived from absorption lines on the spatial distribution of the ions in an absorber. In particular, we investigate four varieties of coverage by the absorber of the background source: the familiar homogeneous partial coverage (HPC), and three functional forms that parameterize inhomogeneous coverage: a powerlaw, an ellipse quandrant, and a Gaussian distribution. We calculate the residual line intensities obtained from our inhomogeneous coverage models and then use these intensities as ``observed'' quantities to compute the optical depth and covering factors assuming HPC. We find that the resulting spatially-averaged optical depths are comparable (within a factor of $\lesssim 1.5$) to the average optical depths of the input distributions, as long as the input distributions do not contain spatially narrow ``spikes.'' Such spikes (very large optical depths over a small coverage area) can profoundly affect the average optical depth in the absorber but have little impact on the observed intensities. We also study the converse approach: we start with HPC as the assumed physical model and then infer the parameters of the inhomogeneous coverage models through a doublet analysis. Again the resulting average optical depths are comparable to the corresponding quantity of the input distribution. Finally, we construct a more realistic two-dimensional optical depth distriubution based on a random distribution of absorbing clouds, and we use that to calculate observed intensities. A doublet analysis applied to those intensities shows all four of our simple analytic functions yield an accurate estimate of the true average optical depth, while the powerlaw yields the best approximation to the intial distribution.
Cosmology in extended theories of gravity is considered assuming the Palatini variational principle, for which the metric and connection are independent variables. The field equations are derived to linear order in perturbations about the homogeneous and isotropic but possibly spatially curved background. The results are presented in a unified form applicable to a broad class of gravity theories allowing arbitrary scalar-tensor couplings and nonlinear dependence on the Ricci scalar in the gravitational action. The gauge-ready formalism exploited here makes it possible to obtain the equations immediately in any of the commonly used gauges. Of the three type of perturbations, the main attention is on the scalar modes responsible for the cosmic large-scale structure. Evolution equations are derived for perturbations in a late universe filled with cold dark matter and accelerated by curvature corrections. Such corrections are found to induce effective pressure gradients which are problematical in the formation of large-scale structure. This is demonstrated by analytic solutions in a particular case. A physical equivalence between scalar-tensor theories in metric and in Palatini formalisms is pointed out.
We study the evolution of the [O/Fe]-[Fe/H] relation and the dependence of the iron abundance on distance from the galactic plane z in a one-zone model for a disk galaxy, starting from the beginning of star formation (Wiebe98). We obtain good agreement with the observational data, including, for the first time, agreement for the [Fe/H]-z relation out to heights of 16 kpc. We also study the influence of the presence of dark matter in the galaxies on the star-formation rate. Comparison of the observed luminosity of the Galaxy with the model prediction places constraints on the fractional mass of dark matter, which cannot be much larger than the fractional mass of visible matter, at least within the assumed radius of the Galaxy, 20 kpc. We studied the evolution of disk galaxies with various masses, which should obey the Tully-Fisher relation, M \propto R^2. The Tully-Fisher relation can be explained as a combination of a selection effect related to the observed surface brightnesses of galaxies with large radii and the conditions for the formation for elliptical galaxies.
We identify the fading X-ray afterglow of GRB 001025A from XMM-Newton observations obtained 1.9-2.3 days, 2 years, and 2.5 years after the burst. The non-detection of an optical counterpart to an upper limit of R=25.5, 1.20 days after the burst, makes GRB 001025A a ``dark'' burst. Based on the X-ray afterglow spectral properties of GRB 001025A, we argue that some bursts appear optically dark because their afterglow is faint and their cooling frequency is close to the X-ray band. This interpretation is applicable to several of the few other dark bursts where the X-ray spectral index has been measured. The X-ray afterglow flux of GRB 001025A is an order of magnitude lower than for typical long-duration gamma-ray bursts. The spectrum of the X-ray afterglow can be fitted with an absorbed synchrotron emission model, an absorbed thermal plasma model, or a combination thereof. For the latter, an extrapolation to optical wavelengths can be reconciled with the R-band upper limit on the afterglow, without invoking any optical circumburst absorption, provided the cooling frequency is close to the X-ray band. Alternatively, if the X-ray afterglow is due to synchrotron emission only, seven magnitudes of extinction in the observed R-band is required to meet the R-band upper limit, making GRB 001025A much more obscured than bursts with detected optical afterglows. Based on the column density of X-ray absorbing circumburst matter, an SMC gas-to-dust ratio is insufficient to produce this amount of extinction. The X-ray tail of the prompt emission enters a steep temporal decay excluding that the tail of the prompt emission is the onset of the afterglow (abridged).
Type Ia supernovae (SN Ia) have provided the first evidence for an accelerating universe and for the existence of an unknown ``dark energy'' driving this expansion. The 5-year Supernova Legacy Survey (SNLS) will deliver \~700 type Ia supernovae and as many type II supernovae with well-sampled light curves in 4 filters g', r', i' and z'. The current status of the project will be presented, along with the real time processing leading to the discovery and spectroscopic observation of the supernovae. We also present an offline selection of the SN candidates which aims at identifying and eliminating potential selection biases.
The expected distributions of eclipse-depth versus period for eclipsing binaries of different luminosities are derived from large-scale population synthesis experiments. Using the rapid Hurley et al. BSE binary evolution code, we have evolved several hundred million binaries, starting from various simple input distributions of masses and orbit-sizes. Eclipse probabilities and predicted distributions over period and eclipse-depth (P/dm) are given in a number of main-sequence intervals, from O-stars to brown dwarfs. The comparison between theory and Hipparcos observations shows that a standard (Duquennoy & Mayor) input distribution of orbit-sizes (a) gives reasonable numbers and P/dm-distributions, as long as the mass-ratio distribution is also close to the observed flat ones. A random pairing model, where the primary and secondary are drawn independently from the same IMF, gives more than an order of magnitude too few eclipsing binaries on the upper main sequence. For a set of eclipsing OB-systems in the LMC, the observed period-distribution is different from the theoretical one, and the input orbit distributions and/or the evolutionary environment in LMC has to be different compared with the Galaxy. A natural application of these methods are estimates of the numbers and properties of eclipsing binaries observed by large-scale surveys like Gaia.
We report the results of a deep Chandra survey of the Sculptor dwarf spheroidal galaxy. We find five X-ray sources with $L_X$ of at least $6\times10^{33}$ ergs/sec with optical counterparts establishing them as members of Sculptor. These X-ray luminosities indicate that these sources are X-ray binaries, as no other known class of Galactic point sources can reach 0.5-8 keV luminosities this high. Finding these systems proves definitively that such objects can exist in an old stellar population without stellar collisions. Three of these objects have highly evolved optical counterparts (giants or horizontal branch stars), as do three other sources whose X-ray luminosities are in the range which includes both quiescent low mass X-ray binaries and the brightest magnetic cataclysmic variables. We predict that large area surveys of the Milky Way should also turn up large numbers of quiescent X-ray binaries.
Solar-like oscillations detected in both components of the binary system alpha Centauri provide strong constraints on the fundamental parameters of the stellar system. We model alpha Centauri by means of a Levenberg-Marquardt minimization algorithm including seismic and classical constraints. Computations, that were perfomed decreasing significanly the weight of alpha Cen B seismic data in the calibration procedure, predict small separations in good agreement with new observations of solar-like oscillations in alpha Cen B by Bedding (these proceedings).
We calculate the X-ray luminosity and light curve for the stellar binary system Eta Carinae for the entire orbital period of 5.54 years. By using a new approach we find, as suggested before, that the collision of the winds blown by the two stars can explain the X-ray emission and temporal behavior. Most X-ray emission results from the shocked secondary stellar wind. The observed rise in X-ray luminosity just before minimum is due to a decrease in the radiative cooling time of the shocked fast secondary wind. Although absorption cannot explain the abrupt decline and the 70 day X-ray flat minimum observed every period, our model predicts that just before the minimum, when the X-rays are produced relatively close to the primary, only 10% of the soft X-rays and 60% of the hard X-rays can escape the dense primary wind. The 70 day flat minimum, on the other hand, is assumed to result from the collapse of the collision region of the two winds onto the secondary star. This process is assumed to shut down the secondary wind, hence the main X-ray source. We show that this assumption results in a successful model for the X-ray minimum.
The central 2x1 kpc of the starburst galaxy NGC 253 has been imaged using the Submillimeter Array at a 60 pc resolution in the J=2-1 transitions of 12CO, 13CO, and C18O as well as in the 1.3 mm continuum. Molecular gas and dust are mainly in the circumnuclear disk of ~500 pc radius, with warm (~40 K) and high area-filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have ~100 pc diameters and have large velocity widths of 80-100 km/s, suggestive of expansion at ~50 km/s. Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed kinetic energy of ~1E46 J as well as mean mechanical luminosity of ~1E33 W for its formation. The large energy allows each to be called a superbubble. A ~10^6 Msun super star cluster can provide the luminosity, and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to globally disturbed in the form of the well-known superwind.
It is known that the shape of a planet (oblateness, rings, etc.) slightly modifies the shape of the transit light curve. The forthcoming space missions (Corot, Kepler), able to detect the transit of Earth-like planets, could a fortiori also detect the transit of artificial planet-size objects if their shape is significantly different from a natural (planetary) object. Multiple artificial objects would also produce transit light curves easily recognizable from natural transits. Artificial transits, especially of multiple objects, could be used for the transmission of clear attention-getting signals, with a sky coverage (efficiency) comparable to that of the laser pulse method. Although out of reach of current human technologies, the building of an Earth-size 1-micron thick mask would require energy and bulk material amounts already managed on Earth today. The migration of the mask toward an inner orbit and its protection against asteroids or meteoroids are also briefly discussed.
Very long (172 ks effective exposure time) observations of the BALQSO LBQS 2212-1759 with XMM-Newton yield a stringent upper-limit on its 0.2-10 keV (rest- frame 0.64-32.2 keV) flux, F < 6 E-17 erg/cm2/s, while simultaneous UV and optical observations reveal a rather blue spectrum extending to 650 A in the source rest frame. These results are used to set a tight upper-limit on its optical to X-ray spectral index alpha_{ox} < -2.56. Given the HI-BAL nature of LBQS 212-1759, its X-ray weakness is most likely due to intrinsic absorption. If this is the case, and assuming that the intrinsic alpha_{ox} of LBQS 2212-1759 is -1.63 - a value appropriate for a radio-quiet quasar of this luminosity - one can set a lower limit on the X-ray absorbing column N_{H} > 3.4 E25 cm-2. Such a large column has a Thomson optical depth to electron scattering tau > 23, sufficient to extinguish the optical and UV emission. The problem only gets worse if the gas is neutral since the opacity in the Lyman continuum becomes extremely large, > 2 E8, conflicting with the source detection below 912 A. This apparent contradiction probably means that our lines-of-sight to the X-ray and to the UV emitting regions are different, such that the gas covers completely the compact X-ray source but only partially the more extended source of ultraviolet photons. An extended (~ 1') X-ray source is detected 2' to the south-east of the QSO. Given its thermal spectrum and temperature (1.5 < T < 3.0 keV}, it is probably a foreground (0.29 < z < 0.46) cluster of galaxies.
Quasars are thought to be powered by the infall of matter onto a supermassive black hole at the centre of massive galaxies. As the optical luminosity of quasars exceeds that of their host galaxy, disentangling the two components can be difficult. This led in the 1990's to the controversial claim of the discovery of 'naked' quasars. Since then, the connection between quasars and galaxies has been well established. Here we report on the observation of a quasar lying at the edge of a gas cloud, whose size is comparable to that of a small galaxy, but whose spectrum shows no evidence for stars. The gas cloud is excited by the quasar itself. If a host galaxy is present, it is at least six times fainter than would normally be expected for such a bright quasar. The quasar is interacting dynamically with a neighbouring galaxy - which matter might be feeding the black hole.
We obtained N- and Q-band observations of the Apollo-type asteroid 25143 Itokawa during its close Earth approach in July 2004 with TIMMI2 at the ESO 3.6 m telescope. Our photometric measurement, in combination with already published data, allowed us to derive a radiometric effective diameter of 0.32+/-0.03 km and an albedo of 0.19 +0.11/-0.03 through a thermophysical model. This effective diameter corresponds to a slightly asymmetrical and flattened ellipsoid of the approximate size of 520(+/-50) x 270(+/-30) x 230(+/-20) m, based on the Kaasalainen et al. (2005) shape model. Our studies show that the thermal observations lead to size estimates which are about 15% smaller than the radar results (Ostro et al. 2005), slightly outside the stated radar uncertainties of +/-10%. We determined a rather high thermal inertia of 750 Jm-2s-0.5K-1. This is an indication for a bare rock dominated surface, a thick dust regolith can be excluded as well as a metallic surface. From our data we constructed a 10.0 micrometer thermal lightcurve which is nicely matched in amplitude and phase by the shape and spin vector solution in combination with our TPM description. The assumed S-type bulk density in combination with radiometric size lead to a total mass estimate of 4.5 +2.0/-1.8 x 10^10 kg.
Through polarization observations water masers are excellent probes of magnetic fields in the maser region. Magnetic field strengths, such as those in the water masers regions of the envelopes of late-type stars and star-forming regions, are typically determined using a direct relation between the field strength and the observed circular polarization. Here it is shown that velocity and magnetic field gradients along the maser have a significant effect on the field strengths obtained from circular polarization observations. Due to velocity gradients the actual magnetic field strength could be up to 100% higher than the field strength derived from the observations. Additionally, when a magnetic field gradient is present, the resulting circular polarization derived is caused predominantly by the average magnetic field in the unsaturated maser core. Measurements of the fractional linear polarization are not affected by velocity or magnetic field strength gradients, though changes in the magnetic field angle along the maser do quench the linear polarization intensity when the maser saturates.
We present optical studies of the physical and wind properties, plus CNO chemical abundances, of 25 O9.5-B3 Galactic supergiants. We employ non-LTE, line blanketed, extended model atmospheres, which provide a modest downward revision in the effective temperature scale of early B supergiants of up to 1-2kK relative to previous non-blanketed results. The so-called `bistability jump' at B1 (Teff ~ 21kK) from Lamers et al. is rather a more gradual trend (with large scatter) from v_inf/v_esc ~ 3.4 for B0--0.5 supergiants above 24kK to v_inf/v_esc ~ 2.5 for B0.7-1 supergiants with 20kK < Teff < 24kK, and v_inf/v_esc ~ 1.9 for B1.5-3 supergiants below 20kK. This, in part, explains the break in observed UV spectral characteristics between B0.5 and B0.7 subtypes as discussed by Walborn et al. We compare derived (homogeneous) wind densities with recent results for Magellanic Cloud B supergiants and generally confirm theoretical expectations for stronger winds amongst Galactic supergiants. However, winds are substantially weaker than predictions from current radiatively driven wind theory, especially at mid-B subtypes, a problem which is exacerbated if winds are already clumped in the H-alpha line forming region. In general, CNO elemental abundances reveal strongly processed material at the surface of Galactic B supergiants, with mean N/C and N/O abundances 10 and 5 times higher than the Solar value, respectively, with HD 2905 (BC0.7 Ia) indicating the lowest degree of processing in our sample, and HD 152236 (B1.5 Ia+) the highest.
We present the review of various methods of detection of magnetic field strengths in the nearest regions of the active galaxy nuclei (AGN) which are the high energetic machines. Original spectropolarimetric method developed in the Pulkovo Observatory allows us to determine the magnitude and geometry of the magnetic field in the region of the optical and more hard electromagnetic radiation. The results of theoretical calculations are compared to the results of spectropolarimetric observations of AGN. We used the method of determining magnetic fields through the spectrum synchrotron radiation in the region of synchrotron self-absorption. As the magnitude of magnetic field of the extragalactic source depends very strongly on the angular size of extragalactic source and therefore on the photometric distance the calculated magnetic field magnitudes depends very strongly on the definite cosmological model. This result allows us to present the new method for determination of the most important cosmological parameters including dark matter and dark energy parameters.
In this paper we explore the relation between dust extinction and stellar light distribution in disks of spiral galaxies. Extinction influences our dynamical and photometric perception of disks, since it can distort our measurement of the contribution of the stellar component. To characterize the total extinction by a foreground disk, Gonzalez et al. (1998) proposed the ``Synthetic Field Method'' (SFM), which uses the calibrated number of distant galaxies seen through the foreground disk as a direct indication of extinction. The method is described in Gonzalez et al. (1998) and Holwerda et al. (2005a). To obtain good statistics, the method was applied to a set of HST/WFPC2 fields Holwerda et al. (2005b) and radial extinction profiles were derived, based on these counts. In the present paper, we explore the relation of opacity with surface brightness or color from 2MASS images, as well as the relation between the scalelengths for extinction and light in the I band. We find that there is indeed a relation between the opacity (A_I) and the surface brightness, particularly at the higher surface brightnesses. No strong relation between near infrared (H-J, H-K) color and opacity is found. The scalelengths of the extinction are uncertain for individual galaxies but seem to indicate that the dust distribution is much more extended than the stellar light. The results from the distant galaxy counts are also compared to the reddening derived from the Cepheids light-curves Freedman et al. (2001). The extinction values are consistent, provided the selection effect against Cepheids with higher values of A_I is taken into account. The implications from these relations for disk photometry, M/L conversion and galaxy dynamical modeling are briefly discussed.
We have used the available samples of Gigahertz Peaked Spectrum (GPS) galaxies to investigate their evolution properties in the framework of the ``youth'' scenario. Care was taken to properly allow for the different selection criteria used to define the samples. We find that the observed redshift and peak frequency distributions can be satisfactorily accounted for in terms of simple luminosity evolution of individual sources, along the lines discussed by Fanti et al. (1995) and Begelman (1996, 1999), although the derived parameter values have large uncertainties due to ambiguities in the selection of GPS sources and to the incompleteness of redshift measurements. However the simplest self-similar model, whereby the evolution is controlled only by the radial profile of the density of the ambient medium is not good enough and one additional parameter needs to be introduced. The fit requires a decrease of the emitted power and of the peak luminosity with source age or with decreasing peak frequency, at variance with the Snellen et al. (2000) model. On the other hand, our analysis confirms the rather flat slope of the luminosity function, found by Snellen et al. (2000) who also report indications of a high luminosity break, not required by the data sets we have used. Our results suggest that the GPS galaxies are the precursors of extended radio sources with luminosities below the break of the luminosity function. No cosmological evolution of the GPS galaxy population is required by presently available data.
We investigate the generation of galactic outflows by supernova feedback in the context of SPH cosmological simulations. We use a modified version of the code GADGET-2 which includes chemical enrichment and energy feedback by Supernova. We find that energy feedback plays a fundamental role in the evolution of galaxies, heating up the cold material in the centre of the haloes and triggering outflows which efficiently transport gas from the centre to the outskirts of galaxies. The impact of feedback is found to depend on the virial mass of the system with smaller systems, such as dwarf galaxies, being more strongly affected. The outflows help to establish a self-regulated star formation process, and to transport a significant amount of metals into the haloes and even out of the systems. According to our results, energy feedback by supernovae could be the mechanism responsible for the chemical enrichment of the intergalactic medium.
Local-box hydrodynamical model atmospheres provide statistical information about the spatial dependence, as well as temporal evolution, of a star's emergent radiation field. Here, we consider late-type stellar atmospheres for which temporal changes of the radiative output are primarily related to convective (granular) surface flows. We derived relations for evaluating the granulation-induced, disk-integrated thus observable fluctuations of the stellar brightness and location of the photocenter from radiation intensities available from a local model. Apart from their application in the context of hydrodynamical stellar atmospheres, these formulae provide some broader insight into the nature of the fluctuations under consideration. Brightness fluctuations scale inversely proportional to the square root of the number of convective cells (the statistically independently radiating surface elements) present on the stellar surface and increase with more pronounced limb-darkening. Fluctuations of the stellar photocentric position do not depend on the number of cells and are largely insensitive to the degree of limb-darkening. They amount to a small fraction of the typical cell size, and can become a limiting factor for high-precision astrometry in the case of extreme giants. The temporal brightness and positional fluctuations are statistically uncorrelated but closely related in magnitude.
We study stellar wind properties of selected late O stars in the Small Magellanic Cloud (SMC). We calculate NLTE line-driven wind models for these stars and compare predicted wind parameters with observed values. We found satisfactory agreement between theoretical and observed terminal velocities. On the other hand, predicted and observed mass-loss rates are in a good agreement only for higher mass-loss rates. For mass-loss rates lower than approximately 10^{-7} M_sun / year we found large discrepancy between theoretical and observed values. We propose a new explanation of this effect based on dynamical decoupling of some atoms. Finally, we study the dependence of wind terminal velocities and mass-loss rates on metallicity.
We study the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes (single and multishell) in various phases of the interstellar medium. Using the formalism of Draine and Lazarian for the rotational dynamics of these molecules in various astrophysical environments, we find effective rotation rates in the range 1-65 GHz with a trend toward lower rotational frequency as the radius of the molecule increases. Owing to the moderately polar nature of the C--H bond, hydrogenated fullerenes (fulleranes) are expected to have a net dipole moment and produce electric dipole radiation. Adopting the same size distribution proposed for fullerenes in the study of the UV extinction bump (2175 \AA) we predict the dipole electric emission of mixtures of fulleranes for various levels of hydrogenation. We find that these molecules could be the carriers of the anomalous microwave emission recently detected by Watson et al. in the Perseus molecular complex.
Around 200 bright stars (V < 6) have been monitored with the two-inch star tracker on the WIRE satellite since observations started in 1999. Here we present new results for the solar-like star Procyon A, the two Delta Scuti stars Altair and Epsilon Cephei, and the triple system Lambda Scorpii which consist of two B-type stars -- one of which we find to be an eclipsing binary.
We present a Monte Carlo study of an underwater neutrino telescope based on the detection of acoustic signals generated by neutrino induced cascades. This provides a promising approach to instrument large detector volumes needed to detect the small flux of cosmic neutrinos at ultra-high energies (E > 1 EeV). Acoustic signals are calculated based on the thermo-acoustic model. The signal is propagated to the sensors taking frequency dependent attenuation into account, and detected using a threshold trigger, where acoustic background is included as an effective detection threshold. A simple reconstruction algorithm allows for the determination of the cascade direction and energy. Various detector setups are compared regarding their effective volumes. Sensitivity estimates for the diffuse neutrino flux are presented.
We report the first comprehensive investigation of the line profile variation caused by non-radial pulsation in a magnetic oscillating chemically peculiar star. Spectrum variation of the well-known roAp star HR 3831 is detected using very high-resolution high signal-to-noise spectroscopic time-series observations and are followed through the whole rotation cycle of the star. We confirm outstanding diversity of pulsational behaviour of different lines in the HR 3831 spectrum and attribute this phenomenon to an interplay between extreme vertical chemical inhomogeneity of the HR 3831 atmosphere and a running pulsation wave, propagating towards the upper photospheric layers with increasing amplitude. Rapid profile variation of the NdIII 6145 A line is characterized by measuring changes of its equivalent width and the first three moments. We demonstrate that rotational modulation of the radial velocity oscillations cannot be fully explained by an oblique axisymmetric dipole (ell=1, m=0) mode, implied by the classical oblique pulsator model of roAp stars. Pulsational variation of the higher moments reveal substantial contribution of the high-order (ell=3) spherical harmonics which appear due to distortion of pulsations in HR 3831 by the global magnetic field. We interpret observations with a novel numerical model of the pulsational variation and rotational modulation of the line profile moments in roAp stars. The comparison between observed and computed amplitudes and phases of the radial velocity and line width variation is used to establish parameters of the oblique pulsator model of HR 3831. Furthermore, definite detection of pulsational variation in lines of light and iron-peak elements enables the first 3-D mapping of pulsations in non-radially oscillating star.
(Abridged) We study the gravitational radiation from gravitational collapses of rapidly rotating neutron stars induced by a phase-transition from normal nuclear matter to a mixed phase of quark and nuclear matter in the core of the stars. The study is based on self-consistent three dimensional hydrodynamic simulations with Newtonian gravity and a high resolution shock capturing scheme, and the quadrupole formula of gravitational radiation. The quark matter of the mixed phase is described by the MIT bag model and the normal nuclear matter is described by an ideal fluid equation of state (EOS). 1. We determined the magnitudes of the emitted gravitational waves for several collapse scenarios. 2. We determined the types and frequencies of the fluid oscillation modes excited by the process. In particular, we find that the gravitational wave signals produced by the collapses are dominated by the fundamental quadrupole and quasi-radial modes of the final equilibrium configurations. In some collapse scenarios, we find that the oscillations are damped out within a few dynamical timescales due to the growth of differential rotations and the formation of strong shock waves. 3. We showed that the spectrum of the gravitational wave signals is sensitive to the EOS, implying that the detection of such gravitational waves could provide useful constraints on the EOS of newly born quark stars. 4. For the range of rotation periods we have studied, we found no sign of the development of nonaxisymmetric dynamical instabilities in the collapse process.
This is the first of a series of three papers exploring the connection between the multiwavelength properties of AGNs in nearby early-type galaxies and the characteristics of their hosts. We selected two samples, both with high resolution 5 GHz VLA observations available and providing measurements down to 1 mJy level, reaching radio-luminosities as low as 10^19 W/Hz. We focus on the 116 radio-detected galaxies as to boost the fraction of AGN with respect to a purely optically selected sample. Here we present the analysis of the optical brightness profiles based on archival HST images, available for 65 objects. We separate early-type galaxies on the basis of the slope of their nuclear brightness profiles, into core and power-law galaxies following the Nuker's scheme, rather than on the traditional morphological classification (i.e. into E and S0 galaxies). Our sample of AGN candidates is indistinguishable, when their brightness profiles are concerned, from galaxies of similar optical luminosity but hosting weaker (or no) radio-sources. We confirm previous findings that relatively bright radio-sources (L_r > 10^(21.5) W/Hz) are uniquely associated to core galaxies. However, below this threshold in radio-luminosity core and power-law galaxies coexist and they do not show any apparent difference in their radio-properties. Not surprisingly, since our sample is deliberately biased to favour the inclusion of active galaxies, we found a higher fraction of optically nucleated galaxies. Addressing the multiwavelength properties of these nuclei will be the aim of the two forthcoming papers.
The standard assumption in interpretation of stellar oscillation spectra from
photometry is that the excited mode have low angular degrees, typically $\ell<
3$. Considering the case of FG Vir, the $\delta$ Scuti star with the richest
known oscillation spectrum, we show that this assumption is not justified for
low amplitude peaks. The $\ell<3$ identifications have been found for 12
dominant peaks from pulsation amplitudes and phases. However, we show that for
the rest of the peaks (55), whose amplitudes are typically below 1 mmag, much
higher $\ell$'s are most likely.
We argue that improving amplitude resolution to the micromagnitude level, as
expected from space observations, is not likely to be rewarded with a credible
mode identifications because the spectra will be dominated by high-$\ell$ modes
of unknown azimuthal order, $m$.
Experiments were performed at a proton accelerator and an infrared laser acility to investigate the sound generation caused by the energy deposition of pulsed particle and laser beams in water. The beams with an energy range of 1 PeV to 400 PeV per proton beam spill and up to 10 EeV for the laser pulse were dumped into a water volume and the resulting acoustic signals were recorded with pressure sensitive sensors. Measurements were performed at varying pulse energies, sensor positions, beam diameters and temperatures. The data is well described by simulations based on the thermo-acoustic model. This implies that the primary mechanism for sound generation by the energy deposition of particles propagating in water is the local heating of the media giving rise to an expansion or contraction of the medium resulting in a pressure pulse with bipolar shape. A possible application of this effect would be the acoustical detection of neutrinos with energies greater than 1 EeV.
Rich oscillation spectra of dwarf-like pulsators contain a wealth of information about the object interiors and, in particular, about macroscopic transport processes, which is the most difficult aspect of stellar physics. Examples of extracting such information from data on solar-like and opacity driven pulsators are given. Problems in understanding new oscillation spectra are discussed. Importance of employing various data on excited mode is emphasized.
We review the science case for the Laser Guide Star system being built for the William Herschel Telescope (WHT) on La Palma. When used in combination with the NAOMI Adaptive Optics system and the OASIS visible-wavelength Integral Field Spectrograph, we demonstrate that there are substantial, exciting areas of astrophysical research in which the WHT can contribute.
Calibration sources are an indispensable tool for all detectors. In acoustic particle detection the goal of a calibration source is to mimic neutrino signatures as expected from hadronic cascades. A simple and promising method for the emulation of neutrino signals are piezo ceramics. We will present results of measruements and simulations on these piezo ceramics.
With new generation spectrographs integral field spectroscopy is becoming a widely used observational technique. The Integral Field Unit of the VIsible Multi-Object Spectrograph on the ESO-VLT allows to sample a field as large as 54" x 54" covered by 6400 fibers coupled with micro-lenses. We are presenting here the methods of the data processing software developed to extract the astrophysical signal of faint sources from the VIMOS IFU observations. We focus on the treatment of the fiber-to-fiber relative transmission and the sky subtraction, and the dedicated tasks we have built to address the peculiarities and unprecedented complexity of the dataset. We review the automated process we have developed under the VIPGI data organization and reduction environment (Scodeggio et al. 2005), along with the quality control performed to validate the process. The VIPGI-IFU data processing environment is available to the scientific community to process VIMOS-IFU data since November 2003.
Acoustic neutrino detection is a promising approach to instrument the large detector volumes needed for the detection of the small neutrino fluxes expected at ultra-high energies (E > 1 EeV). We report on several studies investigating the feasibility of such an acoustic detector. High-precision lab measurements using laser and proton beams aiming at the verification of the thermo-acoustic model have been performed. Different types of acoustic sensors have been developed and characterized. An autonomous acoustic system, attached to the ANTARES prototype string "Line0", has been deployed and operated successfully at 2400 m depth, allowing for in-situ studies of the acoustic background in the Mediterranean Sea.
Core-collapse supernovae are among the most energetic explosions in the
universe marking the catastrophic end of massive stars. In spite of rigorous
studies for several decades, we still don't understand the explosion mechanism
completely. Since they are related to many astrophysical phenomena such as
nucleosynthesis, gamma-ray bursts and acceleration of cosmic rays,
understanding of their physics has been of wide interest to the astrophysical
community.
In this article, we review recent progress in the study of core-collapse
supernovae focusing on the explosion mechanism, supernova neutrinos, and the
gravitational waves. As for the explosion mechanism, we present a review paying
particular attention to the roles of multidimensional aspects, such as
convection, rotation, and magnetic fields, on the neutrino heating mechanism.
Next, we discuss supernova neutrinos, which is a powerful tool to probe not
only deep inside of the supernovae but also intrinsic properties of neutrinos.
For this purpose, it is necessary to understand neutrino oscillation which has
been established recently by a lot of experiments. Gravitational astronomy is
now also becoming reality.
We present an extensive review on the physical foundations and the emission
mechanism of gravitational waves in detail, and discuss the possibility of
their detections.
We have used NIRSPEC on Keck II to obtain $K$-band spectroscopy of several magnetic cataclysmic variables. These data reveal that the secondary stars in these binary systems have spectra that are consistent with normal, late-type dwarfs in both their atomic and molecular line strengths, as well as in the slopes of their continuua. This result is in stark contrast to the infrared spectra of their non-magnetic cousins, nearly all of which show peculiar abundances, especially of CNO species and their isotopes. It appears that the evolutionary path taken by the secondary stars in magnetic systems differs from that for the non-magnetic systems. We discuss the implications of this result.
We report on the properties of LISM O VI absorption observed with 20 km/s resolution FUSE observations of 39 white dwarfs (WDs) ranging in distance from 37 to 230 pc with a median distance of 109 pc. LISM O VI is detected with >2sigma significance along 24 of 39 lines of sight. The column densities range from log N(O VI) = 12.38 to 13.60 with a median value of 13.10. The line of sight volume density, n(O VI) = N(O VI)/d exhibits a large dispersion ranging from (0.68 to 13.0)x10(-8) cm(-3) with an average value 3.6x10(-8) cm(-3) twice larger than found for more distant sight lines in the Galactic disk. The narrowest profiles are consistent with thermal Doppler broadening of O VI near its temperature of peak abundance, 2.8x10(5) K. Comparison of the average velocities of O VI and C II absorption reveals 10 cases where the O VI absorption is closely aligned with the C II absorption as expected if the O VI is formed in a condensing interface between the cool and warm absorption and a hot exterior gas. The comparison also reveals 13 cases where O VI absorption is displaced to positive velocity by 7 to 29 km/s from the average velocity of C II. The positive velocity O VI appears to be tracing the evaporative flow of O VI from a young interface between warm gas and a hot exterior medium. However, it is possible the positive velocity O VI is instead tracing cooling hot Local Bubble (LB) gas. The properties of the O VI absorption in the LISM are broadly consistent with the expectations of the theory of conductive interfaces caught in the old condensing phase and possibly in the young evaporative phase of their evolution.
We examine whether the ultraluminous infrared galaxies that contain a type I Seyfert nucleus (a type I ULIRG) are in the transition stage from ULIRGs to quasi-stellar objects (QSOs). To inspect this issue, we compare the black hole (BH) mass, the bulge luminosity and the far infrared luminosity among type I ULIRGs, QSOs and elliptical galaxies. As a result, we find the following results; (1) The type I ULIRGs have systematically smaller BH masses in spite of the comparable bulge luminosity relative to QSOs and elliptical galaxies. (2) The far-infrared luminosity of most type I ULIRGs is larger than the Eddington luminosity. We show that above results do not change significantly for 3 type I ULIRGs that we can estimate the visual extinction from the column density. Also, for all 8 type I ULIRGs, we investigate the effect of uncertainties of BH mass measurments and our sample bias, so that it turns out that our results do not alter even if we consider above two effects. In addition, Anabuki (2004) revealed that their X-ray properties are similar to those of the narrow line Seyfert 1 galaxies. These would indicate that active galactic nuclei (AGNs) with a high mass accretion rate exist in the type I ULIRGs. Based on all of these findings, we conclude that it would be a natural interpretation that type I ULIRGs are the early phase of BH growth, namely the missing link between ULIRGs and QSOs. Moreover, by comparing our results with a theoretical model of a coevolution scenario of a QSO BH and a galactic bulge, we show clearly that this explanation would be valid.
We critically analyze the role of clusters of galaxies as probes for precision cosmology. Using synthetic observations of numerically simulated clusters viewed through their X-ray emission and thermal Sunyaev-Zeldovich effect (SZE), we reduce the observations to attain measurements of the cluster gas mass. We utilize both parametric models such as the isothermal cluster model and non-parametric models that involve the geometric deprojection of the cluster emission assuming spherical symmetry. We are thus able to quantify the possible sources of uncertainty and systematic bias associated with the common simplifying assumptions used in reducing real cluster observations including isothermality and hydrostatic equilibrium. We find that intrinsic variations in clusters limit the precision of observational gas mass estimation to ~10% to 80% confidence excluding instrumental effects. For the full cluster sample, methods that use SZE profiles out to roughly the virial radius are the most accurate and precise way to estimate cluster mass. X-ray methods are systematically more precise mass estimators than are SZE methods if merger systems are removed, but X-ray methods slightly overestimate (5-10%) the cluster gas mass on average. SZE methods are more precise and accurate than X-ray methods at mass estimation if the sample is contaminated by merging or disturbed clusters. We also find that cool core clusters in our samples are particularly poor candidates for observational mass estimation, even when excluding emission from the core region. Finally, we find that methods using a universal temperature profile estimate cluster masses to higher precision than those assuming isothermality.
The spectrum of HV 2310, an evolved star in the Large Magellanic Cloud, taken with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope reveals the presence of an optically thin shell of silicate dust with unusual spectral structure in the 10 um feature, with an emission peak at 9.7 um, a saddle at 10.4 um, and an extended shoulder to 11.2 um. This structure is similar to spectra from crystalline silicate grains, and of the available optical constants, forsterite provides the best fit. The spectrum also shows structure at 14 um which may arise from an unidentified dust feature.
We present a precise kinematic study of very young brown dwarfs (BDs) in the ChaI cloud based on radial velocities (RVs) measured with UVES / VLT. This is compared to the kinematics of T Tauri stars (TTS) in the same field based on own measurements and on RVs from the literature. In comparison with a former paper (Joergens & Guenther 2001), additional UVES spectra have been taken, the reduction of the spectra was improved and a re-study of the literature for RVs of TTS in ChaI was done. The result is an improved empirical RV distribution of the BDs as well as of the TTS in ChaI. We find that nine BDs/VLMSs (M6--M8) in ChaI have a RV dispersion of 0.9 km/s measured in terms of standard deviation. This is consistent with the dispersion measured in terms of fwhm of 2.1 km/s by Joergens & Guenther (2001). The studied sample of 25 TTS (G2-M5) has a dispersion of 1.3 km/s (standard deviation). The RV dispersion of the BDs is consistent within the errors with that of the TTS, which is in line with the finding of no mass dependence in some theoretical models of the ejection-scenario for the formation of brown dwarfs. In contrast to current N-body simulations, we do not find a high-velocity tail for the BDs RVs. We find suggestive hints for different kinematics of binaries compared to predominantly single objects in ChaI. The global RV dispersion for ChaI members (1.24 km/s) is significantly lower than for Taurus members (2.0 km/s), despite a larger stellar density in ChaI showing that a fundamental increase of velocity dispersion with stellar density of the star-forming region is observationally not established. The observed RVs of BDs in ChaI are less dispersed than predicted by any of the models for the ejection-scenario.
Photoionization models of the giant HII region 30 Doradus are built and confronted to available UV, optical, IR (ISO) and radio spectra, under black-body or CoStar SEDs for the primary source and various density distributions for the nebular gas. Chemically homogeneous models show very small rms electron temperature fluctuations and fail to reproduce the heavy element optical recombination line (ORL) spectrum of the nebula. Dual abundance models incorporating small-scale chemical inhomogeneities in the form of H-deficient inclusions which are in pressure balance with the normal composition ambient gas, provide a better fit to the observed heavy element ORLs and other nebular lines, while most spectral features are satisfactorily accounted for. The inclusions, whose mass is ~2 per cent of the total gaseous mass, are 2-3 times cooler and denser than the ambient nebula. Their O/H abundance ratio is ~0.9 dex larger than in the normal composition gas and have typical mass fractions of X = 0.687, Y = 0.273 and Z = 0.040. Helium is found to be about as deficient as hydrogen in the inclusions, while elements heavier than Ne, such as S and Ar, are quite possibly enhanced in proportions similar to O. This suggests that the posited H-deficient inclusions may have arisen from partial mixing of matter which was nucleosynthetically processed in a supernova event with gas of normal LMC composition. Attention is drawn to a bias in the determination of HII region helium abundances in the presence of H-deficient inclusions. It is argued that these results provide evidence for incomplete small-scale mixing of the ISM. The case for the existence of abundance inhomogeneities in HII regions is examined in the light of current theoretical considerations regarding the process of homogenization in the ISM.
We present first realistic numerical simulations of 3D radiative convection in the surface layers of main sequence A-type stars with Teff = 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the CO5BOLD radiation hydrodynamics code. The resulting models are used to investigate the structure of the H+HeI and the HeII convection zones in comparison with the predictions of local and non-local convection theories, and to determine the amount of "overshoot" into the stable layers below the HeII convection zone. The simulations also predict how the topology of the photospheric granulation pattern changes from solar to A-type star convection. The influence of the photospheric temperature fluctuations and velocity fields on the shape of spectral lines is demonstrated by computing synthetic line profiles and line bisectors for some representative examples, allowing us to confront the 3D model results with observations.
The Coronet Cluster in the nearby R CrA dark cloud offers the rare opportunity to study at least four "class I" protostellar sources as well as one candidate "class 0" source, a Herbig Ae star, and a candidate brown dwarf within a few square arcminutes - most of them detected at radio- and X-ray wavelengths. These sources were observed with the Very Large Array (VLA) at 3.5cm on nine occasions in 1998, spread over nearly four months. The source IRS 5, earlier shown to emit circularly polarized radio emission, was observed to undergo a flux increase accompanied by changes in its polarization properties. Comparison with VLA measurements taken in January 1997 allows for some analysis of longer-term variability. In addition to this radio monitoring, we analyze archival Chandra and XMM-Newton X-ray data of these sources. Three class I protostars are bright enough for X-ray spectroscopy, and we perform a variability analysis for these sources, covering a total of 154 ksec spread over more than two and a half years. Also in X-rays, IRS 5 shows the most pronounced variability, whilst the other two class I protostars IRS 1 and IRS 2 have more stable emission. X-ray data is also analyzed for the recently identified candidate class 0 source IRS 7E, which shows strong variability as well as for the Herbig Ae star R CrA for which we find extremely hot X-ray-emitting plasma. For IRS 1,2 and 5, the hydrogen column densities derived from the X-ray spectra are at about half the values derived with near-infrared techniques, a situation similar to what has been observed towards some other young stellar objects.
The present catalog is the result of our attempts to collect all published photometric data on GRB afterglows observed in the pre-Swift era by the end of 2004 in order to gain statistical insight on the phenomenology of GRB afterglows. Part I contains all published data on GRB afterglows in filters we used in Zeh, Klose, & Kann (2005) to create reference light curves and derive light curve parameters (mostly R band, but a few bursts have better data in other colors) including the corresponding references. The catalog includes GCN data as well as data published in refereed journals. No data have been omitted or evaluated in any way (with the exception of a very small number of data that turned out to be not related to an afterglow). Part II will contain color information via the observed light curves in the other photometric bands (Kann et al. 2006, in preparation). Using a simple computer program that can handle strings our catalog is easy to use since all tables are provided in TeX format. For an on-line searchable GCN catalog, we refer to the work done by Quimby et al. (2003). Our catalog includes photometric data on 59 bursts (GRB 970228 - GRB 041006) with altogether 4883 data points. Most data are from GRB 030329 (2759 data points), followed by GRB 021004 (393 data points), while 13 bursts have less than 10 data points. In the case of GRB 030329 we have not included the extensive data list which is on-line provided by Lipkin et al. (2004). (Abridged)
The Submillimeter Array (SMA), a collaboration between the Smithsonian
Astrophysical Observatory and the Academica Sinica Institute for Astronomy and
Astrophysics of Taiwan, is an eight-element radio-interferometer designed to
operate throughout the major atmospheric windows from about 180 to 900 GHz. In
an effort to mitigate the effects of atmospheric instabilities which limit the
phase coherence of the array especially in the higher frequency bands, the
array was designed to allow simultaneous operation of a low frequency receiver
(<350 GHz) with a high frequency receiver (>330 GHz). The overlap region of
330-350 GHz was included to facilitate dual polarization measurements in the
frequency range considered to offer the highest sensitivity for continuum
observations with the array.
So far, the array is equipped with working SIS receivers covering the
frequency ranges 176-256 GHz, 260-350 GHz, and 600-700 GHz, and single
frequency operation has been routine in the lower two frequency bands for the
past year. More recently, with the completion of IF hardware required to make
full use of the SMA cross-correlator, dual receiver operation became possible.
We have since made a number of Galactic and extra-galactic astronomical
observations in dual-band mode with the hopes of using the 230 GHz receiver as
a phase reference to enable improved interferometry in the 650 GHz band. We
will present the current antenna and receiver performance, some of the first
interferometric images in the 650 GHz receiver band, and our initial attempts
at phase transfer.
Type IIP (plateau) supernovae are thought to come from stars with initial mass about 8-25 solar masses that end their lives as red supergiants. The expected stellar end points can be found from evolutionary calculations and the corresponding mass loss properties at this point can be estimated from typical values for Galactic stars. The mass loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling can affect the radio emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of a supernova in a metal poor region might show how the mass loss depends on metallicity.
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs in the UV spectra of quasars selected from the SDSS Early Data Release. These quasars have strong intervening MgII-FeII systems which are known signatures of high column density neutral gas. In total, UV observations of Ly-alpha absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW) W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases to 42(+/- 7)% for systems with W2796/W2600 < 2 and MgI W2852 > 0.1 A. (2) The mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36 lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M = 0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is constant for 0.5<z<5.0 to within the uncertainties. This is larger than Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution does not change significantly with redshift. However, the low redshift distribution is marginally flatter due to the higher fraction of high N(HI) systems in our sample. (6) Finally, using the precision of MgII survey statistics, we find that there may be evidence of a decreasing Omega_{DLA} from z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very high columns of neutral gas might be missed by DLA surveys because of their very small cross sections, and therefore, that Omega_{DLA} might not include the bulk of the neutral gas mass in the Universe. (Abridged)