This paper presents the first optical variability study of the Westerlund 1 super star cluster in search of massive eclipsing binary systems. A total of 129 new variable stars have been identified, including the discovery of 4 eclipsing binaries that are cluster members, 1 additional candidate, 8 field binaries, 19 field delta Scuti stars, 3 field W UMa eclipsing binaries, 13 other periodic variables and 81 long period or non-periodic variables. These include the known luminous blue variable, the B[e] star, 11 Wolf-Rayet stars, several supergiants, and other reddened stars that are likely members of Westerlund 1. The bright X-ray source corresponding to the Wolf-Rayet star WR77o (B) is found to be a 3.51 day eclipsing binary. The discovery of a reddened detached eclipsing binary system implies the first identification of main-sequence stars in Westerlund 1.
We use that gravity probes only the total energy momentum tensor to show how this leads to a degeneracy for generalised dark energy models. Because of this degeneracy, Omega_m cannot be measured. We demonstrate this explicitely by showing that the CMB and supernova data is compatible with very large and very small values of Omega_m for a specific family of dark energy models. We also show that for the same reason interacting dark energy is always equivalent to a family of non-interacting models. We argue that it is better to face this degeneracy and to parametrise the actual observables.
We have used the Parkes radio telescope to study the polarized emission from the anomalous X-ray pulsar XTE J1810-197 at frequencies of 1.4, 3.2, and 8.4 GHz. We find that the pulsed emission is nearly 100% linearly polarized. The position angle of linear polarization varies gently across the observed pulse profiles, varying little with observing frequency or time, even as the pulse profiles have changed dramatically over a period of 7 months. In the context of the standard pulsar "rotating vector model," there are two possible interpretations of the observed position angle swing coupled with the wide profile. In the first, the magnetic and rotation axes are substantially misaligned and the emission originates high in the magnetosphere, as seen for other young radio pulsars, and the beaming fraction is large. In the second interpretation, the magnetic and rotation axes are nearly aligned and the line of sight remains in the emission zone over almost the entire pulse phase. We deprecate this possibility because of the observed large modulation of thermal X-ray flux. We have also measured the Faraday rotation caused by the Galactic magnetic field, RM = +77 rad/m^2, implying an average magnetic field component along the line of sight of 0.5 microG.
The recent detection of X-ray flares during the afterglow phase of gamma-ray bursts (GRBs) suggests an inner-engine origin, at radii inside the forward shock. There must be inverse Compton (IC) emission arising from such flare photons scattered by forward shock afterglow electrons when they are passing through the forward shock. We find that this IC emission produces high energy gamma-ray flares, which may be detected by AGILE, GLAST and ground-based TeV telescopes. The anisotropic IC scattering between flare photons and forward shock electrons does not affect the total IC component intensity, but cause a time delay of the IC component peak relative to the flare peak. The anisotropic scattering effect may also weaken, to some extent, the suppression effect of the afterglow intensity induced by the enhanced electron cooling due to flare photons. We speculate that this IC component may already have been detected by EGRET from a very strong burst--GRB940217. Future observations by GLAST may help to distinguish whether X-ray flares originate from late central engine activity or from external shocks.
Dust is formed in the expanding atmosphere during late stages of stellar evolution. Dust influences the dynamics and thermodynamics of the stellar atmosphere by its opacity. The dust opacity depends both on the optical properties of the grain material as well as on the amount of dust present. A rich source of information on some mineral phases of dust in AGB stars comes from the study of presolar grains from meteorites. This paper presents a short overview of presolar grains studies and describes how the optical properties of dust grains are obtained in the laboratory.
The first search for ultra-high energy (UHE) neutrinos using a radio
telescope was conducted by Hankins, Ekers and O'Sullivan (1996). This was a
search for nanosecond duration radio Cherenkov pulses from electromagnetic
cascades initiated by ultra-high energy (UHE) neutrino interactions in the
lunar regolith, and was made using a broad-bandwidth receiver fitted to the
Parkes radio telescope, Australia. At the time, no simulations were available
to convert the null result into a neutrino flux limit. Since then, similar
experiments at Goldstone, USA, and Kalyazin, Russia, have also recorded null
results, and computer simulations have been used to model the experimental
sensitivities of these two experiments and put useful limits on the UHE
neutrino flux.
Proposed future experiments include the use of broad-bandwidth receivers,
making the sensitivity achieved by the Parkes experiment highly relevant to the
future prospects of this field. We have therefore calculated the effective
aperture for the Parkes experiment and found that when pointing at the lunar
limb, the effective aperture at all neutrino energies was superior to
single-antenna, narrow-bandwidth experiments, and that the detection threshold
was comparable to that of the double-antenna experiment at Goldstone. However,
because only a small fraction of the observing time was spent pointing the
limb, the Parkes experiment places only comparatively weak limits on the UHE
neutrino flux. Future efforts should use multiple telescopes and
broad-bandwidth receivers.
The X-ray light curves of hundreds of bursts are now available, thanks to the X-ray Telescope on board the Swift satellite, on time scales from ~1 minute up to weeks and in some cases months from the burst explosion. These data allow us to investigate the physics of the highly relativistic fireball outflow and its interaction with the circumburst environment. Here we review the main results of the XRT observations, with particular regard to the evolution of the X-ray light curves in the early phases. Unexpectedly, they are characterised by different slopes, with a very steep decay in the first few hundred of seconds, followed by a flatter decay and, a few thousand of seconds later, by a somewhat steeper decay. Often strong flare activity up to few hours after the burst explosion is also seen. These flares, most likely, are still related to the central engine activity, that last much longer than expected and it is still dominating the X-ray light curve well after the prompt phase, up to a few thousand of seconds. The real afterglow emission (external shock) is dominating the X-ray light curve only after the flatter phase ends. The flatter phase is probably the combination of late-prompt emission and afterglow emission. When the late-prompt emission ends the light curve steepens again. Some flare activity can still be detected during these later phases. Finally, even the late evolution of the XRT light curves is puzzling, in particular many of them do not show a ``jet-break''. There are various possibilities to explain these observations (e.g.time evolution of the microphysical parameters, structured jet). However, a clear understanding of the formation and evolution of the jet and of the afterglow emission is still lacking.
The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. The 511 keV line is clearly detected from the Galactic disk; in addition, there is a tantalizing hint at possible halo-like emission. The available data do not yet permit to discern whether the emission around the bulge region originates from a halo-like component or from a disk component that is very extended in latitude.
Planets orbiting in the habitable zone (HZ) of M-Dwarf stars are subject to high levels of galactic cosmic rays (GCRs) which produce nitrogen oxides in earthlike atmospheres. We investigate to what extent this NOx may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources) . Our model results suggest that such signals are robust, changing in the M-star world atmospheric column by up to 20% due to the GCR NOx effects compared to an M-star run without GCR effects and can therefore survive at least the effects of galactic cosmic rays. We have not however investigated stellar cosmic rays here. CH4 levels are about 10 times higher than on the Earth related to a lowering in hydroxyl (OH) in response to changes in UV. The increase is less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH4 fluxes compared to those studies. Unlike on the Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the M-star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M-star world differs considerably compared with the Earth.
We announce the forthcoming public release of Version 1.1 of MGGPOD, a
user-friendly suite of Monte Carlo codes built around the widely used GEANT
(Version 3.21) package. MGGPOD is capable of simulating ab initio the physical
processes relevant for the production of instrumental backgrounds. These
processes include the build-up and delayed decay of radioactive isotopes as
well as the prompt de-excitation of excited nuclei, both of which give rise to
a plethora of instrumental gamma-ray background lines in addition to continuum
backgrounds. A detailed qualitative and quantitative understanding of
instrumental backgrounds is crucial for most stages of high-energy astronomy
missions.
Improvements implemented in Version 1.1 of the proven MGGPOD Monte Carlo
suite include: additional beam geometry options, the capability of modelling
polarized photons, additional output formats suitable e.g. for event
reconstruction algorithms, improved neutron interaction cross sections, and
improved treatment of the radioactive decay of isomeric nuclear states.
The MGGPOD package and documentation are publicly available for download from
this http URL
We report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the two observations the source exhibits a powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have shown that bulk (dynamical) Comptonization of soft photons can be a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure originated at the surface of the neutron star (NS). As a result the high radiation pressure stops the bulk inflow and consequently this radiation feedback of the NS surface leads to quenching the bulk Comptonization.
We present local simulations that verify the linear streaming instability that arises from aerodynamic coupling between solids and gas in protoplanetary disks. This robust instability creates enhancements in the particle density in order to tap the free energy of the relative drift between solids and gas, generated by the radial pressure gradient of the disk. We confirm the analytic growth rates found by Youdin & Goodman (2005) using grid hydrodynamics to simulate the gas and, alternatively, particle and grid representations of the solids. Since the analytic derivation approximates particles as a fluid, this work corroborates the streaming instability when solids are treated as particles. The idealized physical conditions -- axisymmetry, uniform particle size, and the neglect of vertical stratification and collisions -- provide a rigorous, well-defined test of any numerical algorithm for coupled particle-gas dynamics in protoplanetary disks. We describe a numerical particle-mesh implementation of the drag force, which is crucial for resolving the coupled oscillations. Finally we comment on the balance of energy and angular momentum in two-component disks with frictional coupling. A companion paper details the non-linear evolution of the streaming instability into saturated turbulence with dense particle clumps.
We present simulations of the non-linear evolution of streaming instabilities in protoplanetary disks. The two components of the disk, gas treated with grid hydrodynamics and solids treated as superparticles, are mutually coupled by drag forces. We find that the initially laminar equilibrium flow spontaneously develops into turbulence in our unstratified local model. Marginally coupled solids (that couple to the gas on a Keplerian time-scale) trigger an upward cascade to large particle clumps with peak overdensities above 100. The clumps evolve dynamically by losing material downstream to the radial drift flow while receiving recycled material from upstream. Smaller, more tightly coupled solids produce weaker turbulence with more transient overdensities on smaller length scales. The net inward radial drift is decreased for marginally coupled particles, whereas the tightly coupled particles migrate faster in the saturated turbulent state. The turbulent diffusion of solid particles, measured by their random walk, depends strongly on their stopping time and on the solids-to-gas ratio of the background state, but diffusion is generally modest, particularly for tightly coupled solids. Angular momentum transport is too weak and of the wrong sign to influence stellar accretion. Self-gravity and collisions will be needed to determine the relevance of particle overdensities for planetesimal formation.
The CMB power spectra are studied for different families of single field new and chaotic inflation models in the effective field theory approach to inflation. We implement a systematic expansion in 1/N_e where N_e ~ 50 is the number of e-folds before the end of inflation. We study the dependence of the observables (n_s, r and dn_s/d\ln k) on the degree of the potential (2 n) and confront them to the WMAP3 and large scale structure data: this shows in general that fourth degree potentials (n=2) provide the best fit to the data; the window of consistency with the WMAP3 and LSS data narrows for growing n. New inflation models yield a good fit to the r and n_s data in a wide range of field and parameter space. Small field inflation yields r<0.16 while large field inflation yields r>0.16 (for N_e=50). All members of the new inflation family predict a small but negative running -4(n+1)10^{-4}< dn_s/d\ln k <-2 10^{-4}. A reconstruction program is carried out suggesting quite generallythat for n_s consistent with the WMAP3 and LSS data and r<0.1 the symmetry breaking scale for new inflation is | phi_0|~10 M_{Pl} while the field scale at Hubble crossing is |\phi_{50}|~M_{Pl}.The family of chaotic models feature r>0.16 (for N_e=50) and only a restricted subset of chaotic models are consistent with the combined WMAP3 bounds on r, n_s,dn_s/d\ln k with a narrow window in field amplitude around |\phi_{50}|~15 M_{Pl}.A measurement of r<0.16 (for N_e =50) will distinctly rule out a large class of chaotic scenarios and will favor new inflationary models. As a general consequence, new inflation emerges more favoured than chaotic inflation.
It is generally acknowledged that the mass loss of Asymptotic Giant Branch (AGB) stars undergoes variations on different time scales. We constructed models for the dust envelopes for a sample of AGB stars to assess whether mass-loss variations influence the spectral energy distribution (SED). To constrain the variability, extra observations at millimetre wavelengths (1.2 mm) were acquired. From the analysis of the dust models, two indications for the presence of mass-loss variations can be found, being (1) a dust temperature at the inner boundary of the dust envelope that is far below the dust condensation temperature and (2) an altered density distribution with respect to rho(r) \propto r^{-2} resulting from a constant mass-loss rate. For 5 out of the 18 studied sources a two-component model of the envelope is required, consisting of an inner region with a constant mass-loss rate and an outer region with a less steep density distribution. For one source an outer region with a steeper density distribution was found. Moreover, in a search for time variability in our data set at 1.2 mm, we found that WX Psc shows a large relative time variation of 34% which might partially be caused by variable molecular line emission.
In the context of the Galactic Emission Mapping collaboration, a galactic survey at 5GHz is in preparation to properly characterize the galactic foreground to the Cosmic Microwave Background Radiation (CMBR). For the North sky survey, a new receiver is being developed. This 5GHz heterodyne polarimeter has a high gain IF (intermediate frequency) chain using the latest RF technology and microstrip design that we describe in the present article. Working at 600MHz central frequency, it consists of a pre-amplifier with an integral band-pass filter followed by an amplifier with digitally controlled gain and a frequency converter to zero-IF that feeds the ADC of a four channel digital correlator (outside the scope of the present article). This paper focuses the design options and constraints and presents the simulations and experimental results of a circuit prototype.
We investigate the structure and stability of hypercritical accretion flows around stellar-mass black holes, taking into account neutrino cooling, lepton conservation, and firstly a realistic equation of state in order to properly treat the dissociation of nuclei. We obtain the radial distributions of physical properties, such as density, temperature and electron fraction, for various mass accretion rates $0.1\sim 10M_{\odot}{\rm s}^{-1}$. We find that, depending on mass accretion rates, different physics affect considerably the structure of the disk; most important physics is (1) the photodissociation of nuclei around $r\sim 100r_g$ for relatively low mass accretion rates ($\dot{M}\sim 0.01-0.1M_{\odot} {\rm s}^{-1}$), (2) efficient neutrino cooling around $r\sim 10-100r_g$ for moderately high mass accretion rate ($\dot{M}\sim 0.2-1.0M_{\odot}{\rm s}^{-1}$), and (3) neutrino trapping ($r\sim 3-10r_g$) for very high mass accretion rate ($\dot{M}\gtrsim 2.0M_{\odot}{\rm s}^{-1}$). We also investigate the stability of hypercritical accretion flows by drawing the thermal equilibrium curves, and find that efficient neutrino cooling makes the accretion flows rather stable against both thermal and viscous modes.
We present optical lightcurves of 428 periodic variable stars coincident with ROSAT X-ray sources, detected using the first run of the SuperWASP photometric survey. Only 68 of these were previously recognised as periodic variables. A further 30 of these objects are previously known pre-main sequence stars, for which we detect a modulation period for the first time. Amongst the newly identified periodic variables, many appear to be close eclipsing binaries, their X-ray emission is presumably the result of RS CVn type behaviour. Others are probably BY Dra stars, pre-main sequence stars and other rapid rotators displaying enhanced coronal activity. A number of previously catalogued pulsating variables (RR Lyr stars and Cepheids) coincident with X-ray sources are also seen, but we show that these are likely to be misclassifications. We identify four objects which are probable low mass eclipsing binary stars, based on their very red colour and light curve morphology.
The observation of photons with energies above 10^18 eV would open a new window in cosmic-ray research, with possible impact on astrophysics, particle physics, cosmology and fundamental physics. Current and planned air shower experiments, particularly the Pierre Auger Observatory, offer an unprecedented opportunity to search for such photons and to complement efforts of multimessenger observations of the universe. We summarize motivation, achievements, and prospects of the search for ultra-high energy photons.
We examine the Swift/X-ray Telescope (XRT) light curves from the first ~150 gamma-ray burst (GRB) afterglows. Although we expected to find jet breaks at typical times of 1-2 days after the GRB, we find that these appear to be extremely rare. Typical light curves have a break in the slope at about 10^4s, followed by a single power-law decay whose slope is much too shallow to be consistent with expectations for jet breaks. X-ray light curves typically extend out to ~10 days without any further breaks, until they become too faint for the XRT to detect. In some extreme cases, light curves extend out to more than two months without evidence for jet breaks. This raises concerns about our understanding of afterglow and jet dynamics, and of GRB energetics.
The rotation-activity connection explains stellar activity in terms of rotation and convective overturn time. It is well established in stars of spectral types F--K and in M-type stars of young clusters. It is not established in field M-dwarfs, because they rotate very slowly, and detecting rotation periods or rotational line broadening is a challenge. In field M-dwarfs, saturation sets in below v_rot = 5 km/s, hence they are expected to populate the non-saturated part of the rotation-activity connection. This work for the first time shows intrinsically resolved spectral lines of slowly rotating M-dwarfs and makes a first comparison to estimates of convective velocities. By measuring rotation velocities in a sample of mostly inactive M-dwarfs, the unsaturated part of the rotation-activity connection is followed into the regime of very low activity. Spectra of ten M-dwarfs are taken at a resolving power of R = 200 000 in the molecular FeH absorption band. The intrinsically narrow lines are compared to model calculations and rotational broadening is measured. In one star, an upper limit of vsini = 1 km/s was found, significant rotation was detected in the other nine objects. All inactive stars show rotation below or equal to 2 km/s. In the two active stars AD Leo and YZ CMi, rotation velocities are found to be 40-50% below the results from earlier studies. The rotation activity connection holds in field early-M stars, too. Activity and rotation velocities of the sample stars are well in agreement with the relation found in earlier and younger stars. The intrinsic absorption profiles of molecular FeH lines are consistent with calculations from atomic Fe lines. Investigation of FeH line profiles is a very promising tool to measure convection patterns at the surfaces of M-stars.
In the course of our survey of the outer halo of the Andromeda Galaxy we have discovered a remote, possible satellite of that system at a projected 162 kpc (11.7 degrees) radius. If mass follows light, the fairly elongated (0.31 +/- 0.09 ellipticity) dwarf has an estimated 3.3 (+3.1,-1.8) 10^7 Solar masses (d/784 kpc) mass, M/L of 165 (+156/-92} M/L_solar (d/784 kpc), and 1.07 kpc (d/784 kpc) limiting radius, where the satellite distance, d, is estimated at 630-850 kpc from the tip of the red giant branch. The newfound galaxy, ``Andromeda XIV'' (``AndXIV''), distinguishes itself from other Local Group galaxies by its extreme dynamics: Keck/DEIMOS spectroscopy reveals it to have a large heliocentric radial velocity (-478 km/s), or -203 km/s velocity relative to M31. Even at its projected radius AndXIV already is at the M31 escape velocity based on the latest M31 mass models. If AndXIV is bound to M31, then recent models with reduced M31 virial masses need revision upward. If not bound to M31, then AndXIV is just now falling into the Local Group for the first time and represents a dwarf galaxy that formed and spent almost its entire life in isolation.
We discuss causes of the formation of the observed kinematics and morphology of cones of ionized matter in the neighborhood of the nuclei of Seyfert galaxies. The results of linear stability analysis of an optically thin conic jet where radiation cooling and gravity play an important part are reported. The allowance for radiation cooling is shown to result in strong damping of all acoustic modes and to have insignificant effect on unstable surface Kelvin--Helmholtz modes. In the case of waveguide--resonance internal gravity modes radiative cooling suppresses completely the instability of waves propagating away from the ejection source and, vice versa, reduces substantially the growth time scale of unstable sourceward propagating modes. The results obtained can be used to study ionization cones in Seyfert galaxies with radio jets. In particular, our analysis shows that surface Kelvin--Helmholtz modes and volume harmonics are capable of producing regular features observed in optical emission-line images of such galaxies.
Despite significant strides made towards understanding accretion, outflow, and emission processes in the Galactic Center supermassive black hole Sagittarius A*, the presence of jets has neither been rejected nor proven. We investigate here whether the combined spectral and morphological properties of the source at radio through near infrared wavelengths are consistent with the predictions for inhomogeneous jets. In particular, we construct images of jets at a wavelength of 7mm based on models that are consistent with the spectrum of Sgr A*. We then compare these models through closure quantities with data obtained from the Very Long Baseline Array at 7mm. We find that the best-fit jet models give comparable or better fits than best-fit Gaussian models for the intrinsic source found in previous analyses. The best fitting jet models are bipolar, are highly inclined to the line of sight ($\theta \ge$ 75 degrees), may favor a position angle on the sky of 105 degrees, and have compact bases with sizes of a few gravitational radii.
The early, highly time-variable X-ray emission immediately following GRBs exhibits strong spectral variations that are unlike the temporally smoother emission which dominates after $t\sim 10^3$s. The ratio of hard channel (1.3-10.0 keV) to soft channel (0.3-1.3 keV) counts in the Swift X-ray telescope provides a new measure delineating the end time of this emission. We define $T_{H}$ as the time at which this transition takes place and measure for 59 events a range of transition times that span $10^2$s to $10^{4}$s, on average 5 times longer than the prompt $T_{90}$ duration observed in the Gamma-rays. Using the transition time $T_{H}$ as the delineation between the GRB and afterglow emission, we calculate that the kinetic energy in the afterglow shock is typically a factor of 10 lower than that released in the GRB. This worsens an efficiency problem for the conversion of shock kinetic energy into Gamma-ray's in the fireball model, unless the afterglow is driven not by the GRB but by a broadly-beamed, late-time or low Lorentz factor injection of energy on the order of the energy contained in the GRB. Furthermore, from a typically unchanging hardness ratio after $T_H$, we infer that the mysterious light curve plateau phase is produced by a mechanism separate from that which produced the earlier emission. We favor energy injection scenarios with a linearly increasing input energy versus time for six well sampled events with nearly flat light curves at $t\approx 10^3-10^4$s. There are a handful of cases of very late time $t>10^4$s hardness evolution, which may point to residual central engine activity at very late time.
In Part I of this paper we described an equilibrium model of a jet in the gravitational field corresponding to the rigid-rotation region of the galactic disk. We used linear stability analysis to find the waveguide-resonance instability of internal gravity waves due to the superreflection of these waves from the jet boundary. In this part of the paper, we perform nonlinear numerical 2D and 3D simulations of the development of this instability. We show that the shocks produced by this instability in the ambient medium of the jet are localized inside a cone with a large opening angle and are capable of producing features that are morphologically similar to those observed in galaxies with active nuclei (NGC 5252 for example).
We analyzed deep Chandra observations of the Galactic plane region centered at l=28.55, b=-0.03 with the aim to obtain the best possible constraints on the contribution of weak point sources to the Galactic ridge X-ray emission (GRXE) in this region. We demonstrate that the vast majority of the detected sources are Galactic in origin and are probably cataclysmic variables and coronally active stars. We use the number-flux function of detected sources to constrain the luminosity function of Galactic X-ray sources in the range 1e30--1e32 erg/s and find good agreement with the luminosity function of sources in the Solar vicinity. The fraction of the total flux at energies 1--7 keV resolved into point sources at the current sensitivity level is ~25%. Excluding the expected contribution of extragalactic sources, ~19% of the GRXE is due to point Galactic sources with interstellar absorption corrected fluxes higher than 1.2e-15 erg/s/cm^2in the energy band 1-7 keV.
We calculate the linear momentum flux from merging black holes (BHs) with arbitrary masses and spin orientations, using the effective-one-body (EOB) model. This model includes an analytic description of the inspiral phase, a short merger, and a superposition of exponentially damped quasi-normal ringdown modes of a Kerr BH. By varying the matching point between inspiral and ringdown, we can estimate the systematic errors generated with this method. Within these confidence limits, we find close agreement with previously reported results from numerical relativity. Using a Monte Carlo implementation of the EOB model, we are able to sample a large volume of BH parameter space and estimate the distribution of recoil velocities. For a range of mass ratios 1 <= m_1/m_2 <= 10, spin magnitudes of a_{1,2}=0.9, and uniform random spin orientations, we find that a fraction f_{500}=0.12^{+0.06}_{-0.05} of binaries have recoil velocities greater than 500 km/s and f_{1000}=0.027^{+0.021}_{-0.014} have kicks greater than 1000 km/s. These velocities likely are capable of ejecting the final BH from its host galaxy. Limiting the sample to comparable-mass binaries with m_1/m_2 <= 4, the typical kicks are even larger, with f_{500}=0.31_{-0.12}^{+0.13} and f_{1000}=0.079^{+0.062}_{-0.042}.
We illustrate the constraints that a possible detection of a non-trivial spatial topology may place on the cosmological density parameters by considering the $\Lambda$CDM model Poincar\'e dodecahedal space (PDS) topology as a circles-in-the-sky detectable topology. To this end we reanalyze the type Ia supernovae constraints on the density parameter plane $\Omega_k$--$\Omega_{\Lambda}$ and show that a circles-in-the-sky detectable PDS topology gives rise to important constraints on this parameters plane.
We combine newly measured rotation velocities, velocity dispersions, and stellar masses to construct stellar mass Tully-Fisher relations (M*TFRs) for 544 galaxies with strong emission lines at 0.1<z<1.2 from the All Wavelength Extended Groth Strip International Survey (AEGIS) and the Deep Extragalactic Evolutionary Probe 2 Survey (DEEP2). The conventional M*TFR using only rotation velocity (Vrot) shows large scatter (~1.5 dex in velocity). The scatter and residuals are correlated with morphology in the sense that disturbed, compact, and major merger galaxies have lower velocities for their masses. We construct an M*TFR using the kinematic estimator S_0.5 which is defined as sqrt(0.5Vrot^2 + sigma_g^2) and accounts for disordered or non-circular motions through the gas velocity dispersion (sigma_g). The new M*TFR, termed S_0.5/M*TFR, is remarkably tight over 0.1<z<1.2 with no detectable evolution of its intercept or slope with redshift. The average best fit relation has 0.47 dex scatter in stellar mass, corresponding to ~1.2 `magnitudes,' assuming a constant mass-to-light ratio. Interestingly, the S_0.5/M*TFR is consistent with the absorption-line based stellar mass Faber-Jackson relation for nearby elliptical galaxies in terms of slope and intercept, which might suggest a physical connection between the two relations.
We describe results from a new ground-based monitoring campaign on NGC 5548, the best studied reverberation-mapped AGN. We find that it was in the lowest luminosity state yet recorded during a monitoring program, namely L(5100) = 4.7 x 10^42 ergs s^-1. We determine a rest-frame time lag between flux variations in the continuum and the Hbeta line of 6.3 (+2.6/-2.3) days. Combining our measurements with those of previous campaigns, we determine a weighted black hole mass of M_BH = 6.54 (+0.26/-0.25) x 10^7 M_sun based on all broad emission lines with suitable variability data. We confirm the previously-discovered virial relationship between the time lag of emission lines relative to the continuum and the width of the emission lines in NGC 5548, which is the expected signature of a gravity-dominated broad-line region. Using this lowest luminosity state, we extend the range of the relationship between the luminosity and the time lag in NGC 5548 and measure a slope that is consistent with alpha = 0.5, the naive expectation for the broad line region for an assumed form of r ~ L^alpha. This value is also consistent with the slope recently determined by Bentz et al. for the population of reverberation-mapped AGNs as a whole.
This paper presents new images and spectroscopy of NGC 34 (Mrk 938) obtained with the du Pont 2.5-m and Baade 6.5-m telescopes at Las Campanas, plus photometry of an HST archival V image. This Mv = -21.6 galaxy has often been classified as a Seyfert 2, yet recently published infrared spectra suggest a dominant central starburst. We find that the galaxy features a single nucleus, a main spheroid containing a blue central disk, and tidal tails indicative of two former disk galaxies. These galaxies appear to have completed merging. The remnant shows three clear optical signs that the merger was gas-rich ("wet") and accompanied by a starburst: (1) It sports a rich system of young star clusters, of which 87 have absolute magnitudes -10.0 > Mv > -15.4. Five clusters with available spectra have ages in the range 0.1-1.0 Gyr, photometric masses between 2x10^6 and 2x10^7 Msun, and are gravitationally bound young globulars. (2) The blue central disk appears to be young. It is exponential, can be traced to >10 kpc radius, and has a smooth structure and colors suggest- ing a dominant, ~400 Myr old poststarburst population. And (3), the center of NGC 34 drives a strong outflow of cool, neutral gas, as revealed by broad blueshifted Na I D lines. The mean outflow velocity of this gas is -620 km/s, while the maximum velocity reaches -1050 km/s. We suggest that NGC 34 stems from two recently merged gas-rich disk galaxies with an estimated mass ratio between 1/3 and 2/3. The remnant seems to have first experienced a galaxy-wide starburst that then shrank to its current central and obscured state. The strong gaseous outflow came last. (Abridged)
As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [absolute] magnitudes of (M475, M814) > (29.0 [+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the Big Bang; a possible burst at the oldest ages cannot be claimed with high confidence. The peak SFR occurred ~1.5-4 Gyr ago, at a level 5-10 times the current value. Our modelling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which ALL of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to ~8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.
We report the discovery of a very low mass binary system (primary mass <0.1 Msol) with a projected separation of ~5100 AU, more than twice that of the widest previously known system. A spectrum covering the 1-2.5 microns wavelength interval at R ~1700 is presented for each component. Analysis of the spectra indicates spectral types of M6.5V and M8V, and the photometric distance of the system is ~62 pc. Given that previous studies have established that no more than 1% of very low mass binary systems have orbits larger than 20 AU, the existence of such a wide system has a bearing on very low mass star formation and evolution models.
We show that the existence of cosmic strings can be strongly constrained by the next generation of gravitational lensing surveys at radio frequencies. We focus on cosmic string loops, which we expect to be far more numerous than long (horizon-sized) strings, as suggested by simulations. Using simple models of the loop population and minimal assumptions about the lensing cross-section per loop, we estimate the optical depth to lensing and show that extant radio surveys such as CLASS have already ruled out a portion of the cosmic string model parameter space. Future radio interferometers, such as LOFAR and especially SKA, may constrain $G\mu/c^2 < 10^{-9}$ in some regions of parameter space, outperforming current constraints from pulsar timing and the CMB by up to two orders of magnitude. A fundamental advantage of this approach is that it is based on direct detections of cosmic strings; whereas lensing requires only that the loop have a mass, other constraints must rely on estimates of theoretical uncertainties such as the gravitational wave emission of loops or the population dynamics of string networks. This difference is essential since the properties of cosmic strings are essentially unknown.
We present and compare projected distributions of mass, galaxies, and the intracluster medium (ICM) for a sample of merging clusters of galaxies based on the joint weak-lensing, optical, and X-ray analysis. Our sample comprises seven nearby Abell clusters ($0.0542 \le z \le 0.279$), for which we have conducted systematic, deep imaging observations with Suprime-Cam on Subaru telescope. Our seven target clusters, representing various merging stages and conditions, allow us to investigate in details the physical interplay between dark matter, ICM, and galaxies associated with cluster formation and evolution. A1750 and A1758 are binary systems consisting of two cluster-sized components, A520, A754, A1758, and A1914 are on-going cluster mergers, and A2034 and A2142 are cold-front clusters. In the binary clusters, which are presumably in an early phase of mergers, the projected mass, optical light, and X-ray distributions are overall similar and regular without significant substructures. On-going merging clusters, on the other hand, reveal highly irregular mass distributions. Overall the mass distribution appears to be similar to the galaxy luminosity distribution, whereas their distributions are quite different from the ICM distribution in a various ways. The cold front clusters also reveal irregular mass structures that differ from ICM structures. In particular, all of three cold fronts are found to be associated with a mass substructure located ahead of the moving cold front, as found in the bullet cluster 1E0657-56.
We have measured the host-galaxy starlight contribution to four lower-luminosity AGNs (NGC 3516, NGC 4593, IC 4329A, and NGC 7469). We include these objects with new broad line region measurements for NGC 4151 and NGC 4593 to present a revised version of the radius-luminosity relationship for AGNs.
We present results from a combined weak lensing and X-ray analysis of the
merging cluster A1914 at a redshift of $z=0.1712$ based on $R_{\rm}$-band
imaging data with Subaru/Suprime-Cam and archival Chandra X-ray data. Based on
the weak-lensing and X-ray data we explore the relationships between cluster
global properties, namely the gravitational mass, the bolometric X-lay
luminosity and temperature, the gas mass and the gas mass fraction, as a
function of radius. We found that the gas mass fractions within $r_{2500}$ and
$r_{{\rm vir}}$ are consistent with the results of earlier X-ray cluster
studies and of cosmic microwave background studies based on the WMAP
observations, respectively. However, the observed temperature, $k_B T_{\rm
ave}=9.6\pm0.3 {\rm keV}$, is significantly higher than the virial temperature,
$k_B T_{\rm vir}=4.7\pm0.3 {\rm keV}$ derived from the weak lensing distortion
measurement. The X-ray bolometric luminosity and temperature
($L_X-T$) relation is consistent with the $L_X-T$ relation derived by
previous statistical X-ray studies of galaxy clusters. Such correlations among
the global cluster properties are invaluable observational tools for studying
the cluster merger physics. Our results demonstrate that the combination of
X-ray and weak-lensing observations is a promising, powerful probe of the
physical processes associated with cluster mergers as well as of their mass
properties.
It has been established that Gamma-Ray Bursts (GRBs) are connected to Supernovae (SNe) explosions of Type Ib/c. We intend to test whether the hypothesis of Type Ib/c SNe from different massive progenitors can reproduce the local GRB rate as well as the GRB rate as a function of redshift. We aim to predict the GRB rate at very high redshift under different assumptions about galaxy formation and star formation histories in galaxies. We assume different star formation histories in galaxies of different morphological type: ellipticals, spirals and irregulars. We explore different hypotheses concerning the progenitors of Type Ib/c SNe. We find an excellent agreement between the observed GRB local rate and the predicted Type Ib/c SN rate in irregular galaxies, when a range for single Wolf-Rayet stars of 40-100 M_sun is adopted. We also predict the cosmic Type Ib/c SN rate by taking into account all the galaxy types in an unitary volume of the Universe and we compare it with the observed cosmic GRB rate as a function of redshift. By assuming the formation of spheroids at high redshift, we predict a cosmic Type Ib/c SN rate, which is always higher than the GRB rate, suggesting that only a small fraction (0.1-1 %) of Type Ib/c SNe become GRBs. In particular, we find a ratio between the cosmic GRB rate and the cosmic Type Ib/c rate in the range 0.001-0.01, in agreement with previous estimates. Finally, due to the high star formation in spheroids at high redshift, which is our preferred scenario for galaxy formation, we predict more GRBs at high redshift than in the hierarchical scenario for galaxy formation, a prediction which awaits to be proven by future observations.
A semi-analytic approach to the computation of the non-linear power-spectrum of dark matter density fluctuations is proposed. The method is based on the Renormalization Group technique and can be applied to any underlying cosmological model. Our prediction on the baryonic acoustic oscillations in a LambdaCDM model accurately fits the results of N-body simulations down to zero redshift, where perturbation theory fails.
We show that a star orbiting close enough to an adiabatically grown supermassive black hole (SMBH) can capture weakly interacting massive particles (WIMPs) at an extremely high rate. The stellar luminosity due to annihilation of captured WIMPs in the stellar core may be comparable to or even exceed the luminosity of the star due to thermonuclear burning. The model thus predicts the existence of unusual stars, essentially WIMP burners, in the vicinity of a SMBH. We find that the most efficient WIMP burners are stars with degenerate electron cores, e.g. white dwarfs (WDs); such WDs may have a very high surface temperature. If found, such stars would provide evidence for the existence of particle dark matter and can possibly be used to establish its density profile. On the other hand, the lack of such unusual stars may provide constraints on the WIMP density near the SMBH, as well as the WIMP-nucleus scattering and pair annihilation cross-sections.
We have determined magnesium isotopic ratios (^{25,26}Mg/Mg) in metal-poor (-2.6 [Fe/H] -1.3) halo dwarfs employing high S/N (90-280) high spectral resolution (R = 10^5) Keck HIRES spectra. Unlike previous claims of an important contribution from intermediate-mass AGB stars at low metallicities, we find that the rise of the AGB contribution in the Galactic halo did not occur until intermediate metallicities ([Fe/H] ~> -1.5).
Generally accepted scheme distinguishes two main classes of supernovae (SNe):
Ia resulting from the old stellar population (deflagration of a white dwarf in
close binary systems), and SNe of type II and Ib/c whose ancestors are young
massive stars (died in a core-collapse explosion). Concerning the latter, there
are suggestions that the SNe II are connected to early B stars, and SNe Ib/c to
isolated O or Wolf-Rayet (W-R) stars. However, little or no effort was made to
further separate SNe Ib from Ic. We have used assumed SN rates for different SN
types in spiral galaxies in an attempt to perform this task. If isolated
progenitor hypothesis is correct, our analysis indicates that SNe Ib result
from stars of main-sequence mass $23 \mathcal{M}_{\odot} \lesssim \mathcal{M}
\lesssim 30 \mathcal{M}_{\odot}$, while the progenitors of SNe Ic are more
massive stars with $\mathcal{M} \gtrsim 30 \mathcal{M}_{\odot}$.
Alternatively, if the majority of SNe Ib/c appear in close binary systems
(CBs) then they would result from the same progenitor population as most of the
SNe II, i.e. early B stars with initial masses of order $\mathcal{M} \sim 10
\mathcal{M}_{\odot}$. Future observations of SNe at high-redshift ($z$) and
their rate will provide us with unique information on SN progenitors and
star-formation history of galaxies. At higher-$z$ (deeper in the cosmic past)
we expect to see the lack of type Ia events, i.e. the dominance of
core-collapse SNe. Better understanding of the stripped-envelope SNe (Ib/c),
and their potential use as distance indicators at high-$z$, would therefore be
of great practical importance.
The main aim of the paper was performing test of our (chemical and kinetic)
codes, which will be used during self-consistent modelling of dynamics and
chemistry in the winds from C-rich AGB stars. We use the thermodynamical
equilibrium code to test the different databases of dissociation constants. We
also calculate the equilibrium content of the gas using the kinetic code, which
includes the chemical network of neutral--neutral reactions studied by Willacy
& Cherchneff (1998). The influence of reaction rates updated using the UMIST
database for Astrochemistry 2005 (UDFA05), was tested.
The local thermodynamical equilibrium calculations have shown that the NIST
database reproduces fairly well equilibrium concentrations of Willacy &
Cherchneff (1998), while agreement in case of Tsuji (1973) dissociation
constants is much worse. The most important finding is that the steady state
solution obtained with the kinetic code for reaction network of Willacy &
Cherchneff (1998) is different from the thermodynamical equilibrium solution.
In particular, CN and C2, which are important opacity sources are underabundant
relative to hermodynamical equilibrium, while O-bearing molecules (like SiO,
H2O, and OH) are overabundant. After updating the reaction rates by data from
the UDFA05 database consistency in O-bearing species becomes much better,
however the disagreement in C-bearing species is still present.
BAT99-129 is a massive eclipsing binary system in the Large Magellanic Cloud
(LMC) which consists of WN3(h) and O5V components. A broad-band MACHO light
curve of the system is studied in the present paper. A dense and extended
atmosphere of the Wolf-Rayet (WR) star does not allow one to analyze the light
curve in terms of standard parametric models of Wilson-Devinney type.
Distributions of brightness and absorption across the WR star disk are restored
using direct solution of integral equations describing eclipses in the system.
As a result, reliable estimates of the orbital inclination and component
parameters have been obtained. The orbital inclination is 78 deg, the orbital
separation is 28.5 solar radii, the radius of the O component is 7.1 solar
radii. The size of the WR component core opaque in optical continuum is 3.4
solar radii. The brightness temperature in the center of the WR disk is about
45000K. Probable errors of the parameters are discussed. Evolutionary status of
the system is discussed.
The shear-current effect in a nonrotating homogeneous turbulent convection with a large-scale constant shear is studied. The large-scale velocity shear causes anisotropy of turbulent convection, which produces the mean electromotive force $\bec{\cal E}^{(W)} \propto {\bf W} {\bf \times} {\bf J}$ and the mean electric current along the original mean magnetic field, where ${\bf W}$ is the background mean vorticity due to the shear and ${\bf J}$ is the mean electric current. This results in a large-scale dynamo even in a nonrotating and nonhelical homogeneous sheared turbulent convection, whereby the $\alpha$ effect vanishes. It is found that turbulent convection promotes the shear-current dynamo instability, i.e., the heat flux causes positive contribution to the shear-current effect. However, there is no dynamo action due to the shear-current effect for small hydrodynamic and magnetic Reynolds numbers even in a turbulent convection, if the spatial scaling for the turbulent correlation time is $\tau(k) \propto k^{-2}$, where $k$ is the small-scale wave number.
We show that a quasi-spherical (QS) hot accretion flow is expected to operate in all SMBHs, with its rate being capped at dot m_QS,max~0.001 (M/1E8Msun) in units of the Eddington rate. It is then proposed that AGN jet power is proportional to the product of the hot accretion rate and a SMBH spin-dependent efficiency for energy extraction in the form of jets. Predictions from this model include (1) while radio jets should emerge from all SMBHs, the maximum jet power goes with SMBH mass approximately as 1E43.6(M/1E8Msun)^{2}erg/s, (2) even although there are two separate underlying populations of radio-quiet (RQ) and radio-loud (RL) AGNs, any bimodality in the observed AGN radio loudness distribution is likely due to a selection effect, such as some imposed optical magnitude limits, (3) the RL fraction of quasars is expected to decrease with redshift in the cold dark matter model, (4) host galaxies of RQ and RL quasars should be drawn from the same underlying elliptical galaxy population, although RL quasars may have SMBHs that are somewhat more massive than their RQ counterparts and RL quasars may reside predominantly in core elliptical galaxies, (5) RL low-luminosity AGNs and LINERs may represent the long, declining "trailing" phase following the initial, more luminous AGN phase, (6) a broad anti-correlation between radio- loudness and disk accretion rate is expected, (7) RL AGNs may be expected to be more abundant in Type Ia supernovae than RQ AGNs, (8) among RL AGNs a correlation between radio power and clustering strength is predicted, and (9) RQ and RL AGNs should have, on average, similar IR-optical-UV properties. ~
We assume that the helium-I lines emitted by the massive binary system Eta Carinae are formed in the acceleration zone of the less-massive secondary star. We calculate the Doppler shift of the lines as a function of orbital phase and of several parameters of the binary system. We find that a good fit is obtained if the helium lines are formed in the region where the secondary wind speed is 430 km/sec. The acceptable binary eccentricity is in the range 0.9-0.95, and the inclination angle (the angle between a line perpendicular to the orbital plane and the line of sight) is in the range 40-55 degrees. Lower values of e require higher values of i, and vice versa. The binary system is oriented such that the secondary star is in our direction (closer to us) during periastron passage. The orbital motion can account in part to the Doppler shift of the peak in X-ray emission.
We perform population synthesis calculations of present-day post common
envelope binaries (PCEBs) and zero-age cataclysmic variables (ZACVs) using a
common envelope efficiency parameter, alphaCE, that is a function of secondary
mass, Ms. We investigate three basic possibilities: (1) a standard constant
alphaCE prescription, with alphaCE = 1.0, 0.6, 0.3, 0.2, 0.1 and 0.05, to
provide a baseline for comparison, (2) a power law dependence, alphaCE =
(Ms)^n, with n = 0.5, 1.0 and 2.0, and (3) a dependence in which ?CE approaches
1 for large secondary masses and alphaCE = 0 below some assumed cutoff mass,
alphaCE = 1?Mcut/Ms, where Mcut is the cutoff mass and is equal to 0.0375,
0.075 and 0.15 solar mass. For each population, we compute orbital period,
orbital separation, white dwarf mass and secondary mass distributions.
We find that if alphaCE < 0.2 in our constant ?CE sequence, the predicted
present-day ZACV population is significantly modified compared with our
standard model (alphaCE = 1.0). In our variable alphaCE sequences for ZACVs, we
find that for models in which alphaCE decreases very rapidly for small
secondary masses, the orbital period distribution below the period gap differs
significantly from our standard model. These differences are most evident in
our power law sequence model with n = 2 and in our cutoff mass sequence model
with Mcut = 0.15 solar mass. In these two models, the fraction of CVs forming
with orbital periods below the gap is reduced significantly, the fraction
forming in the gap is increased significantly, and both the short-period peak
and the minimum period in the ZACV orbital period distribution shift to
considerably longer orbital periods compared with our standard model.
Metals in the photospheres of white dwarfs with Teff between 12,000 and 25,000 K should gravitationally settle out of these atmospheres in 1-2 weeks. Temporal variations in the line strengths of these metals could provide a direct measurement of episodic metal accretion. Using archival VLT and Keck spectroscopy, we find evidence that the DAZd white dwarf G29-38 shows significant changes in its Ca II K line strength. At the two best-observed epochs, we find that the Ca line equivalent width (EW) = 165 +- 4 mA (in 1996.885) and 280 +- 8 mA (in 1999.653), which is an increase of 70%. We consider the effect that pulsation has on the Ca EWs for this known variable star, and find that it adds an error of < 1 mA to these measurements. Calcium line strengths at other observational epochs support variations with timescales as short as two weeks. These Ca EW variations indicate that the metal accretion process in G29-38, presumably from its debris disk, is episodic on timescales of a few weeks or less, and thus the accretion is not dominated by Poynting-Robertson drag from an optically thin, continuous disk, which has a timescale of ~1 year.
We explore the evolution of the large-scale anisotropy in the velocity field caused by the gravitational pancaking effect assuming a LCDM universe. The Millennium Run halo catalogs at four different redshifts, z=0, 0.5, 1 and z=2 are analyzed to find that the pancaking effect starts to intervene the hierarchical structure formation at redshift z=2 when a characteristic pancake scale is around 3 Mpc/h. It is also clearly shown how the degree and scale of the pancaking effect changes with time. An analytic model based on the Zel'dovich approximation is presented to explain quantitatively the evolution of the velocity-pancake alignment. A cosmological implication of our finding and a possibility of detecting a signal in real universe are discussed.
Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO+ observations of the galactic star-forming region NGC6334 FIR II are presented, complemented by [C I] 3^P_1--3^P_0 and 3^P_2--3^P_1 data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H II region NGC6334--D, there is a molecular "void". [C I] correlates well with 13^CO and other molecular lines and shows no rim brightening relative to molecular cloud regions farther off the void. While an interpretation in terms of a highly clumped cloud morphology is possible, with photon dominated regions (PDRs) reaching deep into the cloud, the data do not provide any direct evidence for a close association of [C I] with PDRs. Kinetic temperatures are ~40--50K in the molecular cloud and >=200K toward the void. CO and [C I] excitation temperatures are similar. A comparison of molecular and atomic fine structure line emission with the far infrared and radio continuum as well as the distribution of 2.2um H_2 emission indicates that the well-evolved H II region expands into a medium that is homogeneous on pc-scales. If the H_2 emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, "D-type") and the associated shock front farther out (traced by H_2) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of H II regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C18^O/C17^O abundance ratio of 4.13+-0.13 that reflects the 18^O/17^O isotope ratio. This ratio is consistent with values determined in nearby clouds. Right at the edge of the void, however, the oxygen isotope ratio might be smaller.
We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere. To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux. We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of $10^{19}$ Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux.
We calculate isotope mass shift for several light ions using Dirac wave functions and mass shift operator with relativistic corrections of the order of $(\alpha Z)^2$. Calculated relativistic corrections to the specific mass shift vary from a fraction of a percent for Carbon, to 2% for Magnesium. Relativistic corrections to the normal mass shift are typically smaller. Interestingly, the final relativistic mass shifts for the levels of one multiplet appear to be even closer than for non-relativistic operator. That can be important for the astrophysical search for possible $\alpha$-variation, where isotope shift is a source of important systematic error. Our calculations show that for levels of the same multiplet this systematics is negligible and they can be used as probes for $\alpha$-variation.
The Atacama Large Millimeter/submillimeter Array (ALMA) combines large collecting area and location on a high dry site to provide it with unparalleled potential for sensitive millimeter/submillimeter spectral line observations. Its wide frequency coverage, superb receivers and flexible spectrometer will ensure that its potential is met. Since the 1999 meeting on ALMA Science\cite{RefA}, the ALMA team has substantially enhanced its capability for line observations. ALMA's sensitivity increased when Japan joined the project, bringing the 16 antennas of the Atacama Compcat Array (ACA), equivalent to eight additional 12m telescopes. The first four receiver cartridges for the baseline ALMA (Japan's entry has brought two additional bands to ALMA's receiver retinue) have been accepted, with performance above the already-challenging specifications. ALMA's flexibility has increased with the enhancement of the baseline correlator with additional channels and flexibility, and with the addition of a separate correlator for the ACA. As an example of the increased flexibility, ALMA is now capable of multi-spectral-region and multi-resolution modes. With the former, one might observe e.g. four separate transitions anywhere within a 2 GHz band with a high resolution bandwidth. With the latter, one might simultaneously observe with low spectral resolution over a wide bandwidth and with high spectral resolution over a narrow bandwidth; this mode could be useful for observations of pressure-broadened lines with narrow cores, for example. Several science examples illustrate ALMA's potential for transforming millimeter and submillimeter astronomy.
We report the detection of the CO 4-3, 6-5, 9-8, 10-9, and 11-10 lines in the Broad Absorption Line quasar APM08279+5255 at z=3.9 using the IRAM 30m telescope. We also present IRAM PdBI high spatial resolution observations of the CO 4-3 and 9-8 lines, and of the 1.4mm dust radiation as well as an improved spectrum of the HCN(5-4) line. Unlike CO in other QSO host galaxies, the CO line SED of APM08279+5255 rises up to the CO(10-9) transition. The line fluxes in the CO ladder and the dust continuum fluxes are best fit by a two component model, a "cold" component at ~65K with a high density of n(H2)= 1x10^5 cm^-3, and a "warm", ~220K component with a density of 1x10^4 cm^-3. We show that IR pumping via the 14 micron bending mode of HCN is the most likely channel for the HCN excitation. From our models we find, that the CO(1-0) emission is dominated by the {\it dense} gas component which implies that the CO conversion factor is higher than usually assumed for high-z galaxies with alpha~5 SM/(K km/s pc^2). Using brightness temperature arguments, the results from our high-resolution mapping, and lens models from the literature, we argue that the molecular lines and the dust continuum emission arise from a very compact (r~100-300 pc), highly gravitationally magnified (m= 60-110) region surrounding the central AGN. Part of the difference relative to other high-$z$ QSOs may therefore be due to the configuration of the gravitational lens, which gives us a high-magnification zoom right into the central 200-pc radius of APM08279+5255 where IR pumping plays a significant role for the excitation of the molecular lines.
We show that the assumption of a flat universe induces critically large errors in reconstructing the dark energy equation of state at z>~0.9 even if the true cosmic curvature is very small, O(1%) or less. The spuriously reconstructed w(z) shows a range of unusual behaviour, including crossing of the phantom divide and mimicking of standard tracking quintessence models. For 1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching (50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble rate measurements have opposite trends due to the asymmetric influence of the curved geodesics. These results show that including curvature as a free parameter is imperative in any future analyses attempting to pin down the dynamics of dark energy, especially at moderate or high redshifts.
The possibility of the existence of quark stars has been discussed by several authors since 1970. Recently, it has been pointed out that two putative neutron stars, RXJ 1856.5 - 3754 in Corona Australis and 3C58 in Cassiopeia are too small and too dense to be neutron stars; they show evidence of being quark stars. Apart from these two objects, there are several other compact objects which fit neither in the category of neutron stars nor in that of black holes. It has been suggested that they may be quark stars.In this paper it is shown that a black hole cannot collapse to a singularity, instead it may end up as a quark star. In this context it is shown that a gravitationally collapsing black hole acts as an ultrahigh energy particle accelerator, hitherto inconceivable in any terrestrial laboratory, that continually accelerates particles comprising the matter in the black hole. When the energy \textit{E} of the particles in the black hole is $\geq 10^{2}$GeV, or equivalently the temperature \textit{T} of the matter in the black holes is $\geq 10^{15}$K, the entire matter in the black hole will be converted into quark-gluon plasma permeated by leptons. Since quarks and leptons are spin 1/2 particles,they are governed by Pauli's exclusion principle. Consequently, one of the two possibilities will occur; either Pauli's exclusion principle would be violated and the black hole would collapse to a singularity, or the collapse of the black hole to a singularity would be inhibited by Pauli's exclusion principle, and the black hole would eventually explode with a mini bang of a sort. After explosion, the remnant core would stabilize as a quark star.
In this paper we study the object SDSS J1130+0058 which is the only AGN known to have both double-peaked low-ionization broad emission lines, and also X-shaped radio structures. Emission from an accretion disk can reproduce the double-peaked line profile of broad H$\alpha$, but not the radio structure. Under the accretion disk model, the period of the inner emission line region is about 230 years. Using a new method to subtract the stellar component from the data of the SDSS DR4, we obtain an internal reddening factor which is less than previously found. The implied smaller amount of dust disfavors the backflow model for the X-shaped radio structure. The presence of a Binary Black Hole (BBH) system is the most natural way to explain ${\it both}$ the optical and radio properties of this AGN. Under the assumption of the BBH model, we can estimate the BBH system has a separation of less than 0.04 pc with a period less than 59 years, this may pose some problem to the BLRs sizes, still we conclude that the BBH model is favored on the basis of the present limited information.
We report the results of a blind search for 22-GHz water masers in two regions, covering approximately half a square degree, within the giant molecular cloud associated with RCW 106. The complete search of the two regions was carried out with the 26-m Mount Pleasant radio telescope and resulted in the detection of nine water masers, five of which are new detections. Australia Telescope Compact Array (ATCA) observations of these detections have allowed us to obtain positions with arcsecond accuracy, allowing meaningful comparison with infrared and molecular data for the region. We find that for the regions surveyed there are more water masers than either 6.7-GHz methanol, or main-line OH masers. The water masers are concentrated towards the central axis of the star formation region, in contrast to the 6.7-GHz methanol masers which tend to be located near the periphery. The colours of the GLIMPSE point sources associated with the water masers are similar to those of 6.7-GHz methanol masers, but slightly less red. We have made a statistical investigation of the properties of the 13CO and 1.2-mm dust clumps with and without associated water masers. We find that the water masers are associated with the more massive, denser and brighter 13CO and 1.2-mm dust clumps. We present statistical models that are able to predict those 13CO and 1.2-mm dust clumps that are likely to have associated water masers, with a low misclassification rate.
If highly efficient, cosmic ray production can have a significant effect on the X-ray emission from SNRs as well as their dynamical evolution. Using hydrodynamical simulations including diffusive shock acceleration, we produce spectra for both the thermal and nonthermal forward shock emission. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow vs. rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a considerable flattening of the nonthermal spectrum in the soft X-ray band. Spectral index measurements from X-ray observations may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse-Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions and, for the particular parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.
Time-series CCD photometric observations of the intermediate-age open cluster NGC 2099 were performed to search for variable stars. We also carried out BV photometry to study physical properties of variables in the cluster. Using V-band time-series data, we carefully examined light variations of about 12,000 stars in the range of 10 < V < 22 mag. A total of 24 variable stars have been identified; seven stars are previously known variables and 17 stars are newly identified. On the basis of observational properties such as light curve shape, period, and amplitude, we classified the new variable stars as nine delta Scuti-type pulsating stars, seven eclipsing binaries, and one peculiar variable star. Judging from the position of delta Scuti-type stars in the color-magnitude diagram, only two stars are likely to have the cluster membership. One new variable KV10 shows peculiar light variations with a delta Scuti-type short period of about 0.044 day as well as a long period of 0.417 day.
The s-enhanced and very metal-poor star CS 30322-023 shows a puzzling abundance pattern of the neutron-capture elements, i.e. several neutron-capture elements such as Ba, Pb etc. show enhancement, but other neutron-capture elements such as Sr, Eu etc. exhibit deficient with respect to iron. The study to this sample star could make people gain a better understanding of s- and r-process nucleosynthesis at low metallicity. Using a parametric model, we find that the abundance pattern of the neutron-capture elements could be best explained by a star that was polluted by an AGB star and the CS 30322-023 binary system formed in a molecular cloud which had never been polluted by r-process material. The lack of r-process material also indicates that the AGB companion cannot have undergone a type-1.5 supernova, and thus must have had an initial mass below 4.0M$_\odot$, while the strong N overabundance and the absence of a strong C overabundance indicate that the companion's initial mass was larger than 2.0M$_\odot$. The smaller s-process component coefficient of this star illustrates that there is less accreted material of this star from the AGB companion, and the sample star should be formed in the binary system with larger initial orbital separation where the accretion-induced collapse (AIC) mechanism can not work.
The primary nature of the $^{13}$C neutron source is very significant for the studies of the s-process nucleosynthesis. In this paper we present an attempt to fit the element abundances observed in 16 s-rich stars using parametric model of the single neutron exposure. The calculated results indicate that almost all s-elements were made in a single neutron exposure for 9 sample stars. Although a large spread of neutron exposure is obtained, the maximum value of the neutron exposure will reach about 7.0 mbarn$^{-1}$, which is close to the theoretical predictions by the AGB model. The calculated result is a significant evidence for the primary nature of the neutron source. Combining the result obtained in this work and the neutron exposure-initial mass relations, a large spread of neutron exposure can be explained by the different initial stellar mass and their time evolution. The possibility that the rotationally induced mixing process can lead to a spread of the neutron exposure in AGB stars is also existent.
Sub-arcsecond observations using the Very Large Array (VLA) are presented for low-luminosity water maser in M82, M51, and NGC4051. New maser features have been detected within the M82 starburst complex. They are largely associated with star-forming activity, such as optically identified starburst-driven winds, H II regions, or the early phase of star formation in the galaxy. The water maser in M51 consists of blueshifted and redshifted features relative to thesystemic velocity of the galaxy. The redshifted features are measured to the northwest of the nuclear radio source, while the location of the blueshifted counterpart is displaced by about 2" from the radio source. A small velocity gradient closely aligned with the radio jet is detected from the redshifted features. The redshifted maser most likely amplifies the background radio continuum jet, while the blueshifted counterpart marks off-nuclear star formation in the galaxy. All of the detected maser features in the narrow-line Seyfert 1 galaxy NGC 4051 remain unresolved by new VLA observations. Due to the low luminosity of the maser, the maser excitation is not directly related to the active galactic nucleus.
We present an analysis of 3D spectra of Mrk 533, observed with the integral-field spectrograph MPFS and using the Fabry Perot Interferometer (FPI) of the SAO RAS 6-m telescope. We found emissions of gas from the active Sy 2 nucleus in the centre and also from the HII regions in a spiral structure and a circumnuclear region. The gas kinematics shows regular non-circular motions in the wide range of galactocentric distances from 500 pc up to 15 kpc. The maps of inward and outward radial motions of the ionized gas was constructed. We found that the narrow line region (NLR) is composed of at least two (probably three) kinematically separated regions. We detect a stratification in the NLR of Mrk 533 with the outflow velocity ranging from 20-50 km/s to 600-700 km/s, respectively, on the radial distances of ~2.5 and ~1.5 kpc. The maximal outflow velocity comes from the nucleus and corresponds to the position of the observed radio structure, which is assumed to be created in an approaching jet. We suggest that these ionized gas outflows are triggered by the radio jet intrusion in an ambient medium.
The undisputed galactic origin of cosmic rays at energies below the so-called knee implies an existence of a nonthemal population of galactic objects which effectively accelerate protons and nuclei to TeV-PeV energies. The distinct signatures of these cosmic accelerators are high energy neutrinos and gamma-rays produced through hadronic interactions. While gamma-rays can be produced also by directly accelerated electrons, high energy neutrinos provide the most straightforward and unambiguous information about the nucleonic component of accelerated particles. The planned km-cube class high energy neutrino detectors are expected to be sensitive enough to provide the first astrophysically meaningful probes of potential VHE neutrino sources. This optimistic prediction is based on the recent discovery of high energy gamma-ray sources with hard energy spectra extending to 10 TeV and beyond. Amongst the best-bet candidates are two young shell-type supernova remnants - RXJ1713.7-4946 and RXJ0852.0-4622, and perhaps also two prominent plerions - the Crab Nebula and Vela X. Because of strong absorption of TeV gamma-rays, one may expect detectable neutrino fluxes also from (somewhat fainter) compact TeV gamma-ray emitters like the binary systems LS5039 and LS I+61~303, and, hopefully, from hypothetical "hidden" or "orphan" neutrino sources.
Context: Observations indicate that the `quiet' solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear. Aims: We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux. Methods: We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary. Results: Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G. Conclusions: A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface.
We explore the epoch dependence of number density and star-formation rate for submm galaxies found at 850 um. The study uses a sample of 38 submm galaxies in the GOODS-N field, for which cross-waveband identifications have been obtained for 35/38 members together with redshift measurements or estimates. A maximum-likelihood analysis is employed, along with the `single-source-survey' technique. We find a highly significant diminution in both space density and star formation rate at z > 3, closely mimicking the redshift cut-offs found for QSOs selected in different wavebands. The data further indicate that two separately-evolving populations are present, with distinct luminosity functions. These results parallel the different evolutionary behaviours of LIRGs and ULIRGs, and represent another instance of `cosmic down-sizing'.
The results of hydrodynamic simulations of the Virgo and Perseus clusters suggest that thermal conduction is not responsible for the observed temperature and density profiles. As a result it seems that thermal conduction occurs at a much lower level than the Spitzer value. Comparing cavity enthalpies to the radiative losses within the cooling radius for seven clusters suggests that some clusters are probably heated by sporadic, but extremely powerful, AGN outflows interspersed between more frequent but lower power outflows.
[Context] The broadband spectral energy distribution of microquasars has proven to be a valuable tool for assessing the roles of jets and accretion flows in microquasars, as well as the coupling between the two. The coupling might manifest itself observationally in the correlated radio and X-ray variabilities of a source. Such a radio/X-ray correlation has indeed been seen in several microquasars during the low-hard state and subsequently been claimed to be universal for all. If proven, the universal correlation would have profound implications on theoretical models. However, there is already observational evidence that suggests otherwise. [Aims] In this paper, we critically examine the radio/X-ray correlation in a sample of microquasars, in the low-hard state as well as during state transitions, with a goal of testing the claimed universality of the correlation on observational grounds. [Methods] We have assembled a comprehensive data set from the simultaneous/contemporaneous radio and X-ray observations of representative microquasars. The data have allowed us to quantify the radio/X-ray correlation on a source by source basis. [Results] We find that the radio/X-ray correlation of microquasars exhibits diverse behaviors, both in the low-hard and transitional states, ranging from being very week (or none at all) to very strong. There is even a hint of spectral dependence of the correlation in some cases. [Conclusions] Our results rule out the claimed universality of the radio/X-ray correlation of microquasars, even for the low-hard state. On the other hand, we do find that the radio and X-ray variabilities are, to varying degrees, correlated in most cases.
We propose a reflection model of the time delays detected during an exceptionally bright, single flare in MCG-6-30-15. We consider a scenario in which the delays of the hard X-rays with respect to the soft X-rays are caused by the presence of the delayed reflection component. We employ a model of the flare, which is accompanied by reprocessed emission. We consider two geometries/thermal states of the reprocessing medium: a partially ionized accretion disk surface and a distribution of magnetically confined, cold blobs. The reprocessing by cold blobs predicts positive time delays and a saturation in the time delay -- energy relation, which is likely present in the data. The model requires a strong reflection component and relies on the apparent pivoting of the combined primary and reflected spectrum. The reflection by the ionized disk surface does not reproduce the observed delays. We discuss the relation between the two reflection scenarios and argue that they are both present in MCG-6-30-15.
We discuss implications of a strong flare event observed in the Seyfert galaxy MCG-6-30-15 assuming that the emission is due to localized magnetic reconnection. We conduct detailed radiative transfer modeling of the reprocessed radiation for a primary source that is elevated above the disk. The model includes relativistic effects and Keplerian motion around the black hole. We show that for such a model setup the observed time-modulation must be intrinsic to the primary source. Using a simple analytical model we then investigate time delays between hard and soft X-rays during the flare. The model considers an intrinsic delay between primary and reprocessed radiation, which measures the geometrical distance of the flare source to the reprocessing sites. The observed time delays are well reproduced if one assumes that the reprocessing happens in magnetically confined, cold clouds.
The Vela Molecular Ridge is one of the nearest intermediate-mass star forming regions, located within the galactic plane and outside the solar circle. Cloud D, in particular, hosts a number of small embedded young clusters. We present the results of a large-scale map in the dust continuum at 1.2 mm of a ~ 1deg x 1deg area within cloud D. The main aim of the observations was to obtain a complete census of cluster-forming cores and isolated (both high- and low-mass) young stellar objects in early evolutionary phases. The bolometer array SIMBA at SEST was used to map the dust emission in the region with a typical sensitivity of ~ 20 mJy/beam. This allows a mass sensitivity of ~ 0.2 Msun. The resolution is 24 arcsec, corresponding to ~ 0.08 pc, roughly the radius of a typical young embedded cluster in the region. The continuum map is also compared to a large scale map of CO(1-0) integrated emission. Using the CLUMPFIND algorithm, a robust sample of 29 cores has been obtained, spanning the size range 0.03 - 0.25 pc and the mass range 0.4 - 88 Msun. The most massive cores are associated both with red IRAS sources and with embedded young clusters, and coincide with CO(1-0) integrated emission peaks. The cores are distributed according to a mass spectrum ~ M^{-alpha} and a mass-versus-size relation ~ D^{x}, with alpha ~ 1.45 - 1.9 and x ~ 1.1 - 1.7. They appear to originate in the fragmentation of gas filaments seen in CO(1-0) emission and their formation is probably induced by expanding shells of gas. The core mass spectrum is flatter than the Initial Mass Function of the associated clusters in the same mass range, suggesting further fragmentation within the most massive cores. A threshold A_V ~ 12 mag seems to be required for the onset of star formation in the gas.
We develop a two-flow model of accretion onto a black hole which incorporates the effect of the local magneto-rotational instability. The flow consists of an accretion disk and an accreting corona, and the local dynamo affects the disk/corona mass exchange. The model is aimed to explain the power spectrum density of the sources in their soft, disk-dominated states. The local perturbations of the magnetic field in the disk are described as in King et al. (2004) and Mayer and Pringle (2005), but the time-dependent local magnetic field is assumed to affect the local supply of the material to the corona. Since the viscous timescale in the corona is much shorter than in the disk, the local perturbations are not smeared in the corona. Simple analytical estimates and full time-dependent computations of the disk-corona system are performed. The accreting corona model can reproduce the broad power spectra of Soft State X-ray binaries and AGN. The model, however, predicts that (i) sources undergoing radiation pressure instability (GRS 1915+105) should have systematically steeper power spectra than other sources, (ii) AGN should have systematically steeper power spectra than GBH, even if their disks are described using viscosity proportional to the gas pressure. More precise measurements of power spectra of Soft State sources are clearly needed.
High resolution UVES spectra of 40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used to derive S, Fe and Zn abundances from lines in the 400 - 950 nm region. For one star we also present novel observations of the SI triplet at 1.046 micron carried out with the ESO VLT CRIRES spectrograph. Comparison of sulphur abundances from the weak and strong SI lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE effects into account, the Galactic halo stars distribute around a plateau at [S/Fe] = +0.2 dex with a scatter of 0.07 dex only. This indicates that sulphur in Galactic halo stars has been made by alpha-capture processes in massive SNe. The observed scatter in S/Fe is, however, much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. [Zn/Fe] is close to zero for metallicities in the range -2.0 < [Fe/H] < -1.0 but increases to a level of [Zn/Fe] = +0.15 dex in the range -2.7 < [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] rises steeply to a value around [Zn/Fe] = +0.5 dex at [Fe/H] = -3.2. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.
The Rossi X-ray Timing Explorer has detected nearly coherent oscillations in the tails of type I X-ray bursts from 17 low-mass X-ray binaries. The oscillations are thought to be generated by brightness fluctuations associated with a surface mode on the rotating neutron star. The mechanism that drives the modes is, however, not understood, since the burning layer is stable to thermal perturbations. We show here via a linear perturbation analysis that, even under conditions when pure thermal perturbations are stable, nonradial surface modes may still be unstable. Specifically, we find that, if helium-burning reactions supply a reasonable fraction of the outgoing flux during burst decay, nonradial surface modes will grow in time. On the other hand, these modes are stable in the presence of hydrogen burning via the rp-process. The result naturally explain why oscillations in the decay phase of type I X-ray bursts are detected only from short-duration bursts.
Dense cores are the simplest star-forming sites that we know, but despite their simplicity, they still hold a number of mysteries that limit our understanding of how solar-type stars form. ALMA promises to revolutionize our knowledge of every stage in the life of a core, from the pre-stellar phase to the final disruption by the newly born star. This contribution presents a brief review of the evolution of dense cores and illustrates particular questions that will greatly benefit from the increase in resolution and sensitivity expected from ALMA
We present details of the second part of the Southern Infrared Proper Motion Survey (SIPS). Here accurate relative astrometry allows us to reduce the minimum proper motion to 0.1 arcseconds per year. This yields 6904 objects with proper motions between our minum cut and half an arcsecond a year. A small overspill sample with proper motions greater than this is also included. We examine our sample to identify interesting individual objects such as common proper motion binaries, potential L dwarfs and candidate nearby stars. Finally we show our survey is incomplete due to many factors, factors which we will take into account when simulating these survey results in the next paper in this series.
We study the evolution of the mass function in young and dense star clusters by means of direct N-body simulations. Our main aim is to explain the recent observations of the relatively flat mass function observed near the centre of the Arches star cluster. In this region, the power law index of the mass function for stars more massive than about 5-6 solar mass, is larger than the Salpeter value by about unity; whereas further out, and for the lower mass stars, the mass function resembles the Salpeter distribution. We show that the peculiarities in the Arches mass function can be explained satisfactorily without primordial mass segregation. We draw two conclusions from our simulations: 1) The Arches initial mass function is consistent with a Salpeter slope down to ~1 solar mass, 2) The cluster is about half way towards core collapse. The cores of other star clusters with characteristics similar to those of the Arches are expected to show similar flattening in the mass functions for the high mass (>5 solar mass) stars.
I present an analysis of the gamma-ray and afterglow energies of the complete sample of 17 short duration GRBs with prompt X-ray follow-up. I find that 80% of the bursts exhibit a linear correlation between their gamma-ray fluence and the afterglow X-ray flux normalized to t=1 d, a proxy for the kinetic energy of the blast wave ($F_{X,1}~F_{gamma}^1.01). An even tighter correlation is evident between E_{gamma,iso} and L_{X,1} for the subset of 13 bursts with measured or constrained redshifts. The remaining 20% of the bursts have values of F_{X,1}/F_{gamma} that are suppressed by about three orders of magnitude, likely because of low circumburst densities (Nakar 2007). These results have several important implications: (i) The X-ray luminosity is generally a robust proxy for the blast wave kinetic energy, indicating nu_X>nu_c and hence a circumburst density n>0.05 cm^{-3}; (ii) most short GRBs have a narrow range of gamma-ray efficiency, with <epsilon_{gamma}>~0.85 and a spread of 0.14 dex; and (iii) the isotropic-equivalent energies span 10^{48}-10^{52} erg. Furthermore, I find tentative evidence for jet collimation in the two bursts with the highest E_{gamma,iso}, perhaps indicative of the same inverse correlation that leads to a narrow distribution of true energies in long GRBs. I find no clear evidence for a relation between the overall energy release and host galaxy type, but a positive correlation with duration may be present, albeit with a large scatter. Finally, I note that the outlier fraction of 20% is similar to the proposed fraction of short GRBs from dynamically-formed neutron star binaries in globular clusters. This scenario may naturally explain the bimodality of the F_{X,1}/F_{gamma} distribution and the low circumburst densities without invoking speculative kick velocities of several hundred km/s.
We present a Bayesian technique based on a maximum entropy method to
reconstruct the dark energy equation of state $w(z)$ in a non--parametric way.
This MaxEnt technique allows to incorporate relevant prior information while
adjusting the degree of smoothing of the reconstruction in response to the
structure present in the data.
After demonstrating the method on synthetic data, we apply it to current
cosmological data, separately analysing type Ia supernovae measurement from the
HST/GOODS program and the first year Supernovae Legacy Survey (SNLS),
complemented by cosmic microwave background and baryonic acoustic oscillations
data. We find that the SNLS data are compatible with $w(z) = -1$ at all
redshifts $0 \leq z \lsim 1100$, with errorbars of order 20% for the most
constraining choice of priors and model. The HST/GOODS data exhibit a slight
(about $1\sigma$ significance) preference for $w>-1$ at $z\sim 0.5$ and a drift
towards $w>-1$ at larger redshifts, which however is not robust with respect to
changes in our prior specifications.
Our method highlights the danger of employing parametric fits for the unknown
equation of state, that can potentially miss or underestimate real structure in
the data.
Using the Submillimeter Array (SMA) we have imaged for the first time the 321.226 GHz, 10_{29}-9_{36} ortho-H2O maser emission. This is also the first detection of this line in the Cepheus A high-mass star-forming region. The 22.235 GHz, 6_{16}-5_{23} water masers were also observed with the Very Large Array 43 days following the SMA observations. Three of the nine detected submillimeter maser spots are associated with the centimeter masers spatially as well as kinematically, while there are 36 22 GHz maser spots without corresponding submillimeter masers. In the HW2 source, both the 321 GHz and 22 GHz masers occur within the region of ~1'' which includes the disk-jet system, but the position angles of the roughly linear structures traced by the masers indicate that the 321 GHz masers are along the jet while the 22 GHz masers are perpendicular to it. We interpret the submillimeter masers in Cepheus A to be tracing significantly hotter regions (600~2000 K) than the centimeter masers.
Direct imaging of exoplanets is limited by bright quasi-static speckles in the point spread function (PSF) of the central star. This limitation can be reduced by subtraction of reference PSF images. We have developed an algorithm to construct an optimized reference PSF image from a set of reference images. This image is built as a linear combination of the reference images available and the coefficients of the combination are optimized inside multiple subsections of the image independently to minimize the residual noise within each subsection. The algorithm developed can be used with many high-contrast imaging observing strategies relying on PSF subtraction, such as angular differential imaging (ADI), roll subtraction, spectral differential imaging, reference star observations, etc. The performance of the algorithm is demonstrated for ADI data. It is shown that for this type of data the new algorithm provides a gain in sensitivity by up to a factor 3 at small separation over the algorithm used in Marois et al. (2006).
Binary and Millisecond pulsars have a great deal to teach us about stellar evolution and are invaluable tools for tests of relativistic theories of gravity. Our understanding of these objects has been transformed by large-scale surveys that have uncovered a great deal of new objects, exquisitely timed by ever-improving instrumentation. Here we argue that there exists a fundamental relation between the spin period of a pulsar and its companion mass, and that this determines many of the observable properties of a binary pulsar. No recycled pulsars exist in which the minimum companion mass exceeds (P/10 ms) Solar Masses. Furthermore, the three fastest disk millisecond pulsars are either single, or possess extremely low-mass companions Mc~0.02 Mo, consistent with this relation. Finally, the four relativistic binaries for which we have actual measurements of neutron star masses, suggest that not only are their spin periods related to the companion neutron star mass, but that the kick imparted to the system depends upon it too, leading to a correlation between orbital eccentricity and spin period. The isolation of the relativistic binary pulsars in the magnetic field-period diagram is used to argue that this must be because the kicks imparted to proto-relativistic systems are usually small, leading to very few if any isolated runaway mildly-recycled pulsars. This calls into question the magnitude of supernova kicks in close binaries, which have been usually assumed to be similar to those imparted to the bulk of the pulsar population. Finally, we review some of the highlights of the Parkes precision timing efforts, which suggest 10 nanosecond timing is obtainable on PSR J1909-3744 that will aid us in searching for cosmological sources of gravitational waves.
I present a novel view on the problem of solar coronal heating. In my picture, coronal heating should be viewed as a self-regulating process that works to keep the coronal plasma marginally collisionless. The self-regulating mechanism is based on the interplay between two effects: (1) Plasma density controls coronal energy release via the transition between the slow collisional Sweet-Parker regime and the fast collisionless reconnection regime; (2) In turn, coronal energy release through reconnection leads to an increase in the ambient plasma density via chromospheric evaporation, which temporarily shuts off any subsequent reconnection involving the newly-reconnected loops.
We use a grid-based shallow water model to simulate the atmospheric dynamics of the transiting hot Jupiter HD 209458b. Under the usual assumption that the planet is in synchronous rotation with zero obliquity, a steady state is reached with a well-localized cold spot centered 76 degrees east of the antistellar point. This represents a departure from predictions made by previous simulations in the literature that used the shallow water formalism; we find that the disagreement is explained by the factor of 30 shorter radiative timescale used in our model. We also examine the case that the planet is in Cassini state 2, in which the expected obliquity is ~90 degrees. Under these circumstances, a periodic equilibrium is reached, with the temperature slightly leading the solar forcing. Using these temperature distributions, we calculate disk-integrated bolometric infrared light curves from the planet. The light curves for the two models are surprisingly similar, despite large differences in temperature patterns in the two cases. In the zero-obliquity case, the intensity at the minimum is 66% of the maximum intensity, with the minimum occuring 72 degrees ahead of transit. In the high-obliquity case, the minimum occurs 54 degrees ahead of transit, with an intensity of 58% of the maximum.
Various studies have claimed that the fraction of obscured AGN drops with luminosity, but contrary results are also reported. We present our recent studies on the fraction of X-ray obscured quasars in Chandra Deep Fields and in the local universe showing that most quasars in CDFs (at redshift of 1 ~ 4) are obscured and a consistent pattern in the local universe.
As adaptive optics technology continues to improve, the stellar coronagraph will play an ever increasing role in ground-based high-contrast imaging observations. Though several different image masks exist for the most common type of coronagraph, the Lyot coronagraph, it is not yet clear what level of wavefront correction must be reached in order to gain, either in starlight suppression or observing efficiency, by implementing a more sophisticated design. In this paper, we model image plane Lyot-style coronagraphs and test their response to a range of wavefront correction levels, in order to identify regimes of atmospheric compensation where the use of hard-edge, Gaussian, and band-limited image masks becomes observationally advantageous. To delineate performances, we calculate the speckle noise floor mean intensity. We find that apodized masks provide little improvement over hard-edge masks until on-sky Strehl ratios exceed $\sim0.88 \: S_{qs}$, where $S_{qs}$ is the intrinsic Strehl ratio provided by the optical system. Above this value, 4th-order band-limited masks out-perform Gaussian masks by generating comparable contrast with higher Lyot stop throughput. Below this level of correction, hard-edge masks may be preferentially chosen, since they are less susceptible to low-order aberration content. The use of higher-order band-limited masks is relegated to situations where quasi-static residual starlight cannot be sufficiently removed from the search area with speckle-nulling hardware.
The matter content of the Universe is generally regarded as a perfect fluid on sufficiently large scales, for all epochs. But the recent cosmological matter distribution, consisting of an (ideally) random distribution of gravitationally collapsed structures, is more accurately described as a collection of discrete grains, than as a fluid. It is well known that granular materials may have very different macroscopic properties than fluids; analogously, we investigate the possibility that pervasive small-scale inhomogeneities in the recent Universe may lead to perturbations of the cosmological expansion on intermediate and/or large scales.
Cosmological gamma ray bursts (GRBs) are the brightest explosions in the Universe. Satellite detectors, such as Beppo-SAX, HETE2 and more recently Swift, have provided a wealth of data, including the localization and redshifts of subsets of GRBs. The redshift distribution has been utilized in several studies in attempts to constrain the evolving star formation rate and to probe GRB rate evolution in the high-redshift Universe. These studies find that the GRB luminosity function and/or the rate density evolve with redshift. We present a short review of the problems of constraining GRB rate evolution in the context of the complex mix of biases inherent in the redshift measurements. To disentangle GRB rate evolution from the biases prevalent in the redshift distribution will require accounting for the incompleteness of the observed redshift sample. We highlight the importance of formulating a `complete GRB selection function' to account for the main sources of bias.
We have carried out an extensive spectroscopic survey with the Keck and VLT telescopes, targeting lensed galaxies in the background of the massive cluster Abell 68. Spectroscopic measurements are obtained for 26 lensed images, including a distant galaxy at z=5.4 . Redshifts have been determined for 5 out of 7 multiply-image systems. Through a careful modeling of the mass distribution in the strongly-lensed regime, we derive a mass estimate of 5.3 x 10^14 Msun within 500 kpc. Our mass model is then used to constrain the redshift distribution of the remaining multiply-imaged and singly-imaged sources. This enables us to examine the physical properties for a subsample of 7 Lyman-alpha emitters at 1.7 < z < 5.5, whose unlensed luminosities of ~ 10^41 ergs/s are fainter than similar objects found in blank fields. Of particular interest is an extended Lyman-alpha emission region surrounding a highly magnified source at z=2.6, detected in VIMOS Integral Field Spectroscopy data. The physical scale of the most distant lensed source at z=5.4 is very small (<300 pc), similar to the lensed z ~ 5.6 emitter reported by Ellis et al. (2001) in Abell 2218. New photometric data available for Abell 2218 allow for a direct comparison between these two unique objects. Our survey illustrates the practicality of using lensing clusters to probe the faint end of the z ~ 2-5 Lyman-alpha luminosity function in a manner that is complementary to blank field narrow-band surveys.
Recent radial velocity observations have indicated that Jovian-type planets can exist in moderately close binary star systems. Numerical simulations of the dynamical stability of terrestrial-class planets in such environments have shown that, in addition to their giant planets, these systems can also harbor Earth-like objects. In this paper, we study the late stage of terrestrial planet formation in such binary-planetary systems, and present the results of the simulations of the formation of Earth-like bodies in their habitable zones. We consider a circumprimary disk of Moon- to Mars-sized objects and numerically integrate the orbits of these bodies at the presence of the Jovian-type planet of the system and for different values of the mass, semimajor axis, and orbital eccentricity of the secondary star. Results indicate that, Earth-like objects, with substantial amounts of water, can form in the habitable zone of the primary star. Simulations also indicate that, by transferring angular momentum from the secondary star to protoplanetary objects, the giant planet of the system plays a key role in the radial mixing of these bodies and the water contents of the final terrestrial planets. We will discuss the results of our simulation and show that the formation of habitable planets in binary-planetary systems is more probable in binaries with moderate to large perihelia.
We report a measurement of the Rossiter--McLaughlin effect in the transiting extrasolar planetary system TrES-1, via simultaneous spectroscopic and photometric observations with the Subaru and MAGNUM telescopes. By modeling the radial velocity anomaly that was observed during a transit, we determine the sky-projected angle between the stellar spin axis and the planetary orbital axis to be $\lambda = 30 \pm 21$ [deg]. This is the third case for which $\lambda$ has been measured in a transiting exoplanetary system, and the first demonstration that such measurements are possible for relatively faint host stars ($V \sim 12$, as compared to $V \sim 8$ for the other systems). We also derive a time of mid-transit and upper limits on the eccentricity of the TrES-1b orbit.
Development of MITSuME is reported. Two 50-cm optical telescopes have been built at Akeno in Yamanashi prefecture and at Okayama Astrophysical Observatory (OAO) in Okayama prefecture. Three CCD cameras for simultaneous g'RcIc photometry are to be mounted on each focal plane, covering a wide FOV of about 30" x 30". The limiting magnitude at V is fainter than 18. In addition to these two optical telescopes, a 91-cm IR telescope with a 1 deg x 1 deg field of view is being built at OAO, which performs photometry in YJHK bands. These robotic telescopes can start the observation of counterparts of a GRB within a minute from an alert. We aim to obtain photometric redshifts exceeding 10 with these telescopes. The performance and the current construction status of the telescopes are presented.
We examine how the location of star formation within disc galaxies depends on environment at intermediate redshift. This is achieved by comparing emission-line (r_em) and restframe B-band (r_B) scalelengths for matched samples of 50 field and 19 cluster star-forming, disc galaxies, with 0.25 < z < 1.0 and M_B < -19.5 mag. We find that at a given r_B the majority of our cluster galaxies have r_em smaller than those in the field, by 25 percent on average. These results are compared with studies of local galaxies, which find a very similar behaviour. From the relations of r_em and r_B versus B-band absolute magnitude (M_B) we infer that the difference between the intermediate-z cluster and field samples is mostly attributable to variation in r_em at a given M_B, while the r_B versus M_B relation is similar for the two samples.
A jet model for Galactic black-hole X-ray binaries will be presented that appears to explain several observational characteristics. In particular, it explains the energy spectrum from radio to hard X-rays, the time-lags as a function of Fourier frequency, the increase of the variability amplitude (QPO and high frequency) with increasing photon energy, and the narrowing of the autocorrelation function with increasing photon energy. On the other hand, there are additional observational constraints that no model has tried to explain yet. It is important that we all try to address these constraints if we are to make any progress in understanding black-hole X-ray sources
Marigo (2002) has highlighted the crucial importance of molecular opacities in modelling the evolution of AGB stars at varying surface C/O ratio. In particular, it has been shown the large inadequacy of solar-scaled opacities when applied to models of carbon stars, and hence the need for correctly coupling the molecular opacities to the current surface chemical composition of AGB stars. The aim of the present follow-up study is to investigate the effects of variable molecular opacities on the evolutionary properties of luminous AGB stars with massive envelopes, i.e. with initial masses from ~3.5 Msun up to 5-8 Msun, which are predicted to experience both the third dredge-up and hot-bottom burning. It is found that if the dredge-up of carbon is efficient enough to lead to an early transition from C/O<1 to C/O>1, then hot-bottom burning may be weakened, extinguished, or even prevented. The physical conditions for this occurrence are analysed and a few theoretical and observational implications are discussed. Importantly, it is found that the inclusion of variable molecular opacities could significantly change the current predictions for the chemical yields contributed by intermediate-mass AGB stars, with M~3.5 - 4.0 Msun that make as much as ~ 30-50 % of all stars expected to undergo hot-bottom burning.
We present a multiwavelength study of the ultra compact HII region associated with IRAS 20178+4046. This enables us to probe the different components associated with this massive star forming region. The radio emission from the ionized gas was mapped at 610 and 1280 MHz using the Giant Metrewave Radio Telescope (GMRT), India. We have used 2MASS $J H K_{s}$ data to study the nature of the embedded sources associated with IRAS 20178+4046. Submillimetre emission from the cold dust at 450 and 850 $\mu$m was studied using JCMT-SCUBA. The high-resolution radio continuum maps at 610 and 1280 MHz display compact spherical morphology. The spectral type of the exciting source is estimated to be $\sim$ B0.5 from the radio flux densities. However, the near-infrared (NIR) data suggest the presence of several massive stars (spectral type earlier than O9) within the compact ionized region. Submillimetre emission shows the presence of two dense cloud cores which are probably at different evolutionary stages. The total mass of the cloud is estimated to be $\sim$ 700 -- 1500 $\rm M_{\odot}$ from the submillimetre emission at 450 and 850 $\mu$m. The multiwavelength study of this star forming complex reveals an interesting scenario where we see the presence of different evolutionary stages in star formation. The ultra compact HII region coinciding with the southern cloud core is at a later stage of evolution compared to the northern core which is likely to be a candidate protocluster.
Proper motions in a sunspot group with a delta-configuration and close to the solar disc center have been studied by employing local correlation tracking techniques. The analysis is based on more than one hour time series of G-band images. Radial outflows with a mean speed of 0.67 km s^{-1} have been detected around the spots, the well-known sunspots moats. However, these outflows are not found in those umbral core sides without penumbra. Moreover, moat flows are only found in those sides of penumbrae located in the direction marked by the penumbral filaments. Penumbral sides perpendicular to them show no moat flow. These results strongly suggest a relation between the moat flow and the well-known, filament aligned, Evershed flow. The standard picture of a moat flow originated from a blocking of the upward propagation of heat is commented in some detail.
The study of the Fe abundance in the intra cluster medium (ICM) provides strong constraints on the integrated star formation history and supernova rate of the cluster galaxies, as well as on the ICM enrichment mechanisms. In this Letter, using chemical evolution models for galaxies of different morphological types, we study the evolution of the Fe content of clusters of galaxies. We assume that the ICM Fe enrichment occurs by means of galactic winds arising from elliptical galaxies and from gas stripped from the progenitors of S0 galaxies via external mechanisms, due to the interaction of the inter stellar medium with the ICM. The Fe-rich gas ejected by ellipticals accounts for the X_Fe,ICM values observed at z > 0.5, whereas the gas stripped from the progenitors of the S0 galaxies accounts for the increase of X_Fe,ICM observed at z<0.5. We tested two different scenarios for Type Ia supernova (SN) progenitors and we model the Type Ia SN rate observed in clusters, finding a good agreement between our predictions and the available observations.
NGC 1380 is a lenticular galaxy located near the centre of the Fornax Cluster northeast of NGC 1399. The globular cluster system of this galaxy was previously studied only from the ground. Recent studies of similar early-type galaxies, specially lenticular ones, reveal the existence of star clusters that apparently break up the traditional open/globular cluster dichotomy. With higher quality photometry from HST/WFPC2 we study the star clusters in NGC 1380, measuring their magnitudes, colours, sizes and projected distances from the centre of the galaxy. We used deep archival HST/WFPC2 in the B and V bands. We built colour magnitude diagrams from which we selected a sample of cluster candidates. We also analysed their colour distribution and measured their sizes. Based on their location in the luminosity-size diagram we estimated probabilities of them being typical globular clusters as those found in the Galaxy. A total of about 570 cluster candidates were found down to V=26.5. We measured sizes for approximately 200 of them. The observed colour distribution has three apparent peaks. Likewise for the size distribution. We identified the smaller population as being mainly typical globular clusters, while the more extended objects have small probabilities of being such objects. Different correlations between absolute magnitudes, sizes, colours and location were inferred for these cluster sub-populations. Most extended clusters (Reff > 4 pc) share similar properties to the diffuse star clusters reported to inhabit luminous early-type galaxies in the Virgo galaxy cluster such as being of low surface brightness and fainter than MV ~ -8. We also report on a small group of (Reff ~ 10 pc), -8< MV < -6, red clusters located near the centre of NGC 1380, which may be interpreted as faint fuzzies.
Various galactic globular clusters display abundance anomalies that affect the morphology of their colour-magnitude diagrams. In this paper we consider the possibility of helium enhancement in the anomalous horizontal branch of NGC 2808. We examine the dynamics of a self-enrichment scenario in which an initial generation of stars with a top-heavy initial mass function enriches the interstellar medium with helium via the low-velocity ejecta of its asymptotic giant branch stars. This enriched medium then produces a second generation of stars which are themselves helium-enriched. We use a direct N-body approach to perform five simulations and conclude that such two-generation clusters are both possible and would not differ significantly from their single-generation counterparts on the basis of dynamics. We find, however, that the stellar populations of such clusters would differ from single-generation clusters with a standard initial mass function and in particular would be enhanced in white dwarf stars. We conclude, at least from the standpoint of dynamics, that two-generation globular clusters are feasible.
Images of the complex circumstellar nebula associated with the famous red supergiant VY CMa show evidence for multiple and asymmetric mass loss events over the past 1000 yrs. Doppler velocities of the arcs and knots in the ejecta showed that they are not only spatially distinct but also kinematically separate from the surrounding diffuse material. In this paper we describe second epoch HST/WFPC2 images to measure the transverse motions which when combined with the radial motions provide a complete picture of the kinematics of the ejecta including the total space motions and directions of the outflows. Our results show that the arcs and clumps of knots are moving at different velocities, in different directions, and at different angles relative to the plane of the sky and to the star, confirming their origin from eruptions at different times and from physically separate regions on the star. We conclude that the morphology and kinematics of the arcs and knots are consistent with a history of mass ejections not aligned with any presumed axis of symmetry. The arcs and clumps represent relatively massive outflows and ejections of gas very likely associated with large -- scale convective activity and magnetic fields.
We use imaging polarimetry taken with the HST/ACS/HRC to explore the three dimensional structure of the circumstellar dust distribution around the red supergiant VY Canis Majoris. The polarization vectors of the nebulosity surrounding VY CMa show a strong centro-symmetric pattern in all directions except directly East and range from 10% - 80% in fractional polarization. In regions that are optically thin, and therefore likely have only single scattering, we use the fractional polarization and photometric color to locate the physical position of the dust along the line-of-sight. Most of the individual arc-like features and clumps seen in the intensity image are also features in the fractioanl polarization map. These features must be distinct geometric objects. If they were just local density enhancements, the fractional polarization would not change so abruptly at the edge of the feature. The location of these features in the ejecta of VY CMa using polarimetry provides a determination of their 3D geometry independent of, but in close agreement with, the results from our study of their kinematics (Paper I).
Context: Faraday rotation measurements of extragalactic radio sources during coronal occultation allow assessment of both the electron density distribution and the three-dimensional magnetic field topology in the outer solar corona. Aims: We simulate the three-dimensional structure of both the coronal magnetic field and the electron density distribution in order to reproduce the excess Faraday rotation measures (RMs) of the occulted radio sources observed during solar activity minimum. In particular, we infer the tilt of the solar magnetic axis with respect to the rotation axis. Methods: We compare the output of the model with Very Large Array (VLA) radio polarimetric measurements of a sample of extragalactic sources observed in May 1997. Information on the magnetic field geometry can be retrieved by fine-tuning the set of model free parameters that best describe the observations. Results: We find that predicted and observed Faraday rotation measures are in excellent agreement, thus supporting the model. Our best-fitting model yields a tilt angle $\theta_{RB}=3.3^{\circ}$ of the solar magnetic axis with respect to the solar rotation axis around Carrington Rotation 1923. This result is consistent with analogous but independent estimates computed from the expansion coefficients of the photospheric field observed at the Wilcox Solar Observatory (WSO).
We present a non-parametric method for decomposition of the light of disk galaxies into disk, bulge and bar components. We have developed and tested the method on a sample of 68 disk galaxies for which we have acquired I-band photometry. The separation of disk and bar light relies on the single assumption that the bar is a straight feature with a different ellipticity and position angle from that of the projected disk. We here present the basic method, but recognise that it can be significantly refined. We identify bars in only 47% of the more nearly face-on galaxies in our sample. The fraction of light in the bar has a broad range from 1.3% to 40% of the total galaxy light. If low-luminosity galaxies have more dominant halos, and if halos contribute to bar stability, the luminosity functions of barred and unbarred galaxies should differ markedly; while our sample is small, we find only a slight difference of low significance.
We describe a system for rapidly measuring the brightness of the night sky using a mosaic of CCD images obtained with a low-cost automated system. The portable system produces millions of independent photometric measurements covering the entire sky, enabling the detailed characterization of natural sky conditions and light domes produced by cities. The measurements are calibrated using images of standard stars contained within the raw data, producing results closely tracking the Johnson V astronomical standard. The National Park Service has collected hundreds of data sets at numerous parks since 2001 and is using these data for the protection and monitoring of the night-sky visual resource. This system also allows comprehensive characterization of sky conditions at astronomical observatories. We explore photometric issues raised by the broadband measurement of the complex and variable night-sky spectrum, and potential indices of night-sky quality.
We estimate the power spectrum of SZ(Sunyaev-Zel'dovich)-effect-induced temperature fluctuations on sub-degree scales by using the cross correlation between the three-year WMAP maps and 2MASS galaxy distribution. We produced the SZ effect maps by hydrodynamic simulation samples of the $\Lambda$CDM model, and show that the SZ effect temperature fluctuations are highly non-Gaussian. The PDF of the temperature fluctuations has a long tail. More than 70% power of the SZ effect temperature fluctuations attributes to top $\sim 1%$ wavelet modes (long tail events). On the other hand, the CMB temperature fluctuations basically are Gaussian. Although the mean power of CMB temperature fluctuations on sub-degree scales is much higher than that of SZ effect map, the SZ effect temperature fluctuations associated with top 2MASS clusters is comparable to the power of CMB temperature fluctuations on the same scales. Thus, from noisy WMAP maps, one can have a proper estimation of the SZ effect power at the positions of the top 2MASS clusters. The power spectrum given by these top wavelet modes is useful to constrain the parameter of density fluctuations amplitude $\sigma_8$. We find that the power spectrum of these top wavelet modes of SZ effect on sub-degree scales basically is consistent with the simulation maps produced with $\sigma_8=0.84$. The simulation samples of $\sigma_8=0.74$ show, however, significant deviation from detected SZ power spectrum. It can be ruled out with confidence level 99% if all other cosmological parameters are the same as that given by the three-year WMAP results.
We study the apparent lack of power on large angular scales in the WMAP data. We confirm that although there is no apparent lack of power at large angular scales for the full-sky maps, the lowest multipoles of the WMAP data happen to have the magnitudes and orientations, with respect to the Galactic plane, that are needed to make the large scale power in cut-sky maps surprisingly small. Our analysis shows that most of the large scale power of the observed CMB anisotropy maps comes from two regions around the Galactic plane (~9% of the sky). One of them is a cold spot within ~40 degrees of the Galactic center and the other one is a hot spot in the vicinity of the Gum Nebula. If the current full-sky map is correct, there is no clear deficit of power at large angular scales and the alignment of the l=2 and l=3 multipoles remains the primary intriguing feature in the full-sky maps. If the full-sky map is incorrect and a cut is required, then the apparent lack of power remains mysterious. Future missions such as Planck, with a wider frequency range and greater sensitivity, will permit a better modeling of the Galaxy and will shed further light on this issue.
We study the survival of gas planets around stars with masses in the range 1-5 Msun, as these stars evolve off the Main Sequence. We show that planets with masses smaller than one Jupiter mass do not survive the Planetary Nebula phase if located initially at orbital distances smaller than (3-5) AU. Planets more massive than two Jupiter masses around low mass (1 Msun on the Main Sequence) stars survive the Planetary Nebula stage down to orbital distances of 3 AU. As the star evolves through the Planetary Nebula phase, an evaporation outflow will be established at the planet's surface. Evaporating planets may be detected using spectroscopic observations. Planets around white dwarfs with masses M_WD > 0.7 Msun are generally expected to be found at orbital radii r > 15 AU. If planets are found at smaller orbital radii around massive white dwarfs, they had to form as the result of the merger of two white dwarfs.
A full spectral survey was carried out towards the Giant Molecular Cloud complex, Sagittarius B2 (Sgr B2), using the ISO Long Wavelength Spectrometer Fabry-Perot mode. This provided complete wavelength coverage in the range 47-196 um (6.38-1.53 THz) with a spectral resolution of 30-40 km/s. This is an unique dataset covering wavelengths inaccessible from the ground. It is an extremely important region of the spectrum as it contains both the peak of the thermal emission from dust, and crucial spectral lines of key atomic (OI, CII, OIII, NII and NIII) and molecular species (NH3, NH2, NH, H2O, OH, H3O+, CH, CH2, C3, HF and H2D+). In total, 95 spectral lines have been identified and 11 features with absorption depth greater than 3 sigma remain unassigned. Most of the molecular lines are seen in absorption against the strong continuum, whereas the atomic and ionic lines appear in emission (except for absorption in the OI 63 um and CII 158 um lines). Sgr B2 is located close to the Galactic Centre and so many of the features also show a broad absorption profile due to material located along the line of sight. A full description of the survey dataset is given with an overview of each detected species and final line lists for both assigned and unassigned features.
We present the first results using the Reuven Ramaty High-Energy Solar Spectroscopic Imager, RHESSI, to observe solar X-ray emission not associated with active regions, sunspots or flares (the quiet Sun). Using a newly developed chopping technique (fan-beam modulation) during seven periods of offpointing between June 2005 to October 2006, we obtained upper limits over 3-200 keV for the quietest times when the GOES12 1-8A flux fell below $10^{-8}$ Wm$^{-2}$. These values are smaller than previous limits in the 17-120 keV range and extend them to both lower and higher energies. The limit in 3-6 keV is consistent with a coronal temperature $\leq 6$ MK. For quiet Sun periods when the GOES12 1-8A background flux was between $10^{-8}$ Wm$^{-2}$ and $10^{-7}$ Wm$^{-2}$, the RHESSI 3-6 keV flux correlates to this as a power-law, with an index of $1.08 \pm 0.13$. The power-law correlation for microflares has a steeper index of $1.29 \pm 0.06$. We also discuss the possibility of observing quiet Sun X-rays due to solar axions and use the RHESSI quiet Sun limits to estimate the axion-to-photon coupling constant for two different axion emission scenarios.
The cosmic background due to the Sunyaev-Zeldovich (SZ) effect is expected to
be the largest signal at mm and cm wavelengths at a resolution of a few
arcminutes. We investigate some simple statistics of SZ maps and their scaling
with the normalization of the matter power spectrum, sigma_8, as well as the
effects of the unknown physics of the intracluster medium on these statistics.
We show that the SZ background provides a significant background for SZ cluster
searches, with the onset of confusion occurring around 10^{14} h^{-1} solar
masses in a cosmology-dependent way, where confusion is defined as typical
errors in recovered flux larger than 20%. The confusion limit, corresponds to
the mass at which there are roughly ten clusters per square degree, with this
number nearly independent of cosmology and cluster gas physics. Typical errors
grow quickly as lower mass objects are included in the catalog.
We also point out that there is nothing in particular about the rms of the
filtered map that makes it especially well-suited for capturing aspects of the
SZ effect, and other indicators of the one-point SZ probability distribution
function are at least as well suited for the task. For example, the full width
at half maximum of the one point probability distribution has a field-to-field
scatter that is about 60% that of the rms.
The simplest statistics of SZ maps are largely unaffected by cluster physics
such aspreheating, although the impact of preheating is clear by eye in the
maps.Studies aimed at learning about the physics of the intracluster medium
will apparently require more specialized statistical indicators.
In general relativity, the energy conditions are invoked to restrict general energy-momentum tensors $T_{\mu\nu}$ in different frameworks, and to derive general results that hold in a variety of general contexts on physical grounds. We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach, where the equation of state of the cosmological fluid is unknown, the energy conditions provide model-independent bounds on the behavior of the distance modulus of cosmic sources as a function of the redshift for any spatial curvature. We use the most recent type Ia supernovae (SNe Ia) observations, which include the new Hubble Space Telescope SNe Ia events, to carry out a model-independent analysis of the energy conditions violation in the context of the standard cosmology. We show that both the null (NEC) and dominant (DEC) conditions seem to have been violated only recently ($z \lesssim 0.2$), whereas the condition for attractive gravity, i.e., the strong energy condition (SEC) was firstly violated billions of years ago, at $z \gtrsim 1$. If the redshift of the first SNe Ia events that violate the SEC is taken as the beginning of the epoch of cosmic acceleration, then the Universe switched from an early decelerated to the present accelerating phase earlier ($z>1$) than predicted by the current standard concordance flat $\Lambda$CDM scenario ($z\simeq 0.67$).
This note is concerned with potentially misleading concepts in the treatment of cosmological magnetic fields by magnetohydrodynamical (MHD) modelling. It is not a criticism of MHD itself but rather a cautionary comment on the validity of its use in cosmology. Now that cosmological data are greatly improved compared with a few decades ago, and even better data are imminent, it makes sense to revisit original modelling assumptions and examine critically their shortcomings in respect of modern science. Specifically this article argues that ideal MHD is a poor approximation around recombination, since it inherently restricts evolutionary timescales, and is often misapplied in the existing literature.
Several problematical epochs in cosmology, including the recent period of structure formation (and acceleration), require us to understand cosmic evolution during times when the basis of FRW expansion, the cosmological principle, does not completely hold true. We consider that the breakdown of isotropy and homogeneity at such times may be an important key towards understanding cosmic evolution. To study this, we examine fluctuations in the high-z supernova data to search for signs of large-scale anisotropy in the Hubble expansion. Using a cosmological-model-independent statistical analysis, we find mild evidence of real anisotropy in various circumstances. We consider the significance of these results, and the importance of further searches for violations of the cosmological principle.
We investigate charge accretion in vicinity of a slowly rotating compact star with a non-stationary electromagnetic field. Exact solutions to the general relativistic Maxwell equations are obtained for a star formed of a highly degenerate plasma with a gravitational field given by the linearized Kerr metric. These solutions are used to formulate and then to study numerically the equations of motion for a charged particle in star's vicinity using the gravitoelectromagnetic force law. The analysis shows that close to the star charge accretion does not always remain ordered. It is found that the magnetic field plays the dominant role in the onset of chaos near the star's surface.
Filled arrays of bolometers are currently being employed for use in astronomy from the far-infrared through millimeter parts of the electromagnetic spectrum. Because of the large range of wavelengths for which such detectors are applicable, the number of modes supported by a pixel will vary according to the specific application of a given available technology. We study the dependence of image fidelity and induced polarization on the size of the pixel by employing a formalism in which diffraction due to the pixel boundary is treated by propagating the second-order statistical correlations of the radiation field through a model optical system. We construct polarized beam pattern images of square pixels for various ratios of p/\lambda where p is the pixel size and \lambda is the wavelength of the radiation under consideration. For the limit in which few modes are supported by the pixel (p/\lambda<1), we find that the diffraction due to the pixel edges is non-negligible and hence must be considered along with the telescope diffraction pattern in modeling the ultimate spatial resolution of an imaging system. For the case in which the pixel is over-moded (p/\lambda>1), the geometric limit is approached as expected. This technique gives a quantitative approach to optimize the imaging properties of arrays of planar detectors in the few-mode limit.
The stationary spherically symmetric accretion flow in the Schwarzschild metric has been set up as an autonomous first-order dynamical system, and it has been studied completely analytically. Of the three possible critical points in the flow, the one that is physically realistic behaves like the saddle point of the standard Bondi accretion problem. One of the two remaining critical points exhibits the strange mathematical behaviour of being either a saddle point or a centre-type point, depending on the values of the flow parameters. The third critical point is always unphysical and behaves like a centre-type point. The treatment has been extended to pseudo-Schwarzschild flows for comparison with the general relativistic analysis.
An accident of misidentification has brought to light the interesting system HD 191588, a new RS CVn-type spectroscopic binary. A radial-velocity study of the primary star, the only seen component, carried out at the Observatoire de Haute-Provence with the Coravel instrument and subsequently at the Cambridge Observatories with a similar one, reveals two orbital motions: a short-period orbit (60 days) and a long-period one (about 4.5 years), so this star is a triple system. The following orbital elements are obtained: (1) for the long-period orbit P = 1667+/-17 days, T = 50901 +/-67 MJD, Gamma = +2.09 +/-0.07 km/s, K = 2.51 +/-0.13 km/s, e = 0.18 +/-0.04, omega = 228deg +/- 14 deg, a1 sin i = 56.7 +/- 3.0 Gm, f(m) = 0.0026 +/-0.0004 M_sun, and (2) for the short-period orbit P = 60.0269 +/-0.0016 days, T = 50482.6 +/-3.3 MJD, gamma is var., K = 24.03 +/- 0.09 km/s, e = 0.012 +/-0.004, omega = 233 deg +/-19deg, a1 sin i = 19.83 +/-0.07 Gm, f(m) = 0.0865 +/-0.0009 M_sun. From near-infrared observations we refine the classification of the primary component and we found a spectral type of K2.5 III, and a spectrum obtained in the blue--near-UV spectral region reveals strong H and K emission lines of Ca II. The unseen secondary should be a solar-type star (F or G V); the minimum mass of the third body is that of a dwarf M star. Probably, the primary component rotates in synchronism with the orbital motion in the inner orbit; a model, based upon that hypothesis, is proposed for the system, and finally the connection of the inner binary to the long-period RS CVn group (Hall 1976) is discussed.
Recent proper motion and parallax measurements for the pulsar PSR B1508+55 gave the highest (transverse) velocity (~1100 km/s) ever measured for a neutron star (Chatterjee et al. 2005). The spin-down characteristics of PSR B1508+55 (typical of non-recycled pulsars) imply that the high velocity of this pulsar cannot be solely due to disruption of a tight massive binary system. A possible way to account for the high velocity of PSR B1508+55 is to assume that at least a part of this velocity is due to a natal kick or a post-natal acceleration (Chatterjee et al. 2005). We propose an alternative explanation for the origin of hyperfast pulsars based on the idea that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star (or its helium core) which attained its peculiar velocity (similar to that of the pulsar) in the course of a strong three or four body dynamical encounter in the core of the parent young massive star cluster. Our proposal implies that the dense cores of young massive star clusters (located either in the Galactic disk or near the Galactic centre) could also produce the so-called hypervelocity stars (Brown et al. 2005) -- the ordinary stars moving with a speed of ~1000 km/s.
During 2001-2002 the optically violent variable (OVV) blazar 3C 279 un- derwent the most intense outburst seen during the entire fourteen year history that this quasar has been studied at Colgate University's Foggy Bottom Obser- vatory (FBO). This study concentrates on ~1600 R-filter images taken during this period of activity. This data set includes twenty-nine nights of microvari- ability coverage. The outburst began in March 2001, after 3C 279 had faded to its faintest level, R = 15.5, in four years. The source reached its brightest level, R = 12.5, in the fourteen years of our study in August 2001, at which time it became unobservable due to its proximity to the Sun. Upon becoming observable again in mid-December 2001, 3C 279 fluctuated between R = 13.9 and R = 14.7, until a dramatic decrease in flux level in June-July 2002 brought the source back down to a level comparable to its pre-outburst state. The source exhibited numerous week-long flares of approximately one magnitude during the outburst period. Superposed on these flares were night-to-night variations of up to one half magnitude and intra-night microvariability of up to 0.13 magnitude in three hours. We use visual inspection of the light curve as well as numerical timescale analysis tools (the autocorrelation function, the structure function, and the power spectrum) to characterize the multiple timescales of variability ranging from 1.5 years to several hours.
The main goal of this analysis is to present a new method to estimate the physical properties of diffuse cloud of atomic hydrogen observed at high Galactic latitude. This method, based on a comparison of the observations with fractional Brownian motion simulations, uses the statistical properties of the integrated emission, centroid velocity and line width to constrain the physical properties of the 3D density and velocity fields, as well as the average temperature of HI. We applied this method to interpret 21 cm observations obtained with the Green Bank Telescope of a very diffuse HI cloud at high Galactic latitude located in Firback North 1. We first show that the observations cannot be reproduced solely by highly-turbulent CNM type gas and that there is a significant contribution of thermal broadening to the line width observed. To reproduce the profiles one needs to invoke two components with different average temperature and filling factor. We established that, in this very diffuse part of the ISM, 2/3 of the column density is made of WNM and 1/3 of thermally unstable gas (T ~2600 K). The WNM gas is mildly supersonic (<M>~1) and the unstable phase is definitely sub-sonic (<M>~0.3). The density contrast (i.e., the standard deviation relative to the mean of density distribution) of both components is close to 0.8. The filling factor of the WNM is 10 times higher that of the unstable gas, which has a density structure closer to what would be expected for CNM gas. This field contains a signature of CNM type gas at a very low level (N_H ~ 3 x 10^19) which could have been formed by a convergent flow of WNM gas.
The spectrometer aboard INTEGRAL, SPI, has the capability to detect the signature of polarised emission from a bright gamma-ray source. GRB 041219a is the most intense burst localised by INTEGRAL and is an ideal candidate for such a study. Polarisation can be measured using multiple events scattered into adjacent detectors because the Compton scatter angle depends on the polarisation of the incoming photon. A search for linear polarisation in the most intense pulse of duration 66 seconds and in the brightest 12 seconds of GRB 041219a was performed in the 100-350keV, 100-500keV and 100keV-1MeV energy ranges. The multiple event data from the spectrometer was analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT 4 INTEGRAL mass model. The chi^2 distribution between the real and simulated data as a function of the percentage polarisation and polarisation angle was calculated for all three energy ranges. The degree of linear polarisation in the brightest pulse of duration 66s was found to be 63+/-31% at an angle of 70+/-14 degrees in the 100-350keV energy range. The degree of polarisation was also constrained in the brightest 12s of the GRB and a polarisation fraction of 96+/-40% at an angle of 60+/-14 degrees was determined over the same energy range. However, despite extensive analysis and simulations, a systematic effect that could mimic the weak polarisation signal could not be definitively excluded. Our results over several energy ranges and time intervals are consistent with a polarisation signal of about 60% at a low level of significance (2 sigma). We conclude that the procedure described here demonstrates the effectiveness of using SPI as a polarimeter, and is a viable method of measuring polarisation levels in intense gamma--ray bursts.
We present the X-ray properties and scaling relations of a flux-limited morphology-unbiased sample of 12 X-ray luminous galaxy clusters at redshift around 0.2 based on XMM-Newton observations. The scaled radial profiles are characterized by a self-similar behavior at radii outside the cluster cores (>0.2 r500) for the temperature, surface brightness, entropy, gas mass and total mass. The cluster cores contribute up to 70% of the bolometric X-ray luminosity. The X-ray scaling relations and their scatter are sensitive to the presence of the cool cores. Using the X-ray luminosity corrected for the cluster central region and the temperature measured excluding the cluster central region, the normalization agrees to better than 10% for the cool core clusters and non-cool core clusters, irrelevant to the cluster morphology. No evolution of the X-ray scaling relations was observed comparing this sample to the nearby and more distant samples. With the current observations, the cluster temperature and luminosity can be used as reliable mass indicators with the mass scatter within 20%. Mass discrepancies remain between X-ray and lensing and lead to larger scatter in the scaling relations using the lensing masses (e.g. ~40$% for the luminosity--mass relation) than using the X-ray masses (<20%) due to the possible reasons discussed.
Our knowledge of the blazar surface densities and luminosity functions, which are fundamental parameters, relies still on samples at relatively high flux limits. As a result, our understanding of this rare class of active galactic nuclei is mostly based on relatively bright and intrinsically luminous sources. We present the radio number counts, evolutionary properties, and luminosity functions of the faintest blazar sample with basically complete (~ 95%) identifications. Based on the Deep X-ray Radio Blazar Survey (DXRBS), it includes 129 flat-spectrum radio quasars (FSRQ) and 24 BL Lacs down to a 5 GHz flux and power ~ 50 mJy and ~ 10^{24} W/Hz, respectively, an order of magnitude improvement as compared to previously published (radio-selected) blazar samples. DXRBS FSRQ are seen to evolve strongly, up to redshift ~ 1.5, above which high-power sources show a decline in their comoving space density. DXRBS BL Lacs, on the other hand, do not evolve. High-energy (HBL) and low-energy (LBL) peaked BL Lacs share the same lack of cosmological evolution, which is at variance with some previous results. The observed luminosity functions are in good agreement with the predictions of unified schemes, with FSRQ getting close to their expected minimum power. Despite the fact that the large majority of our blazars are FSRQ, BL Lacs are intrinsically ~ 50 times more numerous. Finally, the relative numbers of HBL and LBL in the radio and X-ray bands are different from those predicted by the so-called "blazar sequence" and support a scenario in which HBL represent a small minority (~ 10%) of all BL Lacs.
We continue to see a range of values for the Hubble constant obtained from gravitationally lensed multiple image time delays when assuming an isothermal lens despite a robust value from the HST key project (72 +- 8 km s^-1 Mpc^-1. One explanation is that there is a variation in Hubble constant values due to a fundamental heterogeneity in lens galaxies present in groups. Our goal is to see if a variety of group interactions between the most massive group members can result in significant changes in the galaxy density profiles over the scale probed by strong lensing (<15 kpc). While stripping of the outer parts of the halo can be expected, the impact on inner regions where the luminous component is important is less clear in the context of lensing, though still crucial, as a steepened density profile within this inner region allows these lens systems to be consistent with current HST/WMAP estimates on H_0. We employ the particle-mesh code SUPERBOX to carry out the group interaction simulations. We simulate interactions between group members, comparing the density profile for the satellite before and after interaction for the mass range of 10^11 to 10^13 M_sun. Our investigations show a significant steepening of the density profile in the region of < 5-20 kpc, i.e. that which dominates strong lensing in lens galaxies. Additionally, the steepening in the inner region is transient in nature, with consecutive interactions returning the profile to an isothermal state within a timeframe of ~ 0.5 - 2.0 Gyr. This factor may help explain why lens galaxies that produce lower values of H_0 (i.e. those with possibly steeper profiles) are far fewer in number than those which agree with both the HST key project value for H_0 and isothermality, since one would have to observe the lens galaxy during this transient steepened phase.
We present results from Chandra and XMM-Newton observations of the low-ionization broad absorption line (LoBAL) quasar H 1413+117. Our spatial and spectral analysis of a recent deep Chandra observation confirms a microlensing event in a previous Chandra observation performed about 5 years earlier. We present constraints on the structure of the accretion flow in H 1413+117 based on the time-scale of this microlensing event. Our analysis of the combined spectrum of all the images indicates the presence of two emission peaks at rest-frame energies of 5.35 keV and 6.32 keV detected at the > 98% and > 99% confidence levels, respectively. The double peaked Fe emission line is fit well with an accretion-disk line model, however, the best-fitting model parameters are neither well constrained nor unique. Additional observations are required to constrain the model parameters better and to confirm the relativistic interpretation of the double peaked Fe Kalpha line. Another possible interpretation of the Fe emission is fluorescent Fe emission from the back-side of the wind. The spectra of images C and D show significant high-energy broad absorption features that extend up to rest-frame energies of 9 keV and 15 keV respectively. We propose that a likely cause of these differences is significant variability of the outflow on time-scales that are shorter than the time-delays between the images. The Chandra observation of H 1413+117 has made possible for the first time the detection of the inner regions of the accretion disk and/or wind and the high ionization component of the outflowing wind of a LoBAL quasar.
We have obtained a time series of 81 high-cadence circular polarization observations of the rapidly oscillating Ap star HD 24712 with the new ESPaDOnS spectropolarimeter at CFHT. We used the high-S/N, high-resolution Stokes I and V spectra to investigate possible variation of the mean longitudinal field over the pulsation cycle in this roAp star. Our multiline magnetic field and radial velocity measurements utilized 143 spectral lines of rare-earth elements, attaining precision better than 13 G and 19 m/s, respectively. A multiperiodic radial velocity variation with an amplitude of 40-136 m/s is clearly detected at the known pulsation frequencies of HD 24712. At the same time, no evidence for pulsational changes of the magnetic field can be found. We derive a 3sigma upper limit of 10 G, or about 1% of the mean longitudinal field strength, for magnetic field oscillations in the upper atmosphere of HD 24712. The absence of detectable pulsational variability of the magnetic field provides a valuable constraint for the interaction between pulsations and magnetic field in roAp stars and is compatible with the recent predictions of detailed theoretical models of stellar magnetoacoustic oscillations.
We report the discovery of an X-ray source coincident with the nuclear star cluster at the dynamical center of the nearby late-type spiral galaxy NGC 2403. The X-ray luminosity of this source varies from below detection levels, ~1e35 erg/s in the 0.5-8.0 keV band, to 7e38 erg/s on timescales between observations of less than 2 months. The X-ray spectrum is well-fit by an accretion disk model consisting of multiple blackbody components and corresponding physically to a compact object mass of greater than approximately 5 solar masses. No pulsations nor aperiodic behavior is evident in its X-ray light curve on the short timescales of the individual observations. The X-ray properties of the source are more similar to those of the nuclear source X-8 in M33, believed to be a low-mass X-ray binary, then to those of the low-luminosity active galactic nucleus in NGC 4395. The brightness of the nuclear star cluster, M_I ~ -11.8 mag, is typical of clusters in late-type spirals but its effective radius, r_e \~ 12 pc, is several times larger than average indicating a relatively relaxed cluster and a low probability of a central massive object. The cluster has a mass 3e6 solar masses and an age of 1.4 Gyr estimating from its observed colors and brightness.
We present a 5'x5' integrated intensity map of 12CO (J=3-2) emission from the rho-Ophiuchi cloud core that traces low-luminosity outflow emission from two protostars: Elias 29 and, most likely, LFAM 26. The morphology of the outflow from Elias 29 is bipolar and has a curved axis that traces the S-shaped symmetry seen in H_2 emission. The outflow from LFAM 26 is a new detection and oriented in the east/west direction near the plane of the sky with most of the blue-shifted emission being absorbed by intervening clouds. The outflow axis of this object also appears to intersect a knot of H_2 emission previously attributed to Elias 29. LFAM 26 is a low luminosity source (L_bol = 0.06 L_sun) which, in combination with the observed outflow, makes it a candidate Very Low Luminosity Object (VeLLO). We derive lower limits to the gas column densities and energetics for both outflows. The mechanical luminosities for Elias 29 and LFAM 26 are 6.4 and 10.3 x 10^{-3} L_sun, respectively.
We present new Very Large Array (VLA) radio images at 74 and 324 MHz of the SNR W44. The VLA images, obtained with unprecedented angular resolution and sensitivity, have been used in combination with existing 1442 MHz radio data, Spitzer IR data, and ROSAT and Chandra X-ray data to investigate morphological and spectral properties of this SNR. The spatial spectral study revealed that the bright filaments, both around and across the SNR, have a straight spectrum between 74 and 1442 MHz, with alpha ~ -0.5, with two clear exceptions: a short portion of the SNR limb to the southeast, with alpha varying between 0 and +0.4 and a bright arc to the west where the spectrum breaks around 300 MHz and looks concave down. We conclude that at the shell and along the internal filaments, the electrons responsible for the synchrotron emission were accelerated at the shock according to a simple diffusive shock model; the positive spectrum corresponds to a location where the SN shock is running into a molecular cloud and where the line of sight intersects the photo dissociation region of an HII region and a young stellar object is present. The curved spectrum on the westernmost bright arc is explained as the consequence of strong post-shock densities and enhanced magnetic fields after the interaction of the SN shock with a collindant molecular cloud.
We study the saturation near threshold of the axisymmetric magnetorotational instability (MRI) of a viscous, resistive, incompressible fluid in a thin-gap Taylor-Couette configuration. A vertical magnetic field, Keplerian shear and no-slip, conducting radial boundary conditions are adopted. The weakly non-linear theory leads to a real Ginzburg-Landau equation for the disturbance amplitude, like in our previous idealized analysis. For small magnetic Prandtl number (P \ll 1), the saturation amplitude scales as P^{2/3} while the magnitude of angular momentum transport scales as P^{4/3}. The difference with the previous scalings (~P^{1/2} and P respectively) is attributed to the emergence of radial boundary layers. Away from those, steady-state non-linear saturation is achieved through a modest reduction in the destabilizing shear. These results will be useful to understand MRI laboratory experiments and associated numerical simulations.
Context: The location of coronal heating in magnetic loops has been the
subject of a long-lasting controversy: does it occur mostly at the loop
footpoints, at the top, is it random, or is the average profile uniform?
Aims: We try to address this question in model loops with MHD turbulence and
a profile of density and/or magnetic field along the loop.
Methods: We use the ShellAtm MHD turbulent heating model described in Buchlin
& Velli (2006), with a static mass density stratification obtained by the
HydRad model (Bradshaw & Mason 2003). This assumes the absence of any flow or
heat conduction subsequent to the dynamic heating.
Results: The average profile of heating is quasi-uniform, unless there is an
expansion of the flux tube (non-uniform axial magnetic field) or the variation
of the kinetic and magnetic diffusion coefficients with temperature is taken
into account: in the first case the heating is enhanced at footpoints, whereas
in the second case it is enhanced where the dominant diffusion coefficient is
enhanced.
Conclusions: These simulations shed light on the consequences on heating
profiles of the complex interactions between physical effects involved in a
non-uniform turbulent coronal loop.
The timing and duration of exoplanet transits has a dependency on observer position due to parallax. In the case of an Earth-bound observer with a 2 AU baseline the dependency is typically small and slightly beyond the limits of current timing precision capabilities. However, it can become an important systematic effect in high-precision repeated transit measurements for long period systems due to its relationship to secular perspective acceleration phenomena. In this short paper we evaluate the magnitude and characteristics of transit parallax in the case of exoplanets using simplified geometric examples. We also discuss further implications of the effect, including its possible exploitation to provide immediate confirmation of planetary transits and/or unique constraints on orbital parameters and orientations.
Using a three-dimensional nonlinear hydrodynamic code, we examine the
dynamical stability of more than twenty self-gravitating, compressible,
ellipsoidal fluid configurations that initially have the same velocity
structure as Riemann S-type ellipsoids. Our focus is on ``adjoint''
configurations, in which internal fluid motions dominate over the collective
spin of the ellipsoidal figure; Dedekind-like configurations are among this
group. We find that, although some models are stable and some are moderately
unstable, the majority are violently unstable toward the development of $m=1$,
$m=3$, and higher-order azimuthal distortions that destroy the coherent, $m=2$
bar-like structure of the initial ellipsoidal configuration on a dynamical time
scale.
The parameter regime over which our models are found to be unstable generally
corresponds with the regime over which incompressible Riemann S-type ellipsoids
have been found to be susceptible to an elliptical strain instability
\citep{LL96}. We therefore suspect that an elliptical instability is
responsible for the destruction of our compressible analogs of Riemann
ellipsoids. The existence of the elliptical instability raises concerns
regarding the final fate of neutron stars that encounter the secular bar-mode
instability and regarding the spectrum of gravitational waves that will be
radiated from such systems.
We put constraints on the velocity power spectrum shape parameter $\Gamma$ in linear theory using the nine bulk--flow and shear moments estimated from five recent peculiar velocity surveys. For each survey, a likelihood function for $\Gamma$ was found after marginalizing over the power spectrum amplitude $\Omega^{0.6}\sigma_8$ using constraints obtained from comparisons between redshift surveys and peculiar velocity data. In order to maximize the accuracy of our analyses, the velocity noise $\sigma_*$ was estimated directly for each survey. A statistical analysis of the differences between the values of the moments estimated from different surveys showed consistency with theoretical predictions, suggesting that all the surveys investigated reflect the same large scale flows. The peculiar velocity surveys were combined into a composite survey yielding the constraint $\Gamma=0.13^{+0.09}_{-0.05}$. This value is lower than, but consistent with, values obtained using redshift surveys and CMB data.
We report on time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) days, and 0.055883(3) for the 2002 May superoutburst, and the 2006 April-May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst is a normal outburst, which is the first recorded normal outburst in V844 Her. We also examined superhump period changes during 2002 May and 2006 April-May superoutbursts, both of which showed increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory in some superoutbursts, we found that V844 Her showed no precursors and rebrightenings. Based on the long-term light curve, we further confirmed V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 days. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively large X-ray luminosity of V844 Her.
We present the results of an optical and near-IR spectroscopic study of giant nebular emission line halos associated with three z > 3 radio galaxies, 4C 41.17, 4C 60.07 and B2 0902+34. Previous deep narrow band Ly-alpha imaging had revealed complex morphologies with sizes up to 100 kpc), possibly connected to outflows and AGN feedback from the central regions. The outer regions of these halos show quiet kinematics with typical velocity dispersions of a few hundred km/s, and velocity shears that can mostly be interpreted as being due to rotation. The inner regions show shocked cocoons of gas closely associated with the radio lobes. These display disturbed kinematics and have expansion velocities and/or velocity dispersions >1000 km/s. The core region is chemically evolved, and we also find spectroscopic evidence for the ejection of enriched material in 4C 41.17 up to a distance of approximately 60 kpc along the radio-axis. The dynamical structures traced in the Ly-alpha line are, in most cases, closely echoed in the Carbon and Oxygen lines. This shows that the Ly-alpha line is produced in a highly clumped medium of small filling factor, and can therefore be used as a tracer of the dynamics of high-z radio galaxies (HzRGs). We conclude that these HzRGs are undergoing a final jet-induced phase of star formation with ejection of most of their interstellar medium before becoming "red and dead" Elliptical galaxies.
We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook & Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color-selection technique, which identifies candidate binaries containing a white dwarf and a low mass stellar (or sub-stellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 28 firm candidates (20 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L type (i.e., potential brown dwarf) companion. Only one previously known WD + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios.
Optical high-resolution spectra of five central stars of planetary nebulae (CSPN) in the Galactic Bulge have been obtained with Keck/HIRES in order to derive their parameters. Since the distance of the objects is quite well known, such a method has the advantage that stellar luminosities and masses can in principle be determined without relying on theoretical relations between both quantities. By alternatively combining the results of our spectroscopic investigation with evolutionary tracks, we obtain so-called spectroscopic distances, which can be compared with the known (average) distance of the Bulge-CSPN. This offers the possibility to test the validity of model atmospheres and present date post-AGB evolution. We analyze optical H/He profiles of five Galactic Bulge CSPN (plus one comparison object) by means of profile fitting based on state of the art non-LTE modeling tools, to constrain their basic atmospheric parameters (Teff, log g, helium abundance and wind strength). Masses and other stellar radius dependent quantities are obtained from both the known distances and from evolutionary tracks, and the results from both approaches are compared. The major result of the present investigation is that the derived spectroscopic distances depend crucially on the applied reddening law. An "average extinction law" leads to a distance of 10.7 +- 1.2 kpc, which is considerably larger than the Galactic Center distance of 8 kpc. However, we find a remarkable internal agreement of the individual spectroscopic distances of our sample objects. Due to the uncertain reddening correction, the analysis presented here cannot yet be regarded as a consistency check for our method, and a rigorous test of the CSPN evolution theory becomes only possible if this problem has been solved.
Ongoing experimental efforts to detect cosmic sources of high energy neutrinos are guided by the expectation that astrophysical accelerators of cosmic ray protons would also generate neutrinos through interactions with ambient matter and/or photons. However there will be a reduction in the predicted neutrino flux if cosmic ray sources accelerate not only protons but also significant number of heavier nuclei, as is indicated by recent air shower data. We consider plausible extragalactic sources such as active galactic nuclei, gamma-ray bursts and starburst galaxies and demand consistency with the observed cosmic ray composition and energy spectrum at Earth after allowing for propagation through intergalactic radiation fields. This allows us to calculate the expected neutrino fluxes from the sources, normalised to the observed cosmic ray spectrum. We find that the likely signals are still within reach of next generation neutrino telescopes such as IceCube.
Star formation (SF) takes place in unusual places such as way out in the intergalactic medium out of material expelled from parent galaxies. We wish to answer whether SF proceeds in this specific environment in a similar way than in galactic disks. We have carried out a multiwavelength analysis of the interacting system NGC 5291, which is remarkable for its extended HI ring hosting numerous intergalactic HII regions. We combined new ultraviolet (GALEX) observations with archival Halpha, 8 mu m (Spitzer Space Telescope) and HI (VLA B-array) images of the system. We have found that the morphology of the star forming regions, as traced by the ultraviolet, Halpha, and 8 mu m emission is similar. There is a clear excess of ultraviolet emission compared to individual HII regions in spirals, i.e. the [8.0]/[NUV] and [Halpha]/[NUV] SFR ratios are on average low although there are some large variations from one region to another, which cannot be explained by variations of the metallicity or the dust extinction along the HI structure. Comparing the observed SFR with a model of the evolution of [Halpha]/[NUV] with time favours young, quasi-instantaneous though already fading starbursts. The total star formation rate measured in the intergalactic medium (which accounts for 80% of the total) surrounding NGC 5291 is up to 1.3 Msun/yr, a value typical for spirals, assuming the standard SFR calibrations are valid. The SFR drops by a factor of 2 to 4 in case the star formation is indeed quasi-instantaneous. (abridged)
We present near-infrared spectroscopic observations for a sample of ten optically faint luminous infrared galaxies (R-[24]> 14) using Keck NIRSPEC and Gemini NIRI. The sample is selected from a 24 micron Spitzer MIPS imaging survey of the NDWFS Bootes field. We measure accurate redshifts in the range 1.3<z<3.4. Based on either emission line widths or line diagnostics, we find that all ten galaxies harbor luminous AGN. Seven sources are type I AGN, exhibiting broad (>1900 km/s) Halpha or Hbeta emission lines; the remaining three are type II AGN. Given their large mid-IR luminosities and faint optical magnitudes, we might expect these sources to be heavily extincted quasars, and therefore only visible as type II AGN. The visibility of broad lines in 70% of the sources suggests that it is unlikely that these AGN are being viewed through the mid-plane of a dusty torus. For four of the sources we constrain the Halpha/Hbeta Balmer decrement and estimate the extinction to the emission line region to be large for both type I and type II AGN, with A_Halpha > 2.4-5 mag. Since the narrow-line region is also extincted and the UV continuum emission from the host galaxies is extremely faint, this suggests that much of the obscuration is contributed by dust on large (~kpc) scales within the host galaxies. These sources may be examples of "host-obscured" AGN which could have space densities comparable or greater to that of optically luminous type I AGN with similar bolometric luminosities.
We make predictions for diffuse stellar mass fractions in dark matter halos from the scales of small spiral galaxies to those of large galaxy clusters. We use an extensively-tested analytic model for subhalo infall and evolution and empirical constraints from galaxy survey data to set the stellar mass in each accreted subhalo to model diffuse light. We add stellar mass to the diffuse light as subhalos become disrupted due to interactions within their host halos. We predict that the stellar mass fraction in diffuse, intrahalo light should rise on average from ~0.5% to approximately 20% from small galaxy halos to poor groups. The trend with mass flattens considerably beyond the group scale to a near-constant fraction of roughly ~20% in galaxy clusters, although this asymptotic value may be as high as ~40%, depending on the empirical model used to populate satellite halos with stars. The mass-dependent diffuse light fraction is governed primarily by the empirical fact that the mass-to-light ratio in galaxy halos must vary as a function of halo mass. Galaxy halos have little diffuse light because they accrete most of their mass in small subhalos that themselves have high mass-to-light ratios; stellar halos around galaxies are built primarily from disrupted dwarf-irregular-type galaxies with M*~10^8.5 M_sun. The diffuse light in group and cluster halos is built from satellite galaxies that form stars efficiently and have correspondingly low mass-to-light ratios; intracluster light is dominated by material liberated from massive galaxies with M*~10^11 M_sun. Our results are consistent with existing observations spanning the galaxy, group, and cluster scale; however, they can be tested more rigorously in future deep surveys for faint diffuse light.
The Galactic O star catalog (GOS) is an ambitious project to provide as much information regarding these types of objects as possible. The first version of the catalog (GOS v1) included data for 378 stars with precise spectral classification. It was intended to be complete up to V < 8, but also included many stars fainter than that limit. In this new version, we include a second list with more than 700 stars that have sometimes been classified as O stars. The catalog includes cross-references to other catalogs, spectral classification (various references for each star on the second list), coordinates (obtained in most cases from Tycho-2 data, but some from 2MASS and USNO B1), astrometric distances for some of the nearer stars, Optical and NIR photometry (Tycho-2, Johnson, Stromgren, Cousin and 2MASS), group membership, runaway character, and multiplicity information.
New observations of CO (J=1->0) line emission from M33, using the 25 element BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK) maps to date of the distribution of molecular gas in the central 5.5 kpc of the galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong function of radius in the galaxy, declining from 60% in the center to 20% at galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly absent, although molecular gas exists. Most (90%) of the emission from low mass clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1< R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1. Inside that radius, the mass distribution is truncated, and clouds more massive than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no significant difference in the grand design spiral arms versus the interarm region. The CO surface brightness ratio for the arm to interarm regions is 1.5, typical of other flocculent galaxies.
We report observations of the ro-vibronic transition of SO at 1.707 microns on Io. These data were taken while Io was eclipsed by Jupiter, on four nights between July 2000 and March 2003. We analyze these results in conjunction with a previously published night to investigate the temporal behavior of these emissions. The observations were all conducted using the near-infrared spectrometer NIRSPEC on the W.M. Keck II telescope. The integrated emitted intensity for this band varies from 0.8 x 10^27 to 2.4 x 10^27 photons/sec, with a possible link to variations in Loki's infrared brightness. The band-shapes imply rotational temperatures of 550-1000K for the emitting gas, lending further evidence to a volcanic origin for sulfur monoxide. An attempt to detect the ro-vibronic transition of SO at 0.97 microns was unsuccessful; simultaneous detection with the 1.707 micron band would permit determination of the SO column abundance.
This paper analytically investigates a series of two-dimensional MHD reconnection solutions over a wide variation of magnetic Reynolds number ($R_{em}^*$). A new series of solutions explains a continuous transition from Petschek-like fast regime to a Sweet-Parker-like slow regime. The inflow region is obtained from a Grad-Shafranov analysis used by Nitta et al. 2002 and the outflow region from a shock-tube approximation used by Nitta 2004, 2006. A single X-point (Petschek-like) solution forms for a sufficiently small $R_{em}^*$. As $R_{em}^*$ gradually increases, the solutions shifts to an X-O-X solution with a magnetic island between two X-points. When $R_{em}^*$ increases further, the island collapses to a new elongated current sheet with Y-points at both ends (Sweet-Parker-like). These reconnection structures expand self-similarly as time proceeds. As $R_{em}^*$ increases, the reconnection rate and the reducible fraction of the initial magnetic energy of the system decrease as power-law functions of $R_{em}^*$.
The afterglows of gamma-ray bursts (GRBs) have been commonly assumed to be due to shocks sweeping up circum-stellar medium. However, most of them have been found to be hosting in dense star-forming regions where a significant fraction of the prompt X-ray emission can be scattered by dust grains. Here we revisit the behaviors of X-ray dust-scattering in GRBs. We find that the features of some X-ray afterglows about minutes to days after the gamma-ray triggers are consistent with scattering of prompt X-ray emission from GRBs off host dust grains. This implies that some of the observed X-ray afterglows (especially those without sharp rising and decaying flares) could be understood in this dust-scattering-driven emission model.
We present an X-ray analysis of the radio-quiet cool-core galaxy group NGC 4325 (z=0.026) based on Chandra and ROSAT observations. The Chandra data were analysed using XSPEC deprojection, 2D spectral mapping and forward-fitting with parametric models. Additionally, a Markov chain Monte Carlo method was used to perform a joint Bayesian analysis of the Chandra and ROSAT data. The results of the various analysis methods are compared, particularly those obtained by forward-fitting and deprojection. The spectral mapping reveals the presence of cool gas displaced up to 10 kpc from the group centre. The Chandra X-ray surface brightness shows the group core to be highly disturbed, and indicates the presence of two small X-ray cavities within 15 kpc of the group core. The XSPEC deprojection analysis shows that the group has a particularly steep entropy profile, suggesting that an AGN outburst may be about to occur. With the evidence of prior AGN activity, but with no radio emission currently observed, we suggest that the group in in a pre-outburst state, with the cavities and displaced gas providing evidence of a previous, weak AGN outburst.
We review the theory of electron-conduction opacity, a fundamental ingredient in the computation of low-mass stellar models; shortcomings and limitations of the existing calculations used in stellar evolution are discussed. We then present new determinations of the electron-conduction opacity in stellar conditions for an arbitrary chemical composition, that improve over previous works and, most importantly, cover the whole parameter space relevant to stellar evolution models (i.e., both the regime of partial and high electron degeneracy). A detailed comparison with the currently used tabulations is also performed. The impact of our new opacities on the evolution of low-mass stars is assessed by computing stellar models along both the H- and He-burning evolutionary phases, as well as Main Sequence models of very low-mass stars and white dwarf cooling tracks.
We describe two peculiar galaxies falling into the massive galaxy clusters
Abell 1689 (z~0.18) and 2667 (z~0.23) respectively.
Hubble Space Telescope images show extraordinary trails composed of bright
blue knots (-16.5<M<-11.5 mag) and stellar streams associated with each of
these systems. Combining optical, near and mid-infrared and radio observations
we prove that while both galaxies show similar extended trails of star-forming
knots, their recent star formation histories are different. One (~L*) is
experiencing a strong burst of star formation, appearing as a rare example of a
luminous infrared cluster galaxy. In comparison, the other (~ 0.1 L*) has
recently ceased its star formation activity. Our model suggests that the
morphologies and star formation in these galaxies have been influenced by the
combined action of tidal interaction (likely with the cluster potential) and of
ram pressure with the intracluster medium. These results can be used to gain
more insights to the origin of S0s, dwarf and ultra-compact dwarf (UCD) cluster
galaxies.
We have analysed the physical properties of z~4 Lyman Break Galaxies observed in the GOODS-S survey, in order to investigate the possible differences between galaxies where the Ly-alpha is present in emission, and those where the line is absent or in absorption. The objects have been selected from their optical color and then spectroscopically confirmed by Vanzella et al. (2005). From the public spectra we assessed the nature of the Ly-alpha emission and divided the sample into galaxies with Ly-alpha in emission and objects without Ly-alpha line (i.e. either absent or in absorption). We have then used the complete photometry, from U band to mid infrared from the GOODS-MUSIC database, to study the observational properties of the galaxies, such as UV spectral slopes and optical to mid-infrared colors, and the possible differences between the two samples. Finally through standard spectral fitting tecniques we have determined the physical properties of the galaxies, such as total stellar mass, stellar ages and so on, and again we have studied the possible differences between the two samples. Our results indicate that LBG with Ly-alpha in emission are on average a much younger and less massive population than the LBGs without Ly-alpha emission. Both populations are forming stars very actively and are relatively dust free, although those with line emission seem to be even less dusty on average. We briefly discuss these results in the context of recent models for the evolution of Lyman break galaxies and Ly-alpha emitters.
The TIR and FUV luminosity functions of galaxies and the related luminosity densities rho(TIR) and rho(FUV) are known to evolve from z=0 to z~1 but with a different rate: the galaxy populations appear brighter in the past at both wavelengths but the evolution in TIR is larger than in FUV. It leads to an increase of the ratio of TIR to FUV luminosity densities rho(TIR)/rho(FUV) which can be interpreted as a global increase of the dust attenuation from z=0 to z~1. Our aim is to understand the origin of this increase. We focus on infrared galaxies with L(TIR>10^{11} Lsun at z~0.7 observed by SPITZER/MIPS and we measure their UV emission at 2310 A from GALEX. These Luminous InfraRed Galaxies (LIRGs) represent the bulk of the TIR luminosity density at intermediate redshift. Some evolution of L(TIR)/L(FUV) and therefore of dust attenuation is found for these galaxies: LIRGs at z=0.7 span a larger range of L(TIR)/L(FUV) ratios than at z=0 and their mean dust attenuation at FUV is found ~0.5 mag lower than for their local counterparts. The decrease of dust attenuation is found lower than that reported in other studies for bright galaxies selected in UV rest-frame at z=1 and 2. From a semi-quantitative analysis we find that the slight decrease of dust attenuation for LIRGs at z=0.7 remains consistent with the increase of rho(TIR)/rho(FUV) with redshift.
In terms of the quark-gluon string model the analysis of the classic procedure to estimate the energy of giant air showers with help of the parameter s(600) (a density of energy deposition in the scintillator at a distance of 600 m from the shower core) have been carried out. Simulations of the signal s(600) with help of the CORSIKA code in terms of the hybrid scheme show energy estimates which are approximately a factor of 1.6 times lower than adopted at the Yakutsk array. The energy estimates calculated with the help of the Cherenkov radiation coincide with the experimental data. Simulations of deposited energy distributions in the atmosphere with help of the GEANT4 code and the CORSIKA code show that more than 20% of this energy may be deposited at distances above 100 m from the shower axis.
A general formalism for calculating the bulk viscosity of strange quark matter is developed. Contrary to the common belief that the non-leptonic processes alone give the dominant contribution to the bulk viscosity, the inclusion of the Urca processes is shown to play an important role at intermediate densities when the characteristic r-mode oscillation frequencies are not too high. The interplay of non-leptonic and Urca processes is analyzed in detail
We carried out a 5-pointing mosaic observation of TeV J2032+4130 at 1.4 and 4.8 GHz with the VLA in April of 2003. The analysis of the 4.8GHz data indicate weak wispy shell-like radio structure(s) which are at least partially non-thermal. The radio data is compatible with one or more young supernova remnants or perhaps the signature of large scale cluster shocks in this region induced by the violent action of the many massive stars in Cyg OB2.
We study two dimensional Fabry-Perot interferometric observations of the nearby face-on late-type spiral galaxy, NGC 628, in order to analyse the ionized gas component of the interstellar medium. Covering the galaxy out to a radius larger than 12 kpc, and with a spatial sampling of 1.6 arcsec, we investigate the large-scale dynamics as well as feedback from individual HII regions into their surrounding medium. We study the role of gravitational perturbations along with that of external triggers which can disturb the kinematics and morphology of NGC 628. We verify the presence of an inner rapidly rotating disc-like component in NGC 628, which we interpret as caused by slow secular evolution of the large-scale spiral arms and oval structure. In combination with auxiliary data, we find indication for that gas is falling in from the outer parts towards the central regions, where a nuclear ring has formed at the location of the inner Lindblad resonance radius of an m=2 perturbation which could help build a pseudo-bulge in NGC 628. Moreover, we calculate radial profiles of the emission-line velocity dispersion which we use to study the role of feedback from individual HII regions. The mean velocity dispersion for the ionized gas (even when excluding pixels belonging to individual HII regions) is almost constant out to 12 kpc, although it varies from 14 to 20 km/s, with a steady decline in the outer parts. The current paper demonstrates a number of tools that we have developed for building a solid frame work for studying the evolution of structure in spiral galaxies using two dimensional kinematic observations.
Radio-quiet gamma-ray pulsars like Geminga may account for a number of the unidentified EGRET sources in the Galaxy. The number of Geminga-like pulsars is very sensitive to the geometry of both the gamma-ray and radio beams. Recent studies of the shape and polarization of pulse profiles of young radio pulsars have provided evidence that their radio emission originates in wide cone beams at altitudes that are a significant fraction (1 -10%) of their light cylinder radius. Such wide radio emission beams will be visible at a much larger range of observer angles than the narrow core components thought to originate at lower altitude. Using 3D geometrical modeling that includes relativistic effects from pulsar rotation, we study the visibility of such radio cone beams as well as that of the gamma-ray beams predicted by slot gap and outer gap models. From the results of this study one can obtain revised predictions for the fraction of Geminga-like, radio quiet pulsars present in the gamma-ray pulsar population.
We report on our analysis of a 20 ksec Chandra X-ray observation of the quasi-persistent neutron star soft X-ray transient (SXT) 1M1716-315 in quiescence. Only one source was detected in the HEAO-I error region. Its luminosity is 1.6E32-1.3E33 erg s-1. In this the range is dominated by the uncertainty in the source distance. The source spectrum is well described by an absorbed soft spectrum, e.g. a neutron star atmosphere or black body model. No optical or near-infrared counterpart is present at the location of the X-ray source, down to a magnitude limit of I> 23.5 and K_s> 19.5. The positional evidence, the soft X-ray spectrum together with the optical and near-infrared non-detections provide strong evidence that this source is the quiescent neutron star SXT. The source is 10-100 times too bright in X-rays in order to be explained by stellar coronal X-ray emission. Together with the interstellar extinction measured in outburst and estimates for the source distance, the reported optical and near-infrared limit give an upper limit on the absolute magnitude of the counterpart of I>8.6 and K_s>5.1. This implies that the system is either an ultra-compact X-ray binary having P_orb<1 hr or the companion star is an M-dwarf. We reconstructed the long term X-ray lightcurve of the source. 1M1716-315 has been active for more than 12 years before returning to quiescence, the reported Chandra observation started 16.9+-4.1 years after the outburst ended.
The penumbra is ideally suited to challenge our understanding of magnetohydrodynamics. The energy transport takes place as magnetoconvection in inclined magnetic fields under the effect of strong radiative cooling at the surface. The relevant processes happen at small spatial scales. In this contribution we describe and elaborate on these small-scale inhomogeneities of a sunspot penumbra. We describe the penumbral properties inferred from imaging, spectroscopic and spectropolarimetric data, and discuss the question of how these observations can be understood in terms of proposed models and theoretical concepts.
We present the complete data set, model and line identification of a survey of the emission from the C-rich protoplanetary nebula CRL 618 performed with the IRAM-30m telescope in the following frequency ranges: 80.25-115.75 GHz, 131.25-179.25 GHz, and 204.25-275.250 GHz. A selection of lines from different species has been used in previous works to derive the structure of the source, its physical conditions and the chemical abundances in the different gas regions. In this work, we have used this information to run a global simulation of the spectrum in order to check the consistency of the model and to ease the task of line identification. The total number of lines that have a correspondence in both data and model is ~3100, although quite often in this object many lines blend into complex features so that the model, that takes into account line blending, is a key tool at this stage of the analysis. Of all the lines that we have been able to label, ~55% of them belong to the different forms of HC3N, and ~18% to those of HC5N. The density of remaining unidentified features above the 3sigma limit is only one per ~2.1 GHz (74 features), which is unprecedented in the analysis of this type of large millimeter-wave line surveys.
We study the extinction efficiencies as well as scattering properties of
particles of different porosity. Calculations are performed for porous
pseudospheres with small size (Rayleigh) inclusions using the discrete dipole
approximation. Five refractive indices of materials covering the range from
$1.20+0.00i$ to $1.75+0.58i$ were selected. They correspond to biological
particles, dirty ice, silicate, amorphous carbon and soot in the visual part of
spectrum. We attempt to describe the optical properties of such particles using
Lorenz-Mie theory and a refractive index found from some effective medium
theory (EMT) assuming the particle is homogeneous. We refer to this as the
effective model.
It is found that the deviations are minimal when utilizing the EMT based on
the Bruggeman mixing rule. Usually the deviations in extinction factor do not
exceed $\sim 5%$ for particle porosity ${\cal P}=0 - 0.9$ and size parameters
$x_{\rm porous} = 2 \pi r_{\rm s, porous}/\lambda \la 25$. The deviations are
larger for scattering and absorption efficiencies and smaller for particle
albedo and asymmetry parameter. Our calculations made for spheroids confirm
these conclusions. Preliminary consideration shows that the effective model
represents the intensity and polarization of radiation scattered by fluffy
aggregates quite well. Thus, the effective models of spherical and
non-spherical particles can be used to significantly simplify computations of
the optical properties of aggregates containing only Rayleigh inclusions.
Over the past two years, the search for low-mass extrasolar planets has led to the detection of seven so-called 'hot Neptunes' or 'super-Earths' around Sun-like stars. These planets have masses 5-20 times larger than the Earth and are mainly found on close-in orbits with periods of 2-15 days. Here we report a system of three Neptune-mass planets with periods of 8.67, 31.6 and 197 days, orbiting the nearby star HD 69830. This star was already known to show an infrared excess possibly caused by an asteroid belt within 1 AU (the Sun-Earth distance). Simulations show that the system is in a dynamically stable configuration. Theoretical calculations favour a mainly rocky composition for both inner planets, while the outer planet probably has a significant gaseous envelope surrounding its rocky/icy core; the outer planet orbits within the habitable zone of this star.
Star clusters projected onto dense stellar fields require field star decontamination to confirm their nature and derive intrinsic photometric and structural fundamental parameters. The present work focusses on the OCs or candidates NGC5715, Lynga4,9, Trumpler23,26 and Czernik37 which are projected against crowded bulge and/or disc fields. We develop a CMD field star decontamination algorithm based on 2MASS data, and respective uncertainties, to detect cluster star excesses over the background. On the other hand, colour-magnitude filters are used to build stellar radial density profiles and mass functions. The results convey compelling evidence that Ly9 and Cz37 are intermediate-age open clusters, and their fundamental parameters are measured for the first time. Tr23 is a particularly challenging object, since besides high background level, its field presents variable absorption in near-IR bands. We confirm it to be an intermediate-age open cluster. These OCs are located 0.9 - 1.6kpc within the Solar Circle, with ages similar to that of the Hyades. Structurally, they are well described by King profiles. In all cases, core and limiting radii are significantly smaller than those of nearby OCs outside the Solar circle. We test the effect of background levels on cluster radii determinations by means of simulations. They indicate that for central clusters, radii may be underestimated by about 10--20%, which suggests that the small sizes measured for the present sample reflect as well intrinsic properties related to dynamical evolution effects. The objects probably have been suffering important tidal effects that may have accelerated dynamical evolution, especially in Cz37, the innermost object.
We analyze 1187 observations of about 860 unique candidate Jovian Trojan asteroids listed in the 3rd release of Sloan Digital Sky Survey (SDSS) Moving Object Catalog. The sample is complete at the faint end to r=21.2 mag (apparent brightness) and H=13.8 (absolute brightness, approximately corresponding to 10 km diameter). A subset of 297 detections of previously known Trojans were used to design and optimize a selection method based on observed angular velocity that resulted in the remaining objects. Using a sample of objects with known orbits, we estimate that the candidate sample contamination is about 3%. The well-controlled selection effects, the sample size, depth and accurate five-band UV-IR photometry enabled several new findings and the placement of older results on a firmer statistical footing. We find that there are significantly more asteroids in the leading swarm (L4) than in the trailing swarm (L5): N(L4)/N(L5)=1.6$\pm$0.1, independently of limiting object's size. The overall counts normalization suggests that there are about as many Jovians Trojans as there are main-belt asteroids down to the same size limit, in agreement with earlier estimates. We find that Trojan asteroids have a remarkably narrow color distribution (root-mean-scatter of only $\sim$0.05 mag) that is significantly different from the color distribution of the main-belt asteroids. The color of Trojan asteroids is correlated with their orbital inclination, in a similar way for both swarms, but appears uncorrelated with the object's size. We extrapolate the results presented here and estimate that Large Synoptic Survey Telescope will determine orbits, accurate colors and measure light curves in six photometric bandpasses for about 100,000 Jovian Trojan asteroids.
Planets in extrasolar systems tend to interact such that their orbits lie near a boundary between apsidal libration and circulation, a "separatrix", with one eccentricity periodically reaching near-zero. One explanation, applied to the Upsilon Andromedae system, assumed three original planets on circular orbits. One is ejected, leaving the other two with near-separatrix behavior. We test that model by integrating hundreds of hypothetical, unstable planetary systems that eject a planet. We find that the probability that the remaining planets exhibit near-separatrix motion is small (< 5% compared with nearly 50% of observed systems). Moreover, while observed librating systems are evenly divided between aligned and anti-aligned pericenter longitudes, the scattering model strongly favors alignment. Alternative scattering theories are proposed, which may provide a more satisfactory fit with observed systems.