We present a model of the stellar populations in the fields observed by one
of the SuperWASP-N cameras in the 2004 observing season. We use the Besancon
Galactic model to define the range of stellar types and metallicities present,
and populate these objects with transiting extra-solar planets using the
metallicity relation of Fischer & Valenti (2005).
We investigate the ability of SuperWASP to detect these planets in the
presence of realistic levels of correlated systematic noise (`red noise'). We
find that the number of planets that transit with a signal-to-noise ratio of 10
or more increases linearly with the number of nights of observations. Based on
a simulation of detection rates across 20 fields observed by one camera, we
predict that a total of 18.6 \pm 8.0 planets should be detectable from the
SuperWASP-N 2004 data alone. The best way to limit the impact of co-variant
noise and increase the number of detectable planets is to boost the
signal-to-noise ratio, by increasing the number of observed transits for each
candidate transiting planet. This requires the observing baseline to be
increased, by spending a second observing season monitoring the same fields.
We report on the identification of the lens responsible for microlensing event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields, we discovered a significant flux excess at the position of the source star for this event. These data, in combination with high resolution near-infrared \textit{Magellan}/PANIC data has allowed us to classify the lens as an early M dwarf in the thick disk of the Milky Way, at a distance of $\sim 2$ kpc. This is only the second microlens to have been identified, the first also being a M dwarf star in the disk. Together, these two events are still consistent with the expected frequency of nearby stars in the Milky Way thin and thick disks acting as lenses.
We examine the origin and evolution of the mass-metallicity (M-Z) relationship for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of the effects of supernovae feedback and subsequent metal enrichment. Our simulations allow us to distinguish between two possible sources that contribute to both the origin of the mass-metallicity relationship and to the low chemical yield observed in low galaxy masses: 1) metal loss due to gas outflow, or 2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies are in excellent agreement with the observed M-Z relationship, both at z=0 and z=2. We find that gas mass loss becomes increasingly important at decreasing galaxy masses for our simulations, This mass loss results in a low effective yield for our lowest mass galaxies in good agreement with observational results. By considering all the gas that has ever belonged to a galaxy (back to z=3), we find the metallicity is unchanged from the measured value from cold gas at z=0, while the effective yield increases to an asymptotic value as would be expected in a closed box model. Hence we show that mass loss does not effect the metallicity of the low mass systems, or subsequently the M-Z relation, while the observed low effective yields for low mass systems are mostly due to mass loss. Instead, low star formation efficiencies, regulated by supernovae feedback, are primarily responsible for the M-Z trend. We find that the shape of the M-Z relation is relatively constant with redshift, but that the normalization increases with time. Simulations with no supernovae feedback fail to reproduce the observed trends.
Using ray tracing for simple analytic profiles, we demonstrate that the lensing cross section for producing giant arcs has distinct contributions due to arcs formed through image distortion only, and arcs form from the merging of two or three images. We investigate the dependence of each of these contributions on halo ellipticity and on the slope of the density profile, and demonstrate that at fixed Einstein radius, the lensing cross section increases as the halo profile becomes steeper. We then compare simulations with and without baryonic cooling of the same cluster for a sample of six clusters, and demonstrate that cooling can increase the overall abundance of giant arcs by factors of a few. The net boost to the lensing probability for individual clusters is mass dependent, and can lower the effective low mass limit of lensing clusters. This last effect can potentially increase the number of lensing clusters by an extra 50%. While the magnitude of these effects may be overestimated due to the well known overcooling problem in simulations, it is evident that baryonic cooling has a non-negligible impact on the expected abundance of giant arcs, and hence cosmological constraints from giant arc abundances may be subject to large systematic errors.
We considered kinematic and virial parameters of a sample of galaxy triplets from the Local Supercluster (LS). The galaxies were selected from the LEDA database, and their radial velocities are no more than 3100 km/s. The sample contains 176 systems selected according to the Karachentsev criterion. We calculated the following median parameters of the LS triplets: rms velocity of galaxies with respect to triplet center, Sv=30 km/s; harmonic mean radius, Rh=160 kpc; virial mass Mvir=3.6x10^11 Msun; and mass-to-light ratio, Mvir/L=35 Msun/Lsun. Although various investigators used different approaches to compile samples of triple systems, general properties of the triplets in all the samples we considered agree rather satisfactorily. The LS triplets have the least mass-to-light ratio as compared to other galaxy triplet samples.
The extended X-ray emission from 'the Cap' region located 11' (11.6 kpc) above the disk of the starburst galaxy M82 has been observed with Suzaku and XMM-Newton. Owing to the good energy resolution and the large collecting area of the XIS on Suzaku, combined with similar properties of the EPIC instrument on XMM-Newton, we have clearly detected K-shell emission lines from O VII, O VIII, Ne X, Mg XI, Mg XII and the Fe-L complex. Two optically-thin thermal plasma components are required to fit the observed X-ray spectra. We have determined the metal abundances of O, Ne, Mg, Si and Fe in this region for the first time. Their metal abundance ratios agree well with those of metal-poor stars and the model prediction of metals synthesized by type-II supernovae, but they are not consistent with the metallicities of type-Ia supernovae. This result is support for the idea that the origin of the metals in the Cap is type-II supernovae explosions occurring in the starburst regions in the M82 galaxy. We discuss the possible contribution from sputtered dust grains to the metals in the Cap. An emission line consistent with the C VI transition of n=4 to 1 at 0.459 keV is marginally detected, although it is not statistically significant at the 99% confidence level; the presence of this line would suggest charge-exchange processes in the Cap.
The purpose of this thesis is to focus attention on the mechanical aspects in
designing an infrared telescope, IRAIT (International Robotic Antarctic
Infrared Telescope), with aperture size of 80 cm, f#=21, entirely robotic and
remote controlled, which must operate at Dome C, on Antarctic Plateau, starting
on Summer 2007. Before illustrating in detail the choice criteria of different
mechanical components, in order to satisfy stress requirements and structural
verification, and the final design solutions we have adopted,firstly a few
tissues must be considered. They mainly concern the preference for Dome C as
probably the best observing site in the world, the scientific targets,
instruments and tools necessary to reach such goals.
The mechanical structure of telescope has been analyzed, and results retrieved
by the static and dynamic analysis through a finite element software are
illustrated. They concern the behavior of single parts, subassemblies and
overall structure to active loads applied. It is shown that, as a matter of
fact, thermal stress can be reckoned as the most influent of all static loads.
A dynamic analysis of some critical subassemblies was used,in order to
determine the frequency response of the system aiming at its best insulation
from vibrations.
We have identified a weak thermal line U42.767, which has been detected only in the directions toward Sgr A and Sgr B2, as the HOCO+ 2_{02}--1_{01} transition. Because of the proximity of this line to the SiO maser line at 42.821 GHz (J=1-0 v=2),it was observable simultaneously in the ~43 GHz SiO maser searches at Nobeyama. From the past data of SiO maser surveys of infrared objects in the Galactic center, we created a map of emission distribution of HOCO+ in the Sgr A molecular cloud as well as maps of the 29SiO J=1-0 v=0 thermal emission and H53alpha emission. The emission distribution of HOCO+ was quite similar to the distribution of 29SiO emission. It suggests that the enhancement of the HOCO+ abundance in the galactic center is induced by shock activities which release the CO2 molecules frozen on grains into gases.
Using available data for C and M giants with Mbol<-3.6 in Magellanic Cloud clusters, we derive limits to the lifetimes of the corresponding evolutionary phases, as a function of stellar mass. The C-star phase is found to have a duration between 2 and 3 Myr for stars in the mass range from 1.5 to 2.8 Msun. There is also an indication that the peak of C-star lifetime shifts to lower masses (from slightly above to slightly below 2 Msun) as we move from LMC to SMC metallicities. The M-giant lifetimes also peak at 2 Msun in the LMC, with a maximum value of about 4 Myr, whereas in the SMC their lifetimes appear much shorter but, actually, they are poorly constrained by the data. These numbers constitute useful constraints to theoretical models of the TP-AGB phase. We show that several models in the literature underestimate the duration of the C-star phase at LMC metallicities.
We address the occurrence of narrow planetary rings and some of their structural properties, in particular when the rings are shepherded. We consider the problem as Hamiltonian {\it scattering} of a large number of non-interacting massless point particles in an effective potential. Using the existence of stable motion in scattering regions in this set up, we describe a mechanism in phase space for the occurrence of narrow rings and some consequences in their structure. We illustrate our approach with three examples. We find eccentric narrow rings displaying sharp edges, variable width and the appearance of distinct ring components (strands) which are spatially organized and entangled (braids). We discuss the relevance of our approach for narrow planetary rings.
In the last few years our knowledge of the physics of massive stars has improved tremendously. However, further investigations are still needed, especially regarding accurate calibrations of their fundamental parameters. To this end, we have constructed a comprehensive grid of NLTE model atmospheres and corresponding synthetic spectra in the massive star domain. The grid covers the complete B type spectral range, extended to late O on the hot side and early A on the cool side, from supergiants to dwarfs and from weak stellar winds to very strong ones. It has been calculated with the latest version of the FASTWIND code. The analysis of an extensive sample of OB stars in the framework of the COROT space mission will lead to accurate calibrations of effective temperatures, gravities, mass loss rates etc. This paper contains a detailed description of the grid, which has been baptised as BSTAR06 and which will be available for further research in the near future.
Using the Millennium N-body simulation we explore how the shape and angular momentum of galaxy dark matter haloes surrounding the largest cosmological voids are oriented. We find that the major and intermediate axes of the haloes tend to lie parallel to the surface of the voids, whereas the minor axis points preferentially in the radial direction. We have quantified the strength of these alignments at different radial distances from the void centres. The effect of these orientations is still detected at distances as large as 2.2 R_void from the void centre. Taking a subsample of haloes expected to contain disc-dominated galaxies at their centres we detect, at the 99.9% confidence level, a signal that the angular momentum of those haloes tends to lie parallel to the surface of the voids. Contrary to the alignments of the inertia axes, this signal is only detected in shells at the void surface (1<R<1.07 R_void) and disappears at larger distances. This signal, together with the similar alignment observed using real spiral galaxies (Trujillo, Carretero & Patiri 2006), strongly supports the prediction of the Tidal Torque theory that both dark matter haloes and baryonic matter have acquired, conjointly, their angular momentum before the moment of turnaround.
Context: The context of this paper is buoyant toroidal magnetic flux ropes, which is a part of flux tube dynamo theory and the framework of solar-like magnetic activity. Aims: The aim is to investigate how twisted magnetic flux ropes interact with a simple magnetized stellar model envelope--a magnetic "convection zone"--especially to examine how the twisted magnetic field component of a flux rope interacts with a poloidal magnetic field in the convection zone. Method: Both the flux ropes and the atmosphere are modelled as idealized 2.5-dimensional concepts using high resolution numerical magneto-hydrodynamic (MHD) simulations. Results: It is illustrated that twisted toroidal magnetic flux ropes can interact with a poloidal magnetic field in the atmosphere to cause a change in both the buoyant rise dynamics and the flux rope's geometrical shape. The details of these changes depend primarily on the polarity and strength of the atmospheric field relative to the field strength of the flux rope. It is suggested that the effects could be verified observationally.
Type Ia supernovae (SNe Ia) are one of the major tools to determine the cosmological parameters. Utilizing them as distance indicators, it is possible to geometrically survey the universe. To this end, the intrinsic scatter in the luminosities of these events needs to be calibrated using empirical relations between observables. A theoretical explanation for these relations is still lacking and can only be provided by a sound understanding of the mechanism of SNe Ia. Recently there has been significant progress in modeling SNe Ia. We report on numerical simulations of the explosion process, compare the results with observations of nearby SNe Ia, and discuss current uncertainties of the models. The presented simulations shed some light on the origin of the diversity of SNe Ia. Such simulations will pave the way towards an understanding of SN Ia diversities and correlations of their properties and ultimately provide a tool to validate the cosmological implications of SN Ia distance measurements.
Context: Sunspot penumbrae harbor highly structured magnetic fields and
flows. The moving flux tube model offers an explanation for several observed
phenomena, e.g. the Evershed effect and bright penumbral grains.
Aims: A wealth of information can be extracted from spectropolarimetric
observations. In order to deduce the structure of the magnetic field in sunspot
penumbrae, detailed forward modeling is necessary. On the one hand, it gives
insight into the sensitivity of various spectral lines to different physical
scenarios. On the other hand, it is a very useful tool to guide inversion
techniques. In this work, we present a generalized 3D geometrical model that
embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere.
Methods: The new semi-analytical geometric model serves as a frontend for a
polarized radiative transfer code. The advantage of this model is that it
preserves the discontinuities of the physical parameters across the flux tube
boundaries. This is important for the detailed shape of the emerging Stokes
Profiles and the resulting net circular polarization (NCP).
Results: (a) The inclination of downflows in the outer penumbra must be
shallower than approximately 15 degrees. (b) Observing the limb-side NCP of
sunspots in the Fe I 1564.8 nm line offers a promising way to identify a
reduced magnetic field strength in flow channels. (c) The choice of the
background atmosphere can significantly influence the shape of the Stokes
profiles, but does not change the global characteristics of the resulting NCP
curves for the tested atmospheric models.
We show that isotropic component of circular polarization of stochastic gravitational wave background can be explored by breaking two dimensional configuration of multiple laser interferometers for correlation analysis. By appropriately selecting orbital parameters for the proposed BBO mission, the circular polarization degree Pi can be measured down to Pi ~ 0.08 (10^{-15}/Omega_{GW})(SNR/5) with slightly (~10%) sacrificing the detection limit for the total intensity Omega_{GW} compared to the standard plane symmetric configuration. This might allow us to detect signature of parity violation in the very early universe.
We present long-term (~10^4 M) axisymmetric simulations of differentially rotating, magnetized neutron stars in the slow-rotation, weak magnetic field limit using a perturbative metric evolution technique. Although this approach yields results comparable to those obtained via nonperturbative (BSSN) evolution techniques, simulations performed with the perturbative metric solver require about 1/4 the computational resources at a given resolution. This computational efficiency enables us to observe and analyze the effects of magnetic braking and the magnetorotational instability (MRI) at very high resolution. Our simulations demonstrate that (1) MRI is not observed unless the fastest-growing mode wavelength is resolved by more than about 10 gridpoints; (2) as resolution is improved, the MRI growth rate converges, but due to the small-scale turbulent nature of MRI, the maximum growth amplitude increases, but does not exhibit convergence, even at the highest resolution; and (3) independent of resolution, magnetic braking drives the star toward uniform rotation as energy is sapped from differential rotation by winding magnetic fields.
Mid-resolution spectra are used to deduce the size and kinematics of the coronal gas in a sample of Seyfert galaxies by means of observations of the [FeXI], [FeX], [FeVII], [SiVI] and [SiVII] lines. These coronal lines (CL) extend from the unresolved nucleus up to a few tens to a few hundreds of parsecs. The region of the highest ionized ions studied, [FeXI] and [FeX], is the least spatially extended, and concentrates at the center; intermediate ionization lines extend from the nucleus up to a few tens to a few hundred parsecs; lower [OIII]-like ions are known to extendin the kpc range. All together indicates a stratification in the ionized gas, usually interpreted in terms of nuclear photoionization as the driving ionization mechanism. However, CL profiles show various peculiarities: they are broader by a factor of two than lower ionization lines, the broadening being in terms of asymmetric blue wings, and their centroid position at the nucleus is blueshifted by a few hundreds of km/s. Moreover, in NGC1386 and NGC1068, a double peak [FeVII] line is detected in the nuclear and extended coronal region, this being the first report in of such type of profile in CL in active galactic nuclei. If interpreted as outflow signatures, the total broadening of the lines at zero intensity levels implies gas velocities up to 2000 km/s. Although the stratification of ions across the coronal region means that photoionization is the main power mechanism, the high velocities deduced from the profiles, the relatively large spatial extension of the emission, and the results from photoionization models indicate that an additional mechanism is at work. We suggest that shocks generated by the outflow could provide the additional required power for line formation.
We compare observations of the high redshift galaxy population to the predictions of the galaxy formation model of Croton et al. (2006). This model, implemented on the Millennium Simulation of the concordance LCDM cosmogony, introduces "radio mode" feedback from the central galaxies of groups and clusters in order to obtain quantitative agreement with the luminosity, colour, morphology and clustering properties of the low redshift galaxy population. Here we compare the predictions of this same model to the observed counts and redshift distributions of faint galaxies, as well as to their inferred luminosity and mass functions out to redshift 5. With the exception of the mass functions, all these properties are sensitive to modelling of dust obscuration. A simple but plausible treatment gives moderately good agreement with most of the data, although the predicted abundance of relatively massive (~M*) galaxies appears systematically high at high redshift, suggesting that such galaxies assemble earlier in this model than in the real Universe. An independent galaxy formation model implemented on the same simulation matches the observed mass functions slightly better, indicating that the discrepancy probably reflects incomplete galaxy formation physics rather than problems with the underlying cosmogony.
This paper presents a novel method for determining the probability that a supernova candidate belongs to a known supernova type (such as Ia, Ibc, IIL, \emph{etc.}), using its photometric information alone. It is validated with Monte Carlo, and both space- and ground- based data. We examine the application of the method to well-sampled as well as poorly sampled supernova light curves. Central to the method is the assumption that a supernova candidate belongs to a group of objects that can be modeled; we therefore discuss possible ways of removing anomalous or less well understood events from the sample. This method is particularly advantageous for analyses where the purity of the supernova sample is of the essence, or for those where it is important to know the number of the supernova candidates of a certain type (\emph{e.g.}, in supernova rate studies).
The bright K1 III-IV star gamma Cep has been reported previously to have a companion in a 2.5-yr orbit that is possibly substellar, and also has a stellar companion at a larger separation that has never been seen. Here we determine for the first time the three-dimensional orbit of the stellar companion accounting also for the perturbation from the closer object. We combine new and existing radial velocity measurements (of both classical precision and high precision) with intermediate astrometric data from the Hipparcos mission (abscissa residuals) as well as ground-based positional observations going back more than a century. The orbit of the secondary star is eccentric and has a period of 66.8 yr. We establish the primary star to be on the first ascent of the giant branch, and to have a mass of 1.18 M_Sun, an effective temperature of 4800 K, and an age around 6.6 Gyr (for an assumed metallicity [Fe/H] = +0.01). The unseen secondary star is found to be an M4 dwarf with a mass of 0.362 M_Sun, and is expected to be about 6.4 mag fainter than the primary in K. The minimum mass of the putative planetary companion is M_p sin i = 1.43 M_Jup. Taking advantage again of the high-precision Hipparcos observations we are able to place a dynamical upper limit on this mass of 13.3 M_Jup (95% confidence level) and 16.9 M_Jup (99.73%, or 3 sigma) thus confirming that it is indeed substellar in nature. (Abridged)
The use of type Ia supernovae (SNe Ia) as cosmological standard candles is
fundamental in modern observational cosmology. In this letter, we derive a
simple empirical photometric redshift estimator for SNe Ia using a training set
of SNe Ia with multiband ($griz$) light-curves and spectroscopic redshifts
obtained by the Supernova Legacy Survey (SNLS). This estimator is analytical
and model-independent; it does not use spectral templates. We use all the
available SNe Ia from SNLS with near maximum photometry in $griz$ (a total of
40 SNe Ia) to train and test our photometric redshift estimator. The difference
between the estimated redshifts $z_{phot}$ and the spectroscopic redshifts
$z_{spec}$, $(z_{phot}-z_{spec})/(1+z_{spec})$, has rms dispersions of 0.031
for 20 SNe Ia used in the training set, and 0.050 for 20 SNe Ia not used in the
training set. The dispersion is of the same order of magnitude as the flux
uncertainties at peak brightness for the SNe Ia. There are no outlyers.
This photometric redshift estimator should significantly enhance the ability
of observers to accurately target high redshift SNe Ia for spectroscopy in
ongoing surveys. It will also dramatically boost the cosmological impact of
very large future supernova surveys, such as those planned for Advanced
Liquid-mirror Probe for Astrophysics, Cosmology and Asteroids (ALPACA), and the
Large Synoptic Survey Telescope (LSST).
(abridged) We use a one-dimensional hydrodynamical code to investigate the effects of preheating on gas accretion and cooling in cold dark matter halos. In the absence of radiative cooling, preheating reduces the amount of gas that can be accreted into a halo, and the accreted gas fraction is determined by the ratio of the initial specific entropy of the gas to the virial entropy of the halo. In the presence of radiative cooling, preheating affects the gas fraction that can cool in two different ways. For small halos with masses <10^12Msun, preheating suppresses gas accretion, but most of the accreted gas can cool. For more massive halos, preheating not only reduces the amount of accreted gas, but also reduces the cooling efficiency. For both small and massive halos, gas cooling is delayed by preheating and in an inside-out fashion if the halo gas is assumed to be a single-phase medium. However, cooling can occur over a wider range of redshifts and radii, if we assume a multi-phase medium. As examples, two specific preheating cases are investigated. In the first case, the preheating entropy is assumed to be proportional to the virial entropy of the halo, as expected from AGN feedback. Such preheating effectively suppresses radiative cooling in halos with M>10^13Msun. We suggest that this may be the reason why the stellar mass function of galaxies breaks sharply at the massive end. Such preheating also helps create the hot diffused halos within which the "radio mode" feedback of AGNs can act effectively. In the second case, we assume the intergalactic medium is warm. Here the total amount of gas that can cool in a halo scales with halo mass as ~M^2, as would be required to match the observed stellar- and HI-mass functions in the current CDM model at the small mass end.
As a neutron star spins down, the nuclear matter continuously is converted into quark matter due to the core density increase and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of change rate of deconfinement baryon number. Numerical solutions to cooling equation are obtained to be very different from the without heating effect. The results show that neutron stars may be heated to higher temperature which is well-matched with pulsar's data despite onset of fast cooling in neutron stars with quark matter core. It is also found that heating effect has magnetic field strength dependence. This feature could be particularly interesting for high temperature of low-field millisecond pulsar at late stage. The inferred temperature from observation for PSR J0437-4715 is just an illustration in quality.
There is still no consensus about progenitor masses of Type IIP supernovae. We study a normal Type IIP SN 1999em in detail and compare it to a peculiar Type IIP SN 1987A. We computed the hydrodynamic and time-dependent atmosphere models interpreting simultaneously both the photometric and spectroscopic observations. The bolometric light curve of SN 1999em and the spectral evolution of its H-alpha line are consistent with a presupernova radius of 500 Rsun, an ejecta mass of 19.0 Msun, an explosion energy of 1.3x10^51 erg, and a radioactive 56Ni mass of 0.036 Msun. A mutual mixing of hydrogen-rich and helium-rich matter in the inner layers of the ejecta guarantees a good fit of the calculated light curve to that observed. Based on the hydrodynamic models in the vicinity of the optimal model, we derive the approximate relationships between the basic physical and observed parameters. We find that the hydrogen recombination in the atmosphere of a normal Type IIP SN 1999em, as well as most likely other Type IIP supernovae at the photospheric epoch, is essentially a time-dependent phenomenon. It is also shown that in normal Type IIP supernovae the homologous expansion of the ejecta in its atmosphere takes place starting from nearly the third day after the supernova explosion. A comparison of SN 1999em with SN 1987A reveals two very important results for supernova theory. First, the comparability of the helium core masses and the explosion energies implies a unique explosion mechanism for these core collapse supernovae. Second, the optimal model for SN 1999em is characterized by a weaker 56Ni mixing up to 660 km/s compared to a moderate 56Ni mixing up to 3000 km/s in SN 1987A, hydrogen being mixed deeply downward to 650 km/s.
We present the first comparison of profiles of H et He resonance lines observed by SUMER with theoretical profiles computed with our non-LTE radiative transfer code. We use the H I Lyman-beta, H I Lyman-epsilon, and He I 584 A lines. Our code allows us to obtain the plasma parameters in prominences in conjunction with a multi-line, multi-element set of observations. The plasma temperature in the prominence core is ~ 8600 K and the pressure is 0.03 dyn/cm^2. The Ly-beta line is formed in a higher temperature region (more than 11000 K).
We estimate the constraints of observational mass and redshift on the properties of equations of state for quarks in the compact stars. We discuss two scenarios: strange stars and hybrid stars. We construct the equations of state utilizing MIT bag model taking medium effect into account for quark matter and relativistic mean field theory for hadron matter. We find that quark may exist in strange stars and the interior of neutron stars, and only these quark matters with stiff equations of state could be consistent with both constraints. The bag constant is main one parameter that affects the mass strongly for strange stars and only the intermediate coupling constant may be the best choice for compatibility with observational constraints in hybrid stars.
The inner parts of black-hole accretion discs shine in X-rays which can be monitored and the observed spectra can be used to trace strong gravitational fields in the place of emission and along paths of light rays. This paper summarizes several aspects of how the spectral features are influenced by relativistic effects. We focus our attention onto variable and broad emission lines, origin of which can be attributed to the presence of orbiting patterns -- spots and spiral waves in the disc. We point out that the observed spectrum can determine parameters of the central black hole provided the intrinsic local emissivity is constrained by theoretical models.
We present the results of the first laboratory experiment of checkerboard shaped pupil binary mask coronagraphs using visible light, in the context of the R&D activities for future mid-infrared space missions such as the 3.5 m SPICA telescope. The primary aim of this work is to demonstrate the coronagraphic performance of checkerboard masks down to a $10^{-6}$ peak-to-peak contrast, which is required to detect self-luminous extra-solar planets in the mid-infrared region. Two masks, consisting of aluminum films on a glass substrates, were manufactured using nano-fabrication techniques with electron beam lithography: one mask was optimized for a pupil with a 30% central obstruction and the other was for a pupil without obstruction. The theoretical contrast for both masks was $10^{-7}$ and no adaptive optics system was employed. For both masks, the observed point spread functions were quite consistent with the theoretical ones. The average contrast measured within the dark regions was $2.7 {\times} 10^{-7}$ and $1.1 {\times} 10^{-7}$. The coronagraphic performance significantly outperformed the $10^{-6}$ requirement and almost reached the theoretical limit determined by the mask designs. We discuss the potential application of checkerboard masks for mid-infrared coronagraphy, and conclude that binary masks are promising for future high-contrast space telescopes.
The aim of this contribution is to present the two first phases of the optical monitoring programme of the Gravitational Lenses group at the Universidad de Cantabria (GLUC, this http URL). In an initial stage (2003 March-June), the Estacion de Observacion de Calar Alto (EOCA) was used to obtain VR frames of SBS 0909+532 and QSO 0957+561. These observations in 2003 led to accurate fluxes of the two components of both double QSOs, which are being compared and complemented with data from other 1-1.5 m telescopes located in the North Hemisphere: Fred Lawrence Whipple Observatory (USA), Maidanak Observatory (Uzbekistan) and Wise Observatory (Israel). On the other hand, the GLUC started the second phase of its monitoring programme in 2005 January. In this second phase, they are using the 2 m fully robotic Liverpool Telescope (LT). The key idea is the two-band photometric follow-up of four lensed QSOs with different main lensing galaxies: SBS 0909+532 (elliptical), QSO 0957+561 (giant cD), B1600+434 (edge-on spiral) and QSO 2237+0305 (face-on spiral). Thus, the light rays associated with the components of the four gravitational mirages cross different galaxy environments, and the corresponding light curves could unveil the content of these environments. While SBS 0909+532 and QSO 0957+561 are the targets for the two first years with the LT (2005-2006), the rest of targets (B1600+434 and QSO 2237+0305) will be monitored starting from 2007.
We present a 20-pc, volume-limited sample of M7-L8 dwarfs created through spectroscopic follow-up of sources selected from the Two Micron All Sky Survey (2MASS) Second Incremental Release Point Source Catalog. In this paper, we present optical spectroscopy of 198 candidate nearby ultracool dwarfs, including 12 late-M and L dwarfs likely to be within 20 pc of the Sun and 94 more distant late-type dwarfs. We have also identified five ultracool dwarfs with spectral signatures of low-gravity. Combining these data with previous results, we define a sample of 99 ultracool dwarfs in 91 systems within 20 pc. These are used to estimate the J- and K-band luminosity functions for dwarfs with optical spectral types between M7 and L8 (10.5<M_J<15, 9.5<M_K<13). We find a space density of 4.9 x 10^-3 pc^-3 for late-M dwarfs (M7-M9.5) and a lower limit of 3.8 x 10^-3 pc^-3 for L dwarfs.
The variational method with constraints recently developed by Verkley and Gerkema to describe maximum-entropy atmospheric profiles is generalized to ideal gases but with temperature-dependent specific heats. In so doing, an extended and non standard potential temperature is introduced that is well suited for tackling the problem under consideration. This new formalism is successfully applied to the atmosphere of Venus. Three well defined regions emerge in this atmosphere up to a height of $100 km$ from the surface: the lowest one up to about $35 km$ is adiabatic, a transition layer located at the height of the cloud deck and finally a third region which is practically isothermal.
We present the results of a deep photometric survey performed with FORS1@VLT aimed at investigating the complex Main Sequence structure of the stellar system omega Centauri. We confirm the presence of a double Main Sequence and identify its blue component (bMS) over a large field of view up to 26' from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other "normal" MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.
Neutrino transport in spherically symmetric models of stellar core collapse and bounce has achieved a technically complete level, rewarded by the agreement among independent groups that a multi-dimensional treatment of the fluid-instabilities in the post-bounce phase is indispensable to model supernova explosions. While much effort is required to develop a reliable neutrino transport technique in axisymmetry, we explore neutrino physics approximations and parameterizations for an efficient three-dimensional simulation of the fluid-instabilities in the shock-heated matter that accumulates between the accretion shock and the protoneutron star. We demonstrate the reliability of a simple parameterization scheme in the collapse phase and extend our 3D magneto-hydrodynamical collapse simulations to a preliminary postbounce evolution. The growth of magnetic fields is investigated.
A new revision of the gamma flux that we expect to detect in Imaging Atmospheric Cherenkov Telescopes (IACTs) from SUSY dark matter annihilation in the Draco dSph is presented using the dark matter density profiles compatible with the latest observations. This revision takes also into account the important effect of the Point Spread Function (PSF) of the Cherenkov telescope. We show that this effect is crucial in the way we will observe and interpret a possible signal profile in the telescope. Given these new considerations, some light can be shed on the recent signal excess reported by the CACTUS experiment.
Because of the similarity of the primary star of HS 1136+6646 to the planetary nebula central star BE Ursae Majoris, we did wide field imaging of the former with an H-alpha filter. No nebulosity was detected. On the other hand, the point spread function of the star appeared extended. A partially-resolved red component is detected in the image with the five-band Sloan Digital Sky Survey. Most importantly, a companion is easily resolved in the HST acquisition image for the published STIS observation. A companion to the pre-cataclysmic binary is present at a separation of 1.349'' at position angle 54.4 deg. Evidence indicates that it is likely of K spectral type. We cannot demonstrate conclusively that this component has common proper motion with the close binary. However, the similar apparent z magnitudes and spectral types of HS 1136+6646B and the resolved component make it likely that we have in reality a hierarchial triple system. In any case, the presence of this component needs to be taken into account in future ground-based studies.
We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-2 in order to derive its afterglow emission parameters and to determine the reason for its optical faintness when compared to other events. Observations were taken in the optical (2.0m Himalayan Chandra Telescope, 1.34m Tautenburg, 4.2m William Herschel Telescope) and in X-rays (Swift/XRT) between 2.7 hours and 10 days after the onset of the event. The data can be interpreted by collimated emission in a jet with a typical value of $p$ = 2.4 which is moving in an homogeneous interstellar medium and with a cooling frequency nu_{c} still above the X-rays at 0.5 days after the burst onset. GRB 051028 can be classified as a ``gray'' or ``potentially dark'' GRB. On the basis of the combined optical and Swift/XRT data, we conclude that the reason for the optical dimness is not extra absorption in the host galaxy, but rather the GRB taking place at high-redshift.We also notice the very striking similarity with the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of 3.967, although GRB 051028 is about 3 mag fainter. We suggest that the bump could be explained by multiple energy injection episodes and that the burst is intrinsically faint when compared to the average afterglows detected since 1997. The non-detection of the host galaxy down to R = 25.1 is also consistent with the burst arising at high redshift, compatible with the published pseudo-z of 3.7 +/- 1.8.
The majority of the highest energy cosmic rays are thought to be electrically charged: protons or nuclei. Charged particles experience angular deflections as they pass through galactic and extra-galactic magnetic fields. As a consequence correlation of cosmic ray arrival directions with potential sources has proved to be difficult. This situation is not helped by current data samples where the number of cosmic rays/source are typically < O(1). Progress will be made when there are significantly larger data samples and perhaps with better catalogs of candidate sources. This paper reports a search for correlations between the RXTE catalog of nearby active galactic nuclei, AGNs, and the published list of ultra-high energy cosmic rays from the AGASA experiment. Although no statistically significant correlations were found, two correlations were observed between AGASA events and the most inclusive category of RXTE AGNs.
We present the UV spectroscopy and timing of three nearby pulsars (Vela, B0656+14 and Geminga) recently observed with the Space Telescope Imaging Spectrograph. We also review the optical and X-ray properties of these pulsars and establish their connection with the UV properties. We show that the multiwavelengths properties of neutron stars (NSs) vary significantly within the sample of middle-aged pulsars. Even larger differences are found between the thermal components of Ge-minga and B0656+14 as compared to those of radio-quiet isolated NSs. These differences could be attributed to different properties of the NS surface layers.
We propose a scalar potential of inflation, motivated by the modular invariant supergravity and computed the angular power spectra of the adiabatic density perturbations. The potential consists of three scalar fields S, Y and T with the two free parameters. By fitting the parameters with the cosmological data at the fixed point T=1, we find the potential behaves as that of the single field S, which slowly rolls down along the minimized trajectory in Y and gives rise the sufficient inflation matching with the recent three-year WMAP data, e.g. the spectral index n_s = 0.951. The TT and TE angular power spectra obtained from our model almost completely coincides with the fitting of the LambdaCDM model. We conclude that our model is considered to be an adequate theory of inflation to explain the present data, although more theoritical foundation of the model should be required.
We present a robust and fast algorithm for performing astrometry and source cross-identification on two dimensional point lists, such as between a catalogue and an astronomical image, or between two images. The method is based on minimal assumptions: the lists can be rotated, magnified and inverted with respect to each other in an arbitrary way. The algorithm is tailored to work efficiently on wide fields with large number of sources and significant non-linear distortions, as long as the distortions can be approximated with linear transformations locally, over the scale-length of the average distance between the points. The procedure is based on symmetric point matching in a newly defined continuous triangle space that consists of triangles generated by an extended Delaunay triangulation. Our software implementation performed at the 99.995% success rate on ~260,000 frames taken by the HATNet project.
IZw18 has been recurrently claimed to be a young galaxy, but stars of increasingly older ages are found every time deeper magnitude levels are reached with high-resolution photometry: from the original few Myrs to, possibly, several Gyrs. We summarize the history of IZw18's age and an HST project which will allow us to derive both its distance and age.
A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (M_{mol}/M_{HI}>2) is considered. The sample galaxies do not differ systematically from other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the total mass of gas in comparison with galaxies of similar linear sizes and disk angular momentum. This suggests that the overabundance of $H_2$ is due to transition of HI to H_2. Galaxies with bars and active nuclei are found more frequently among galaxies which have M_{mol} estimates in CISM. In a small fraction of galaxies, high M_{mol}/M_{HI} ratios are caused by the overestimation of M_{mol} due to a low conversion factor for the translation of CO-line intensities into the number of H_2 molecules along the line of sight. It is argued that the "molecularization" of the bulk of the gas mass could be due 1) to the concentration of gas in the inner regions of the galactic disks, resulting to a high gas pressure and 2) to relatively low star-formation rate per unit mass of molecular gas which indeed takes place in galaxies with high M_{mol}/M_{HI} ratios.
We report observations of gaseous methanol in an edge-on torus surrounding the young stellar object L1551 IRS5. The peaks in the torus are separated by ~ 10,000 AU from L1551 IRS5, and contain ~ 0.03 earth masses of cold methanol. We infer that the methanol abundance increases in the outer part of the torus, probably as a result of methanol evaporation from dust grain surfaces heated by the shock luminosity associated with the shocks associated with the jets of an externally located x-ray source. Any methanol released in such a cold environment will rapidly freeze again, spreading methanol throughout the circumbinary torus to nascent dust grains, planitesimals, and primitive bodies. These observations probe the initial chemical conditions of matter infalling onto the disk.
We have measured the 3.6 micron luminosity and mass evolution of about 1000 galaxies in 32 clusters at 0.2<z<1.25 with a special attention to methodological issues, as emphasized in this proceeding contribution. We find that the luminosity of our galaxies evolves as an old and passively evolving population formed at high redshift without any need for additional redshift-dependent evolution. Models with a prolonged stellar mass growth are rejected by the data with high confidence. The data also reject models in which the age of the stars is the same at all redshifts. Similarly, the characteristic stellar mass evolves, in the last two thirds of the universe age, as expected for a stellar population formed at high redshift. Together with the old age of stellar populations derived from fundamental plane studies, our data seems to suggest that massive early-type cluster galaxies have been completely assembled at high redshift, and not only that their stars are old. The quality of the data allows us to derive the LF and mass evolution homogeneously over the whole redshift range, using a single estimator. The Schechter function describes the galaxy luminosity function well. The characteristic luminosity at z=0.5 is found to be 16.30 mag, with an uncertainty of 10 per cent.
We present observations with the INTEGRAL/IBIS telescope of the wind nebula powered by the young pulsar B1509-58 and we discuss the spatial and spectral properties of the unpulsed emission in the 20-200 keV energy band. The source extension and orientation along the northwest-southeast axis corresponds to the jet emission seen at keV and TeV energies. The hard X-ray spectrum is consistent with the earlier Beppo-SAX measurements. It follows a power law with a photon index alpha = -2.12 pm 0.05 up to 160 keV. A possible break at this energy is found at the 2.9 sigma confidence level. The 0.1-100 keV data are consistent with synchrotron aging of pairs in the jet and yield a magnetic field strength of 22-33 muG for a bulk velocity of 0.3-0.5c. The synchrotron cut-off energy thus corresponds to a maximum electron energy of 400-730 TeV.
We present Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3, which cover a temperature range 3<kT<15 keV. Our analysis is aimed at measuring the Iron abundance in the Intra-Cluster Medium (ICM) out to the highest redshift probed to date. We make use of combined spectral analysis performed over five redshift bins at 0.3<z<1.3 to estimate the average emission weighted Iron abundance. We find that the emission weighted Iron abundance measured within (0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ~2 larger than in hotter clusters, following Z(T) = 0.88 T^{-0.47} Z_o, confirming the trend seen in local samples. We also find a constant average Iron abundance Z_{Fe}~0.25Z_o as a function of redshift, but only for clusters at z>0.5. The emission weighted Iron abundance is significantly higher (Z_{Fe}~0.4 Z_o) in the redshift range z~0.3-0.5, approaching the value measured locally in the inner 0.15 R_{vir} radii for a mix of cool-core and non cool-core clusters at 0.1<z<0.3. The decrease in metallicity with redshift can be parametrized by a power law of the form ~(1+z)^{-1.25}. The observed evolution implies that the average Iron content of the ICM at present epoch is a factor of ~2 larger than at z~1.2. We confirm that the ICM is already significantly enriched (Z_{Fe} ~ 0.25 Z_o) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes which drive the chemical enrichment of the ICM. (abridged)
We present IZJHKL' photometry of the core of the cluster NGC 6611 in the Eagle Nebula. This photometry is used to constrain the Initial Mass Function (IMF) and the circumstellar disk frequency of the young stellar objects. Optical spectroscopy of 258 objects is used to confirm membership and constrain contamination as well as individual reddening estimates. Our overall aim is to assess the influence of the ionizing radiation from the massive stars on the formation and evolution of young low-mass stars and their disks. The disk frequency determined from the JHKL' colour-colour diagram suggests that the ionizing radiation from the massive stars has little effect on disk evolution (Oliveira et al. 2005). The cluster IMF seems indistinguishable from those of quieter environments; however towards lower masses the tell-tale signs of an environmental influence are expected to become more noticeable, a question we are currently addressing with our recently acquired ultra-deep (ACS and NICMOS) HST images.
The far-infrared (FIR) emissivity of dust is an important parameter
characterizing the physical properties of the grains. With the availability of
stellar databases and far-infrared data from Infrared Space Observatory (ISO)
it is possible to compare the optical and infrared properties of dust, and
derive the far-infrared emissivity with respect to the optical extinction.
In this paper we present the results of a systematic analysis of the FIR
emissivity of interstellar clouds observed with ISOPHOT (the photometer onboard
ISO) at least at two infrared wavelengths, one close to ~100um and one at
200um. We constructed FIR emission maps, determined dust temperatures, created
extinction maps using 2MASS survey data, and calculated far-infrared emissivity
for each of these clouds. We present the largest homogeneously reduced database
constructed so far for this purpose. During the data analysis special care was
taken on possible systematic errors. We find that far-infrared emissivity has a
clear dependence on temperature.
The emissivity is enhanced by a factor of usually less than 2 in the low dust
temperature regime of 12K<=T_d<=14K. This result suggests larger grain sizes in
those regions. However, the emissivity increase of typically below 2 restricts
the possible grain growth processes to ice-mantle formation and coagulation of
silicate grains, and excludes the coagulation of carbonaceous particles on the
scales of the regions we investigated.
In the temperature range 14K<=T_d<=16K a systematic decrease of emissivity is
observed with respect to the values of the diffuse interstellar matter.
Possible scenarios for this behaviour are discussed in the paper.
We have modelled the rotational modulation of X-ray emission from T Tauri stars assuming that they have isothermal, magnetically confined coronae. By extrapolating surface magnetograms we find that T Tauri coronae are compact and clumpy, such that rotational modulation arises from X-ray emitting regions being eclipsed as the star rotates. Emitting regions are close to the stellar surface and inhomogeneously distributed about the star. However some regions of the stellar surface, which contain wind bearing open field lines, are dark in X-rays. From simulated X-ray light curves, obtained using stellar parameters from the Chandra Orion Ultradeep Project, we calculate X-ray periods and make comparisons with optically determined rotation periods. We find that X-ray periods are typically equal to, or are half of, the optical periods. Further, we find that X-ray periods are dependent upon the stellar inclination, but that the ratio of X-ray to optical period is independent of stellar mass and radius.
We present our recent work on the conditions under which star formation occurs in a metal-poor environment, the Large Magellanic Cloud ([Fe/H] ~ -0.4). Water masers are used as beacons of the current star formation in HII regions. Comparing their location with the dust morphology imaged with the Spitzer Space Telescope, and additional Halpha imaging and groundbased near-infrared observations, we conclude that the LMC environment seems favourable to sequential star formation triggered by massive star feedback (Oliveira et al. 2006). Good examples of this are 30 Doradus and N 113. There are also HII regions, such as N 105A, where feedback may not be responsible for the current star formation although the nature of one young stellar object (YSO) suggests that feedback may soon start making an impact. The chemistry in one YSO hints at a stronger influence from irradiation effects in a metal-poor environment where shielding by dust is suppressed (van Loon 2005)
We review the theory and observations of star cluster disruption. The three main phases and corresponding typical timescales of cluster disruption are: I) Infant Mortality (~10^7 yr), II) Stellar Evolution (~10^8 yr) and III) Tidal relaxation (~10^9 yr). During all three phases there are additional tidal external perturbations from the host galaxy. In this review we focus on the physics and observations of Phase I and on population studies of Phases II & III and external perturbations concentrating on cluster-GMC interactions. Particular attention is given to the successes and short-comings of the Lamers cluster disruption law, which has recently been shown to stand on a firm physical footing.
The mechanisms governing the star formation rate in spiral galaxies are not
yet clear. The nearby, almost face-on, and interacting galaxy M51 offers an
excellent opportunity to study at high spatial resolutions the local star
formation laws. In this first paper, we investigate the correlation of H2, HI,
and total gas surface densities with the star forming activity, derived from
the radio continuum (RC), along radial averages out to radii of 12kpc.
We have created a complete map of M51 in 12CO 2-1 at a resolution of 450kpc
using HERA at the IRAM-30m telescope. These data are combined with maps of HI
and the radio-continuum at 20cm wavelength. The latter is used to estimate the
star formation rate (SFR), thus allowing to study the star formation efficiency
and the local Schmidt law. The velocity dispersion from CO is used to study the
critical surface density and the gravitational stability of the disk.
The critical gas velocity dispersions needed to stabilize the gas against
gravitational collapse in the differentially rotating disk of M51 using the
Toomre criterion, vary with radius between 1.7 and 6.8 km/s. Observed radially
averaged dispersions derived from the CO data vary between 28 km/s in the
center and 8 km/s at radii of 7 to 9 kpc. They exceed the critical dispersions
by factors Q_gas of 1 to 5. We speculate that the gravitational potential of
stars leads to a critically stable disk.
We construct a complete, semi-empirical donor sequence for CVs with orbital periods less than 6 hrs. All key physical and photometric parameters of CV secondaries (along with their spectral types) are given as a function of P_orb along this sequence. The main observational basis for our donor sequence is an empirical mass-radius relation for CV secondaries. We present an optimal estimate for this relation that ensures consistency with the observed locations of the period gap and the period minimum. We also present new determinations of these periods, finding P_{gap, upper edge} = 3.18 +/- 0.04 hr, P_{gap, lower edge} = 2.15 +/- 0.03 hr and P_{min} = 76.2 +/- 1.0 min. We then test the donor sequence by comparing observed and predicted spectral types (SpTs) as a function of orbital period. To this end, we update the SpT compilation of Beuermann et al. and show explicitly that CV donors have later SpTs than main sequence (MS) stars at all orbital periods. The semi-empirical donor sequence matches the observed SpTs very well, implying that the empirical M2-R2 relation predicts just the right amount of radius expansion. We finally apply the donor sequence to the problem of distance estimation. Based on a sample of 22 CVs with trigonometric parallaxes, we show that the donor sequence correctly traces the envelope of the observed M_{JHK}-P_{orb} distribution. Thus robust lower limits on distances can be obtained from single-epoch infrared observations.
We present new atmosphere models for Wolf-Rayet stars that include a self-consistent solution of the wind hydrodynamics. We demonstrate that the formation of optically thick WR winds can be explained by radiative driving on Fe line opacities, implying a strong dependence on metallicity (Z). Z-dependent model calculations for late-type WN stars show that these objects are very massive stars close to the Eddington limit, and that their formation is strongly favored for high metallicity environments.
We present recent results for galactic WNL stars, obtained with the new Potsdam Wolf-Rayet (PoWR) hydrodynamic model atmospheres. Based on a combination of stellar wind modeling and spectral analysis we identify the galactic WNL subtypes as a group of extremely luminous stars close to the Eddington limit. Their luminosities imply progenitor masses around 120 solar masses or even above, making them the direct descendants of the most massive stars in the galaxy. Because of the proximity to the Eddington limit our models are very sensitive to the L/M ratio, thus allowing for a direct estimate of the present masses of these objects.
We have calculated the H$\alpha$ and Ca {\sc ii} 8542 {\AA} line profiles based on four different atmospheric models, including the effects of nonthermal electron beams with various energy fluxes. These two lines have different responses to thermal and nonthermal effects, and can be used to diagnose the thermal and nonthermal heating processes. We apply our method to an X-class flare that occurred on 2001 October 19. We are able to identify quantitatively the heating effects during the flare eruption. We find that the nonthermal effects at the outer edge of the flare ribbon are more notable than that at the inner edge, while the temperature at the inner edge seems higher. On the other hand, the results show that nonthermal effects increase rapidly in the rise phase and decrease quickly in the decay phase, but the atmospheric temperature can still keep relatively high for some time after getting to its maximum. For the two kernels that we analyze, the maximum energy fluxes of the electron beams are $\sim$ 10$^{10}$ and 10$^{11}$ ergs cm$^{-2}$ s$^{-1}$, respectively. However, the atmospheric temperatures are not so high, i.e., lower than or slightly higher than that of the weak flare model F1 at the two kernels. We discuss the implications of the results for two-ribbon flare models.
Recent results with the Potsdam Wolf-Rayet (PoWR) models have shown that Wolf-Rayet mass loss can be explained by radiative wind driving. An inspection of the galactic WR sample, however, reveals that a significant part of the observed WR stars are in conflict with our models. This group is chiefly formed by intermediate spectral subtypes. Among the population of late-type WN stars we find that the enigmatic WN8 subtypes, which are well-known for their photometric variations, are in disagreement with our models. So are there other driving mechanisms working in these objects?
We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018+4043 in the multi-wavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) erg cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018+4043 field revealed a hard point-like source with the observed 0.5-10 keV flux of 3.4(+0.7)(-0.8) x 10(-12) erg cm(-2) s(-1) (90% confidence level) at alpha = 20h18m38.55s, delta = +40d41m00.4s (with a 4.2" uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index gamma = 1.3 +/- 0.2 and N_H = 6.1(+3.2)(-2.2) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beam-width distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018+4043 are consistent with those expected for an obscured accreting object, i.e. an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018+4043 to the bright gamma-ray source GEV J2020+4023 (3EG J2020+4017) detected by COS B and CGRO EGRET in the gamma-Cygni SNR field.
We search for effects of metallicity on B and Be stars in the Small and Large Magellanic Clouds (SMC and LMC) and in the Milky Way (MW). We extend our previous analysis of B and Be stars populations in the LMC to the SMC. The rotational velocities of massive stars and the evolutionary status of Be stars are examined with respect to their environments. Spectroscopic observations of hot stars belonging to the young cluster SMC-NGC 330 and its surrounding region have been obtained with the VLT-GIRAFFE facilities in MEDUSA mode. We determine fundamental parameters for B and Be stars with the GIRFIT code, taking into account the effect of fast rotation, and the age of observed clusters. We compare the mean vsini obtained by spectral type- and mass-selection for field and cluster B and Be stars in the SMC with the one in the LMC and MW. We find that (i) B and Be stars rotate faster in the SMC than in the LMC, and in the LMC than in the MW; (ii) at a given metallicity, Be stars begin their main sequence life with a higher initial rotational velocity than B stars. Consequently, only a fraction of B stars that reach the ZAMS with a sufficiently high initial rotational velocity can become Be stars; (iii) the distributions of initial rotational velocities at the ZAMS for Be stars in the SMC, LMC and MW are mass- and metallicity-dependent; (iv) the angular velocities of B and Be stars are higher in the SMC than in the LMC and MW; (v) in the SMC and LMC, massive Be stars appear in the second part of the main sequence, contrary to massive Be stars in the MW.
We report on the study of the population of B and Be stars in SMC young clusters, performed with the Wide Field Imager in slitless spectroscopic mode at ESO/T2.2m with a filter centered at Halpha. First, we explain the reduction methods we used and our selection of different types of objects. Second, we present results on the proportion of Be stars in SMC clusters, and we compare this proportion to the one observed in the Milky Way. Finally, we also present results on a statistical study of variability of Be stars with OGLE.
We suggest that the Orion A cloud is gravitationally collapsing on large scales, and is producing the Orion Nebula Cluster due to the focusing effects of gravity acting within a finite cloud geometry. In support of this suggestion, we show how an elliptical rotating sheet of gas with a modest density gradient along the major axis can collapse to produce a structure qualitatively resembling Orion A, with a fan-shaped structure at one end, ridges or filaments along the fan, and a narrow curved filament at the other end reminiscent of the famous integral-shaped filament. The model produces a local concentration of mass within the narrow filament which in principle could form a dense cluster of stars like that of the Orion Nebula. We suggest that global gravitational contraction might be a more common feature of molecular clouds than previously recognized, and that the formation of star clusters is a dynamic process resulting from the focusing effects of gravity acting upon the geometry of finite clouds.
We propose a new approach in determining ages of FR II type radio sources. We apply the assumed dynamical model of Kaiser et al. (1997) to a number of FR II type radio galaxies observed at different radio frequencies, and fit - for each frequency separately - the model free parameters to the observed sources' quantities. Such a procedure, using enlarged in fact a number of observables, enables us to determine relatively precise ages and other crucial characteristics of the analyzed sources. The resulting age estimates agree very well with those obtained by means of `classical' spectral ageing method for objects not older than 10 Myr, for which good-quality spectral data are available. The presented method is however also applicable in the case of the sources older than this, and/or the ones for which the only available low-resolution radio data do not allow for detailed spectral ageing studies. Our analysis indicates that the main factor precluding precise age determination for FR II type radio galaxies regards the poorly known shape of the initial electron energy distribution injected by the jet terminal shocks to the expanding lobes/cocoons. We briefly consider this issue, and conclude that the broad-band single power-law form assumed here may be accurate enough for the presented estimates, although most likely it does not strictly correspond to some well-defined realistic particle acceleration process. Instead, it should be considered as a simplest model approximation of the initial electron continuum, averaged over a very broad energy range and over the age of the source, with the effective spectral index which may be different for different sources.
Aims. We present a systematic theoretical study on 40 polycyclic aromatic hydrocarbons dications (PAHs++) containing up to 66 carbon atoms. Methods. We performed our calculations using well established quantum-chemical techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state properties, and of the time-dependent DFT (TD-DFT) to evaluate the excited\textendash state properties. Results. For each PAH++ considered we computed the absolute visible-UV photo-absorption cross-section up to about 30 eV. We also evaluated their vibrational properties and compared them to those of the corresponding neutral and singly-ionised species. We estimated the adiabatic and vertical second ionisation energy DeltaI through total energy differences. Conclusions. The DeltaI values obtained fall in the energy range 8-13 eV, confirming that PAHs could reach the doubly-ionised state in HI regions. The total integrated IR absorption cross-sections show a marked increase upon ionization, being on the average about two and five times larger for PAHs++ than for PAHs+ and PAHs, respectively. The visible-UV photo-absorption cross-sections for the 0, +1 and +2 charge-states show comparable features but PAHs++ are found to absorb slightly less than their parent neutrals and singly ionized species between ~7 and ~12 eV. Combining these pieces of information we found that PAHs++ should actually be more stable against photodissociation than PAHs and PAHs+,_if_ dissociation tresholds are nearly unchanged by ionization.
In large gradual solar energetic particle (SEP) events, especially the ground-level enhancement (GLE) events, where and how energetic particles are accelerated is still a problem. By using imaging data from TRACE, Yohkoh/HXT, SOHO/MDI and SOHO/EIT, along with the data from the GOES, Apatity NM, and SOHO/LASCO CME catalog, the evolution of the X5.7 two-ribbon flare and the associated SEP event on 14 July 2000 are studied. It is found that the magnetic reconnection in this event consists of two parts, and the induced electric field Erec is temporally correlated with the evolution of hard X-ray and gamma-ray emission. In particular, the first hard X-ray and gamma-ray emission peak occurred at 10:22 UT, corresponding to the magnetic reconnection in the western part of the flare ribbons and the maximum Erec of 9.5 V/cm; the second emission peak at 10:27 UT, corresponding to the eastern part and the maximum Erec of 13.0 V/cm. We also analyze the SEP injection profiles as functions of time and CME-height, and find two-component injection which may result from different acceleration mechanisms. A reasonable conclusion is that reconnection electric field makes a crucial contribution to the acceleration of relativistic particles and to the impulsive component of the large gradual SEP event, while CME-driven shocks play a dominant role in the gradual component.
We explore the possibility of having a composite (self-conserved) dark energy (DE) whose dynamics is controlled by the quantum running of the cosmological parameters. We find that within this scenario it is feasible to find an explanation for the cosmological coincidence problem and at the same time a good qualitative description of the present data.
Aims: To calculate transition rates from ground and excited states in neutral
oxygen atoms due to electron collisions for non-LTE modelling of oxygen in
late-type stellar atmospheres, thus enabling reliable interpretation of oxygen
lines in stellar spectra.
Methods: A 38-state R-matrix calculation in LS-coupling has been performed.
Basis orbitals from the literature (Thomas et al.) are adopted, and a large set
of configurations are included to obtain good representations of the target
wavefunctions. Rate coefficients are calculated by averaging over a Maxwellian
velocity distribution.
Results: Estimates for the cross sections and rate coefficients are presented
for transitions between the seven lowest LS states of neutral oxygen. The cross
sections for excitation from the ground state compare well with existing
experimental and recent theoretical results.
IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs) discovered so far. We report on the aperiodic timing and color analysis of its X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top noise and two harmonically related quasi-periodic oscillations, all of them at very low frequencies (0.01-0.1 Hz), were present in the power spectra during most of the outburst as well as a very high fractional variability (~50%). These properties are atypical not only for AMPs but also for neutron star low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable similarities with the variability observed in black hole systems, reinforcing the connections between these two types of LMXB, as well as some interesting differences. We note finally that the results of this paper are difficult to reconcile with interpretations where any break frequency of power density spectra scales inversely with the mass of the central object at an accuracy sufficient to distinguish between the masses of neutron stars and black holes in LMXBs.
A review of the study of superclusters based on the 2dFGRS and SDSS is given. Real superclusters are compared with models using simulated galaxies of the Millennium Run. We show that the fraction of very luminous superclusters in real samples is about five times greater than in simulated samples. Superclusters are generated by large-scale density perturbations which evolve very slowly. The absence of very luminous superclusters in simulations can be explained either by non-proper treatment of large-scale perturbations, or by some yet unknown processes in the very early Universe.
We discuss a cosmology in which cold dark-matter particles decay into relativistic particles. We argue that such decays could lead naturally to a bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the present hubble age, this bulk viscosity enters the cosmic energy equation as an effective negative pressure. We investigate whether this negative pressure is of sufficient magnitude to account fo the observed cosmic acceleration. We show that a single decaying species in a flat, dark-matter dominated cosmology without a cosmological constant cannot reproduce the observed magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk viscosity, possibly due to a cascade of decaying particles may be able to account for a significant fraction of the apparent cosmic acceleration. Possible candidate nonrelativistic particles for this scenario include sterile neutrinos or gauge-mediated decaying supersymmetric particles.
We have detected radial-velocity variations in two objects that were identified as being likely host stars of transiting exoplanets in the 2004 SuperWASP wide-field transit survey. Using the newly-commissioned radial-velocity spectrograph SOPHIE at the Observatoire de Haute-Provence, we found that both objects exhibit reflex orbital radial-velocity variations with amplitudes characteristic of planetary-mass companions and in-phase with the photometric orbits. Line-bisector studies rule out faint blended binaries as the cause of either the radial-velocity variations or the transits. We perform preliminary spectral analyses of the host stars, which together with their radial-velocity variations and fits to the transit light curves, yield estimates of the planetary masses and radii. WASP-1b and WASP-2b have orbital periods of 2.52 and 2.15 days respectively. Given mass estimates for their F7V and K1V primaries we derive planet masses 0.80 to 0.98 and 0.81 to 0.95 times that of Jupiter respectively. WASP-1b appears to have an inflated radius of at least 1.33 R_Jup, whereas WASP-2b has a radius in the range 0.65 to 1.26 R_Jup.
Large fluctuations in the electron column density can occur during the reionization process. We investigate the possibility of deriving the electron density fluctuations through detailed mapping of the redshifted 21-cm emission from the neutral medium during reionization. We find that the electron-scattering optical depth and 21-cm differential brightness temperature are strongly anti-correlated, allowing optical depth estimates based entirely on redshifted 21-cm measurements. This should help isolate the CMB polarization fluctuations that are due to reionization, allowing both cleaning of the patchy reionization polarization signal as a contaminating source of confusion to other signals and a measurement of the primordial quadrupole that would be measured at various locations in the universe at the epoch of reionization. This latter application in principle allows mapping of the primordial density field at z~1100 over a large fraction of the Hubble volume.
Deep XMM and Chandra observations of ClJ1226.9+3332 at z=0.89 have enabled the most detailed X-ray mass analysis of any such high-redshift galaxy cluster. The XMM temperature profile of the system shows no sign of central cooling, with a hot core and a radially declining profile. A temperature map shows asymmetry with a hot region that appears to be associated with a subclump of galaxies at the cluster redshift, but is not visible in the X-ray surface brightness. This is likely to be result of a merger event in the cluster, but does not appear to significantly affect the overall temperature profile. The XMM temperature profile, and combined Chandra and XMM emissivity profile allowed precise measurements of the global properties of ClJ1226.9+3332; we find kT=10.4+/-0.6keV, Z=0.16+/-0.05\Zsol, and M=5.2^{+1.0}_{-0.8}x10^{14}Msol. We obtain profiles of the metallicity, entropy, cooling time and gas fraction, and find a high concentration parameter for the total density profile of the system. The global properties are compared with the local LT and MT relations, and we are able to make the first observational test of the predicted evolution of the YM relation. We find that departures from these scaling relations are most likely caused by an underestimate of the total mass by ~30% in the X-ray hydrostatic mass analysis due to the apparent recent or ongoing merger activity.
We explore the evolution of thermonuclear supernova explosions when the progenitor white dwarf star ignites asymmetrically off-center. Several numerical simulations are carried out in two and three dimensions to test the consequences of different initial flame configurations such as spherical bubbles displaced from the center, more complex deformed configurations, and teardrop-shaped ignitions. The burning bubbles float towards the surface while releasing energy due to the nuclear reactions. If the energy release is too small to gravitationally unbind the star, the ash sweeps around it, once the burning bubble approaches the surface. Collisions in the fuel on the opposite side increase its temperature and density and may -- in some cases -- initiate a detonation wave which will then propagate inward burning the core of the star and leading to a strong explosion. However, for initial setups in two dimensions that seem realistic from pre-ignition evolution, as well as for all three-dimensional simulations the collimation of the surface material is found to be too weak to trigger a detonation.
We examine whether massive-star accretion disks are likely to fragment due to self-gravity. Rapid accretion and high angular momentum push these disks toward fragmentation, whereas viscous heating and the high protostellar luminosity stabilize them. We find that for a broad range of protostar masses and for reasonable accretion times, massive disks larger than ~150 AU are prone to fragmentation. We develop an analytical estimate for the angular momentum of accreted material, extending the analysis of Matzner and Levin (2005) to account for strongly turbulent initial conditions. In a core-collapse model, we predict that disks are marginally prone to fragmentation around stars of about four to 15 solar masses -- even if we adopt conservative estimates of the disks' radii and tendency to fragment. More massive stars are progressively more likely to fragment, and there is a sharp drop in the stability of disk accretion at the very high accretion rates expected above 110 solar masses. Fragmentation may starve accretion in massive stars, especially above this limit, and is likely to create swarms of small, coplanar companions.
We describe observations of the seventh accretion-powered millisecond pulsar, HETE J1900.1-2455 made with the Rossi X-ray Timing Explorer during the year of activity that followed its discovery in 2005 June. We detected intermittent pulsations at a peak fractional amplitude of 3%, but only in the first two months of the outburst. On three occasions during this time we observed an abrupt increase in the pulse amplitude, approximately coincident with the time of a thermonuclear burst, followed by a steady decrease on a timescale of approx. 10 d. HETE J1900.1-2455 has shown the longest active period by far for any transient accretion-powered millisecond pulsar, comparable instead to the outburst cycles for other transient X-ray binaries. Since the last detection of pulsations, HETE J1900.1-2455 has been indistinguishable from a low-accretion rate, non-pulsing LMXB; we hypothesize that other, presently active LMXBs may have also been detectable initially as millisecond X-ray pulsars.
Time delays between lensed multiple images have been known to provide an interesting probe of the Hubble constant, but such application is often limited by degeneracies with the shape of lens potentials. We propose a new statistical approach to examine the dependence of time delays on the complexity of lens potentials, such as higher-order perturbations, non-isothermality, and substructures. Specifically, we introduce a reduced time delay and explore its behavior as a function of the image configuration that is characterized by the asymmetry and opening angle of the image pair. In particular we derive a realistic conditional probability distribution for a given image configuration. We find that the probability distribution is sensitive to the image configuration such that more symmetric and/or smaller opening angle image pairs are more easily affected by perturbations on the primary lens potential. On average time delays of double lenses are less scattered than those of quadruple lenses. Furthermore, the realistic conditional distribution allows a new statistical method to constrain the Hubble constant from observed time delays. We find that 15 published time delay quasars constrain the Hubble constant to be H_0=70+/-3 km/s/Mpc. While systematic errors coming from the heterogeneous nature of the quasar sample and the uncertainty of the input distribution of lens potentials should be considered, reasonable agreement with other estimates indicates the usefulness of our new approach as a cosmological and astrophysical probe, particularly in the era of large-scale synoptic surveys. (Abridged)
We report the discovery of the two-image gravitationally lensed quasar SDSS J133222.62+034739.9 (SDSS J1332+0347) with an image separation of Delta_theta=1.14". This system consists of a source quasar at z_s=1.445 and a lens galaxy at z_l=0.191. The agreement of the luminosity, ellipticity and position angle of the lens galaxy with those expected from lens model confirms the lensing hypothesis.
We report the discoveries of the two-image gravitationally lensed quasars, SDSS J0746+4403 and SDSS J1406+6126, selected from the Sloan Digital Sky Survey (SDSS). SDSS J0746+4403, which will be included in our lens sample for statistics and cosmology, has a source redshift of z_s=2.00, an estimated lens redshift of z_l~0.3, and an image separation of 1.08". SDSS J1406+6126 has a source redshift of z_s=2.13, a spectroscopically measured lens redshift of z_l=0.27, and an image separation of 1.98". We find that the two quasar images of SDSS J1406+6126 have different intervening MgII absorption strengths, which are suggestive of large variations of absorbers on kpc scales. The positions and fluxes of both the lensed quasar systems are easily reproduced by simple mass models with reasonable parameter values. These objects bring to 18 the number of lensed quasars that have been discovered from the SDSS data.
In this paper we present the results of the past two years' observations on the galactic microquasar LS I +61 303 with the Whipple 10m gamma-ray telescope. The recent MAGIC detection of the source between 200 GeV and 4 TeV suggests that the source is periodic with very high energy (VHE) gamma-ray emission linked to its orbital cycle. The entire 50-hour data set obtained with Whipple from 2004 to 2006 was analyzed with no reliable detection resulting. The upper limits obtained in the 2005-2006 season covered several of the same epochs as the MAGIC Telescope detections, albeit with lower sensitivity. Upper limits are placed on emission during the orbital phases of 0->0.1 and 0.8->1, phases which are not included in the MAGIC data set.
We study the relations existing between fluxes emitted at CO(1-0) line, 60 and 100 micron wavelengths, B and soft X-ray wavebands for galaxies of all morphological types. The large set of data that we created allows to revisit some of known relations existing between the different tracers of the Interstellar Medium (ISM): the link between the FIR flux and the CO line emission, the relation between X-ray emission in non active galaxies and the blue or FIR luminosity. Using catalogues of galaxies and works presented in the literature, we collected fluxes in FIR, 21 cm, CO(0-1) line and soft X-ray for two samples, consisting of normal and interacting galaxies respectively. Joining together these samples, we have data for a total of 2953 galaxies, not all observed in the four above wavebands. All the relations found are discussed in the frame of the star formation activity that is the link for most of them. We note that when an active star formation is present, it may link the galaxy fluxes at almost all wavelengths, from X to microwaves. On the contrary, in early-type galaxies where the current star formation rate has faded out the X-FIR fluxes link disappears. This result obtained for early-type galaxies is discussed and explained in detail in the frame of a suitable theoretical model, obtained coupling chemo-dynamical N-body simulations with a dusty spectrophotometric code of population synthesis.
When taking the holographic principle into account, the vacuum energy will acquire dynamical property that its equation of state is evolving. The current available observational data imply that the holographic vacuum energy behaves as quintom-type dark energy. We adopt the viewpoint of that the scalar field models of dark energy are effective theories of an underlying theory of dark energy. If we regard the scalar field model as an effective description of such a holographic vacuum theory, we should be capable of using the scalar field model to mimic the evolving behavior of the dynamical vacuum energy and reconstructing this scalar field model according to the fits of the observational dataset. We find the generalized ghost condensate model is a good choice for depicting the holographic vacuum energy since it can easily realize the quintom behavior. We thus reconstruct the function $h(\phi)$ of the generalized ghost condensate model using the best-fit results of the observational data.
We present metallicities and ages for 52 red giants in the remote Galactic dwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surface area of Leo II and are radial velocity members. We obtained medium-resolution multi-fiber spectroscopy with ESO/VLT's FLAMES spectrograph. The metallicities were determined based on the near-infrared Ca II triplet. The resulting metallicity distribution (MD) is asymmetric and peaks at [Fe/H]=-1.74 dex on the Carretta & Gratton scale. The full range in metallicities extends from -2.4 to -1.1 dex. As in other dSphs, no extremely metal-poor red giants were found. We compare Leo II's observed MD with model predictions for several other Galactic dSphs from the literature. Leo II clearly exhibits a lack of more metal poor stars, in analogy to the classical G-dwarf problem, which may indicate a comparable `K-giant problem'. Moreover, its evolution appears to have been affected by galactic winds. We use our inferred metallicities as an input parameter for isochrone fits to SDSS photometry and derive approximate ages. The resulting age-metallicity distribution covers the full age range from 2-15 Gyr on our adopted isochrone scale. During the first 7 Gyr relative to the oldest stars [Fe/H] appears to have remained almost constant. The almost constant metallicity at higher ages and a slight drop by about 0.3 dex thereafter may be indicative of rejuvenation by low metallicity gas. Overall, the age-metallicity relation appears to support the formation of Leo II from pre-enriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr. Our findings support earlier photometric findings of Leo II as a galaxy with a prominent old and a dominant intermediate-age population. We do not find a significant radial metallicity gradient nor age gradient in our data.(Abridged)
We present the first tests of a new method, the Correlated Component Analysis (CCA) based on second-order statistics, to estimate the mixing matrix, a key ingredient to separate astrophysical foregrounds superimposed to the Cosmic Microwave Background (CMB). In the present application, the mixing matrix is parameterized in terms of the spectral indices of Galactic synchrotron and thermal dust emissions, while the free-free spectral index is prescribed by basic physics, and is thus assumed to be known. We consider simulated observations of the microwave sky with angular resolution and white stationary noise at the nominal levels for the PLANCK satellite, and realistic foreground emissions, with a position dependent synchrotron spectral index. We work with two sets of PLANCK frequency channels: the low frequency set, from 30 to 143 GHz, complemented with the Haslam 408 MHz map, and the high frequency set, from 217 to 545 GHz. The concentration of intense free-free emission on the Galactic plane introduces a steep dependence of the spectral index of the global Galactic emission with Galactic latitude, close to the Galactic equator. This feature makes difficult for the CCA to recover the synchrotron spectral index in this region, given the limited angular resolution of PLANCK, especially at low frequencies. A cut of a narrow strip around the Galactic equator (|b|<3 deg), however, allows us to overcome this problem. We show that, once this strip is removed, the CCA allows an effective foreground subtraction, with residual uncertainties inducing a minor contribution to errors on the recovered CMB power spectrum.
The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined based on high-resolution spectroscopy. We define carbon-enhanced stars taking account of the carbon abundance ratio ([C/Fe]) and the evolutionary status. Twenty two stars in our sample satisfy our modified definition for Carbon-Enhanced Metal-Poor (CEMP) stars. In addition, we measure Na abundances for nine other carbon-enhanced stars for which abundances of other elements have been previously reported. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. In this paper, we separate the carbon-enhanced objects into Ba-rich and Ba-normal objects, and discuss on (1) the metallicity distributions, (2) the correlation between Ba and C (and C+N) abundances, (3) C abundance distributions, (3) the distributions of evolutionary status, and (5) Na abundances. The implications of these results on the origins of carbon in CEMP stars are discussed.
The X-ray spectra of luminous Seyfert 1 galaxies often appear to be reflection dominated. In a number of Narrow Line Seyfert 1 (NLS1) galaxies and galactic black holes in the very high state, the variability of the continuum and of the iron line are decoupled, the reflected component being often much less variable than the continuum. These properties have been interpreted as effects of gravitational light bending. In this framework, we present detailed Monte-Carlo simulations of the reflection continuum in the Kerr metric. These calculations confirm that the spectra and variability behaviour of these sources can be reproduced by the light bending model. As an alternative to the light bending model, we show that similar observational properties are expected from radiation pressure dominated discs subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two-phase structure. In this model, most of the observed spectral and variability features originate from the complex geometrical structure of the inner regions of near-Eddington accretion flows and are therefore a signature of accretion physics rather than general relativity.
In order to explore the statistical properties of galaxy morphological types in compact groups (CGs), we construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the proportion of early-type galaxies in different redshift bins for the CG sample is statistically higher than that for random group sample, and with growing redshift z this kind of difference becomes more significant. This may be due to the existence of interactions and mergers within a significant fraction of SDSS CGs. We also compare statistical results of CGs with those of more compact groups and pairs, but do not observe as large statistical difference as Hickson (1982)'results.
We present a first analysis of a deep X-ray spectrum of the isolated neutron star RBS1223 obtained with XMM-Newton. Spectral data from four new monitoring observations in 2005/2006 were combined with archival observations obtained in 2003 and 2004 to form a spin-phase averaged spectrum containing 290000 EPIC-pn photons. This spectrum shows higher complexity than its predecessors, and can be parameterised with two Gaussian absorption lines superimposed on a blackbody. The line centers, E_2 ~ 2E_1, could be regarded as supporting the cyclotron interpretation of the absorption features in a field B ~ 4 x 10**13 G. The flux ratio of those lines does not support this interpretation. Hence, either feature might be of truly atomic origin.
We present results based on Spitzer Space Telescope mid-infrared (3.6-30 micron) observations of the nearby IIP supernova 2005af. We report the first ever detection of the SiO molecule in a Type IIP supernova. Together with the detection of the CO fundamental, this is an exciting finding as it may signal the onset of dust condensation in the ejecta. From a wealth of fine-structure lines we provide abundance estimates for stable Ni, Ar, and Ne which, via spectral synthesis, may be used to constrain nucleosynthesis models.
In this paper the N-S asymmetry of the soft X-ray flare index during the solar cycles 21, 22 and 23 has been analyzed. The results show the existence of a real N-S asymmetry which is strengthened during solar minimum. The slope of the regression lines fitted to the daily values of asymmetry time series has been found to be negative in all the three cycles. The yearly asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. The power spectral analysis of daily asymmetry time series reveals the significant periods of around 28.26 days, 550.73 days and 3.72 years.
LS 5039 is the only X-ray binary persistently detected at TeV energies by the Cherenkov HESS telescope. It is moreover a gamma-ray emitter in the GeV and possibly MeV energy ranges. To understand important aspects of jet physics, like the magnetic field content or particle acceleration, and emission processes, such as synchrotron and inverse Compton (IC), a complete modeling of the multiwavelength data is necessary. LS 5039 has been detected along almost all the electromagnetic spectrum thanks to several radio, infrared, optical and soft X-ray detections. However, hard X-ray detections above 20 keV have been so far elusive and/or doubtful, partly due to source confusion for the poor spatial resolution of hard X-ray instruments. We report here on deep (300 ksec) serendipitous INTEGRAL hard X-ray observations of LS 5039, coupled with simultaneous VLA radio observations. We obtain a 20-40 keV flux of 1.1 +/- 0.3 mCrab (5.9 (+/-1.6) X 10^{-12} erg cm^{-2} s^{-1}), a 40-100 keV upper limit of 1.5 mCrab (9.5 x 10^{-12} erg cm^{-2}s^{-1}), and typical radio flux densities of about 25 mJy at 5GHz. These hard X-ray fluxes are significantly lower than previous estimates obtained with BATSE in the same energy range but, in the lower interval, agree with extrapolation of previous RXTE measurements. The INTEGRAL observations also hint to a break in the spectral behavior at hard X-rays. A more sensitive characterization of the hard X-ray spectrum of LS 5039 from 20 to 100 keV could therefore constrain key aspects of the jet physics, like the relativistic particle spectrum and the magnetic field strength. Future multiwavelength observations would allow to establish whether such hard X-ray synchrotron emission is produced by the same population of relativistic electrons as those presumably producing TeV emission through IC.
Recent observations with the Spitzer Space Telescope show clear evidence that star formation takes place in the surrounding of young massive O-type stars, which are shaping their environment due to their powerful radiation and stellar winds. In this work we investigate the effect of ionising radiation of massive stars on the ambient interstellar medium (ISM): In particular we want to examine whether the UV-radiation of O-type stars can lead to the observed pillar-like structures and can trigger star formation. We developed a new implementation, based on a parallel Smooth Particle Hydrodynamics code (called IVINE), that allows an efficient treatment of the effect of ionising radiation from massive stars on their turbulent gaseous environment. Here we present first results at very high resolution. We show that ionising radiation can trigger the collapse of an otherwise stable molecular cloud. The arising structures resemble observed structures (e.g. the pillars of creation in the Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of gravitation we find small regions that can be identified as formation places of individual stars. We conclude that ionising radiation from massive stars alone can trigger substantial star formation in molecular clouds.
We present an analysis of high-resolution FLAMES spectra of approximately 50 early B-type stars in three young clusters at different metallicities, NGC6611 in the Galaxy, N11 in the Large Magellanic Cloud (LMC) and NGC346 in the Small Magellanic Cloud (SMC). Using the TLUSTY non-LTE model atmospheres code, atmospheric parameters and photospheric abundances (C, N, O, Mg and Si) of each star have been determined. These results represent a significant improvement on the number of Magellanic Cloud B-type stars with detailed and homogeneous estimates of their atmospheric parameters and chemical compositions. The relationships between effective temperature and spectral type are discussed for all three metallicity regimes, with the effective temperature for a given spectral type increasing as one moves to a lower metallicity regime. Additionally the difficulties in estimating the microturbulent velocity and the anomalous values obtained, particularly in the lowest metallicity regime, are discussed. Our chemical composition estimates are compared with previous studies, both stellar and interstellar with, in general, encouraging agreement being found. Abundances in the Magellanic Clouds relative to the Galaxy are discussed and we also present our best estimates of the base-line chemical composition of the LMC and SMC as derived from B-type stars. Additionally we discuss the use of nitrogen as a probe of the evolutionary history of stars, investigating the roles of rotational mixing, mass-loss, blue loops and binarity on the observed nitrogen abundances and making comparisons with stellar evolutionary models where possible.
The Baikal Neutrino Telescope NT200 takes data since April 1998. On April 9th, 2005, the 10 Mton scale detector NT200$+$ was put into operation in Lake Baikal. Selected results obtained during 1998-2002 with the neutrino telescope NT200 are presented.
We study the gravitational Faraday rotation, on linearly polarized light rays emitted by a pulsar, orbiting another compact object. We relate the rotation angle to the orbital phase of the emitting pulsar, as well as to other parameters describing its orbit and the orientation of the angular momentum of the binary companion. We give numerical estimates of the effect for the double-pulsar system PSR J0737-3039, and we note that the expected magnitude is exceedingly small, making the effect unlikely to be observed with present technology. It is however interesting per se, since in this phenomenon, gravito-magnetism plays a leading role, unlike what happens, for instance, when studying light bending or gravitational time delay, where it appears as a correction to the gravito-electric contribution. Also, we envisage the possibility that this effect could be relevant, at least in principle, for a pulsar orbiting a non charged black-hole.
We test the Secondary Infall Model (SIM) by direct comparison with the results of N-body simulations. Eight cluster-size and six galactic-size dark matter haloes have been selected at $z=0$ and re-simulated with high resolution. Based on their density profiles at the initial redshift, we compute their evolution by the SIM, assuming a simple prescription for the angular momentum. A comparison of the density profiles obtained by the SIM and the numerical experiments at $z=5$, 1 and 0 shows that, for most of the haloes at most epochs, the SIM reproduces the simulated mater distribution with a typical fractional deviation of less than 40 per cent over more than six order of magnitudes in the density. It is also found that, within the SIM framework, most of the diversity in the shape of the density profiles at $z=0$ arises from the scatter in the primordial initial conditions rather than the scatter in the angular momentum distribution. A crude optimization shows that a similar degree of agreement is obtained for galactic and cluster haloes, but the former seem to require slightly higher amounts of angular momentum. Our main conclusion is that the SIM provides a viable dynamical model for predicting the structure and evolution of the density profile of dark matter haloes.
The technique to realize 3D position sensitivity in a two-phase xenon time projection chamber (XeTPC) for dark matter search is described. Results from a prototype detector (XENON3) are presented.
We present the first NIR spectro-interferometry of the LBV Eta Carinae. The K band observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer using three 8.2m Unit Telescopes with baselines from 42 to 89m. The aim of this work is to study the wavelength dependence of Eta Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution. The medium spectral resolution observations (R=1,500) were performed in the wavelength range around both the HeI 2.059 micron and the Br gamma 2.166 micron emission lines, the high spectral resolution observations (R=12,000) only in the Br gamma line region. In the K-band continuum, a diameter of 4.0 +/-0.2 mas (Gaussian FWHM, fit range 28-89m) was measured for Eta Car's optically thick wind region. If we fit Hillier et al. (2001) model visibilities to the observed AMBER visibilities, we obtain 50 % encircled-energy diameters of 4.2, 6.5 and 9.6mas in the 2.17 micron continuum, the HeI, and the Br gamma emission lines, respectively. In the continuum near the Br gamma line, an elongation along a position angle of 120+/-15 degrees was found, consistent with previous VLTI/VINCI measurements by van Boekel et al. (2003). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. (2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Br gamma line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
We summarize recent numerical results on the control of the star formation efficiency (SFE), addressing the effects of turbulence and the magnetic field strength. In closed-box numerical simulations, the effect of the turbulent Mach number $\Ms$ depends on whether the turbulence is driven or decaying: In driven regimes, increasing $\Ms$ decreases the SFE, while in decaying regimes the converse is true. The efficiencies in non-magnetic cases for realistic Mach numbers $\Ms \sim 10$ are somewhat too high compared to observed values. Including the magnetic field can bring the SFE down to levels consistent with observations, but the intensity of the magnetic field necessary to accomplish this depends again on whether the turbulence is driven or decaying. In this kind of simulations, a lifetime of the molecular cloud (MC) needs to be assumed. Further progress requires determining the true nature of the turbulence driving and the lifetimes of the clouds. Simulations of MC formation by large-scale compressions in the warm neutral medium (WNM) show that the clouds' initial turbulence is produced by the accumulation process that forms them, and that the turbulence is driven for as long as this process lasts, producing realistic velocity dispersions and also thermal pressures in excess of the mean WNM value. In simulations including self-gravity, but neglecting the magnetic field and stellar energy feedback, the clouds never reach an equilibrium state, but rather evolve secularly, increasing their mass and gravitational energy until they engage in generalized gravitational collapse. However, local collapse events begin midways through this process, and produce enough stellar objetcs to disperse the cloud or at least halt its collapse before the latter is completed.
The timescale of de-leptonization by neutrino loss and associated contraction of a proto-neutron star is short compared to the time to progagate a shock through the helium core of a massive star, and so the de-leptonization phase does not occur in the vacuum of space, but within the supernova ambiance whether or not there has been a successful explosion. Dynamical non-axisymmetric instabilities (NAXI) are predicted for sufficiently strongly differentially rotating proto-neutron stars. Some modes are unstable for small values of the ratio of rotational kinetic energy to binding energy, T/|W| > 0.01. The NAXI are likely to drive magnetoacoustic waves into the surrounding time-dependent density structure. These waves represent a mechanism of the dissipation of the free energy of differential rotation of the proto-neutron star, and the outward deposition of this energy may play a role in the supernova explosion process. We estimate the power produced by this process and the associated timescale and discuss the possible systematics of the de-leptonization phase in this context. A likely possibility is that the proto-neutron star will spin down through these effects before de-leptonization and produce substantial but not excessive energy input.
We address the origin and evolutionary status of hot subdwarf stars by studying the optical spectral properties of 58 subdwarf O (sdO) stars. Combining them with the results of our previously studied subdwarf B (sdB) stars, we aim at investigating possible evolutionary links. We analyze high-resolution ESO VLT UVES spectra from the ESO Supernova Ia Progenitor Survey (SPY). Effective temperatures, gravities, and helium abundances are determined simultaneously by fitting the profiles of H and He lines using dedicated synthetic spectra in NLTE. Evidence for cool companions to 8 sdOs as well as a binary consisting of two sdO stars is found. A correlation between He abundances and the presence of carbon and/or nitrogen lines emerges: below solar He abundance, no sdO shows C or N lines. In contrast, C and/or N lines are present in ALL sdOs with super- solar He abundance. We thus use the solar He abundance to divide our sample into He-deficient and He-enriched sdOs. While He-deficient sdOs are scattered in a wide range of the Teff-log(g)-diagram, most of the He-enriched sdOs cluster in a narrow region at Teff = 40,000 ... 50,000K and log(g)=5.5 ... 6.0. An evolu- tionary link between sdBs and sdOs appears plausible only for the He-deficient sdOs indicating that they are the likely successors to sdBs. The properties of He-enriched sdOs cannot be explained with canonical single star evolutionary models. Alternative scenarios (late hot flasher) as well as for binary evolution (white dwarf merger; post-RGB evolution) are tested. While we regard the post-RGB scenario as inappropriate, the white dwarf merger and the late hot flasher scenarios remain viable to explain the origin of He-enriched sdOs.
We study the 36 brightest radio sources in the Subaru/XMM-Newton Deep Field SXDF). Using MIPS 24 micron data from Spitzer we expect to trace accretion activity, even if it is hidden at optical wavelengths, unless the obscuring column is extreme. Our results suggest that in the decade or so below the break in the radio luminosity function that at least half, and potentially nearly all, radio sources are associated with accreting quasar-like objects. This is not true at lower radio luminosities where the quasar-like fraction approaches zero once compact sources are excluded.
We investigate the importance of the far red wing of the Lyman $ \alpha$ line of hydrogen in the atmospheres of cool white dwarfs of pure hydrogen composition. We find that this absorption process dominates all important sources of opacity in the blue part of the optical spectrum of these stars. Our successful fits to the spectra of cool DA/DC white dwarfs indicate that the far red wing of the $\rm Ly \alpha$ line is the source of opacity that had been missing in the models. The observed sequence of cool white dwarfs in color-color diagrams is very well reproduced by our new pure hydrogen atmosphere models, suggesting that the atmospheric composition of the coolest DC white dwarfs must be revisited.
We studied the orbital evolution of Jupiter-family comets (JFCs), Halley-type comets (HTCs), and long-period comets, and probabilities of their collisions with planets. In our runs the probability of a collision of one object with the Earth could be greater than the sum of probabilities for thousands of other objects. Even without a contribution of such a few bodies, the probability of a collision of a former JFC with the Earth was greater than 4$\cdot10^{-6}$. This probability is enough for delivery of all the water to Earth's oceans during formation of the giant planets. The ratios of probabilities of collisions of JFCs and HTCs with Venus and Mars to the mass of a planet usually were not smaller than that for Earth. Among 30,000 considered objects with initial orbits close to those of JFCs, a few objects got Earth-crossing orbits with semi-major axes $a$$<$2 AU and aphelion distances $Q$$<$4.2 AU, or even got inner-Earth ($Q$$<$0.983 AU), Aten, or typical asteroidal orbits, and moved in such orbits for more than 1 Myr (up to tens or even hundreds of Myrs). From a dynamical point of view, the fraction of extinct comets among near-Earth objects can exceed several tens of percent, but, probably, many extinct comets disintegrated into mini-comets and dust during a smaller part of their dynamical lifetimes if these lifetimes were large.
Carbon is one of the most abundant metals in the universe because of its synthesis in the fundamental triple alpha reaction. The knowledge of carbon abundances in different environments is one key ingredient to our understanding of stellar and galactochemical evolution. Studies of luminous OB-type stars allow us to address both topics even in galaxies beyond our own. Unfortunately the history of carbon abundance determinations from these objects in the last three decades is one of limited success. Analyses of the strong and weak line spectra of C II as well as C III tend to be largely discrepant. We present results of quantitative spectral analyses based on a sophisticated model atom for non-LTE line-formation calculations of C II-IV. As a first application, carbon abundances in a sample of B-type dwarfs and giants in nearby associations and in the field are determined. Consistency is finally achieved for all measurable lines (up to 40) from the three ionization stages. This includes in particular the notorious C II 4267 and 6578/6582 A features which are highly important for abundance determinations of fast-rotating and extragalactic objects. The long-standing problem of carbon line-formation can now be regarded as solved, with the previous difficulties related to the use of inaccurate atomic data and stellar parameters. A highly homogeneous and slightly sub-solar present-day carbon abundance from young stars in the solar vicinity of log C/H+12= 8.33+/-0.04 is derived
Aptly named, ice giants such as Uranus and Neptune contain significant amounts of water. While this water cannot be present near the cloud tops, it must be abundant in the deep interior. We investigate the likelihood of a liquid water ocean existing in the hydrogen-rich region between the cloud tops and deep interior. Starting from an assumed temperature at a given upper tropospheric pressure (the photosphere), we follow a moist adiabat downward. The mixing ratio of water to hydrogen in the gas phase is small in the photosphere and increases with depth. The mixing ratio in the condensed phase is near unity in the photosphere and decreases with depth; this gives two possible outcomes. If at some pressure level the mixing ratio of water in the gas phase is equal to that in the deep interior, then that level is the cloud base. Alternately, if the mixing ratio of water in the condensed phase reaches that in the deep interior, then the surface of a liquid ocean will occur. We find that Neptune is both too warm (photospheric temperature too high) and too dry (mixing ratio of water in the deep interior too low) for liquid oceans to exist at present. To have a liquid ocean, Neptune's deep interior water to gas ratio would have to be higher than current models allow, and the density at 19 kbar would have to be ~ 0.8 g/cm^3. Such a high density is inconsistent with gravitational data obtained during the Voyager flyby. As Neptune cools, the probability of a liquid ocean increases. Extrasolar "hot Neptunes," which presumably migrate inward toward their parent stars, cannot harbor liquid water oceans unless they have lost almost all of the hydrogen and helium from their deep interiors.
Using data obtained for twelve galaxies as part of the {\it Spitzer} Infrared Nearby Galaxies Survey (SINGS) and the Westerbork Synthesis Radio Telescope (WSRT)-SINGS radio continuum survey, we study how star formation activity affects the far-infrared (FIR)--radio correlation {\it within} galaxies by testing a phenomenological model, which describes the radio image as a smeared version of the FIR image. The physical basis of this description is that cosmic-ray (CR) electrons will diffuse measurably farther than the mean free path of dust-heating photons before decaying by synchrotron radiation. This description works well in general. Galaxies with higher infrared surface brightnesses have best-fit smoothing scale-lengths of a few hundred parsecs, substantially shorter than those for lower surface brightness galaxies. We interpret this result to suggest that galaxies with higher disk averaged star formation rates have had a recent episode of enhanced star formation and are characterized by a higher fraction of young CR electrons that have traveled only a few hundred parsecs from their acceleration sites in supernova remnants compared to galaxies with lower star formation activity.
We perform test-particle simulations in a 2D, differentially rotating stellar disk, subjected to a two-armed steady state spiral density wave perturbation in order to estimate the influence of a spiral structure on the local velocity field. By using Levenberg-Marquardt least-squares fit we decompose the local velocity field (as a result of our simulations) into Fourier components. Thus we obtain simulated measurements of the Oort constants, A, B, C, and K. We get relations between the Fourier coefficients and some galactic parameters, such as the phase angle of the Solar neighborhood and the spiral pattern speed. We show that systematic errors due to the presence of spiral structure are likely to affect the measurements of the Oort constants. Moderate strength spiral tructure causes errors of order 5 km/s. We find a variation of the Fourier coefficients with velocity dispersion and pattern speed. If our location in the Galaxy is near corotation then we expect a vanishing value for C for all phase angles. As one approaches the 4:1 Lindblad resonances the absolute value of C increases and so does its variation with Galactic longitude. We also ran simulations with a central bar moving at a pattern speed placing the Solar neighborhood just outside the outer Lindblad resonance. For all simulations the absolute value of C, on average, is larger for lower velocity dispersions, contrary to recent measurements.
The origin, evolution and role of magnetic fields in the production and shaping of proto-planetary and planetary nebulae (PPNe, PNe) is a subject of active research. Most PNe and PPNe are axisymmetric with many exhibiting highly collimated outflows, however, it is important to understand whether such structures can be generated by isolated stars or require the presence of a binary companion. Toward this end we study a dynamical, large-scale alpha-Omega interface dynamo operating in a 3.0 M_sun Asymptotic Giant Branch star (AGB) in both an isolated setting and one in which a low-mass companion is embedded inside the envelope. The back reaction of the fields on the shear is included and differential rotation and rotation deplete via turbulent dissipation and Poynting flux. For the isolated star, the shear must be resupplied in order to sufficiently sustain the dynamo. Furthermore, we investigate the energy requirements that convection must satisfy to accomplish this by analogy to the sun. For the common envelope case, a robust dynamo results, unbinding the envelope under a broad range of conditions. Two qualitatively different types of explosion may arise: (i) magnetically dominated, possibly resulting in collimated bipolar outflows and (ii) thermally induced from turbulent dissipation, possibly resulting in quasi-spherical outflows. A range of models is presented for a variety of companion masses.
Using the Hubble Space Telescope, we have obtained the first color-magnitude diagram (CMD) to reach the main-sequence turnoff of the Galactic globular cluster NGC 6388. From a comparison between the cluster CMD and 47 Tucanae's, we find that the bulk of the stars in these two clusters have nearly the same age and chemical composition. On the other hand, our results indicate that the blue horizontal branch and RR Lyrae components in NGC 6388 are intrinsically overluminous, which must be due to one or more, still undetermined, non-canonical second parameter(s) affecting a relatively minor fraction of the stars in NGC 6388.
We discuss the region of transition between galactic and extragalactic cosmic rays. The exact shapes and compositions of these two components contains information about important parameters of powerful astrophysical sources and the conditions in extragalactic space as well as for the cosmological evolution of the sources of high energy cosmic rays. Several types of experimental data, including the exact shape of the ultrahigh energy cosmic rays, their chemical composition and their anisotropy, and the fluxes of cosmogenic neutrinos have to be included in the solution of this problem.
(abridged) It has been suggested that Narrow-Line Seyfert 1 (NLS1) galaxies are evolutionarily young objects, powered by the accretion of gas onto central black holes that are lower in mass than those found in typical broad-line Seyferts. We explore this hypothesis through the analysis of high-spatial resolution, near-IR imaging data for a sample of 11 NLS1s. By employing the correlation between black-hole mass and host galaxy bulge luminosity, we determine the mean black-hole mass for our sample to be, in solar units, <(log M_BH)> = 7.9. Using the correlation between the size of the broad-line region and the monochromatic continuum luminosity, we obtain black-hole mass estimates under the assumption that the emission-line gas is in virial equilibrium. The mean black-hole mass derived from this relation is <(log M_BH)> = 6.4. We explore possible causes for this discrepancy and the ramifications for our understanding of the role played by NLS1s in AGN evolution. Because numerical simulations constrain the start of the AGN duty cycle to a time shortly after a significant gravitational interaction, we examine the morphology and near-IR bulge colors of the NLS1 sample for evidence of recent encounters. The mean bulge color is found to be redder than that of both a matched sample of non-active galaxies and published estimates for broad-line Seyferts. The source of the unusual bulge colors may be an excess of flux, peaking at around 2.2 micron, that has been detected near the centers of some NLS1s such as Mrk 1239. No evidence is found for light asymmetries or an extra stellar component that would indicate NLS1s are young objects. Finally, we postulate that there may be some interesting lines of circumstantial evidence suggesting that secular processes may be relevant in NLS1s.
We estimate the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that {\it a lower limit of} 21 $\pm$ 2% of stars with [Fe/H] $\leq -2.0$ exhibit [C/Fe] $\geq +1.0$. These fractions are higher than have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.
Flat density cores have been obtained for a limited number of clusters of galaxies by strong gravitational lensing. This paper explores the possibility that the degeneracy pressure of fermionic dark matter accounts for the flat top density profiles. This is a case study of A1689 for which the density profile has been obtained from the inner region out to 1Mpc by the combination of strong and weak lensing. In the case that the dark matter consists of the mixture of degenerate relic neutrinos and collisionless cold dark matter particles, the acceptable mass range for relic neutrinos is between 1 and 2 eV, if the ratio of the two kinds of dark matter particles is fixed to its cosmic value.
In many magnetized, dilute astrophysical plasmas, thermal conduction occurs almost exclusively parallel to magnetic field lines. In this case, the usual stability criterion for convective stability, the Schwarzschild criterion, which depends on entropy gradients, is modified. In the magnetized long mean free path regime, instability occurs for small wavenumbers when (dP/dz)(dln T/dz) > 0, which we refer to as the Balbus criterion. We refer to the convective-type instability that results as the magnetothermal instability (MTI). We use the equations of MHD with anisotropic electron heat conduction to numerically simulate the linear growth and nonlinear saturation of the MTI in plane-parallel atmospheres that are unstable according to the Balbus criterion. The linear growth rates measured from the simulations are in excellent agreement with the weak field dispersion relation. The addition of isotropic conduction, e.g. radiation, or strong magnetic fields can damp the growth of the MTI and affect the nonlinear regime. The instability saturates when the atmosphere becomes isothermal as the source of free energy is exhausted. By maintaining a fixed temperature difference between the top and bottom boundaries of the simulation domain, sustained convective turbulence can be driven. MTI-stable layers introduced by isotropic conduction are used to prevent the formation of unresolved, thermal boundary layers. We find that the largest component of the time-averaged heat flux is due to advective motions as opposed to the actual thermal conduction itself. Finally, we explore the implications of this instability for a variety of astrophysical systems, such as neutron stars, the hot intracluster medium of galaxy clusters, and the structure of radiatively inefficient accretion flows.
We investigate the radio spectral energy distributions (SEDs) of young star-forming galaxies and how they evolve with time. The duration and luminosity of the nonthermal radio emission from supernova remnants (SNRs) are constrained by using the observational radio SEDs of SBS 0335-052 and I Zw 18, which are the two lowest-metallicity blue compact dwarf galaxies in the nearby universe. The typical radio ``fluence'' for SNRs in SBS 0335-052, that is the radio energy emitted per SNR over its radiative lifetime, is estimated to be $\sim 6$--$22\times 10^{22} {\rm W Hz^{-1} yr}$ at 5 GHz. On the other hand, the radio fluence in I Zw 18 is $\sim 1$--$3\times 10^{22} {\rm W Hz^{-1} yr}$ at 5 GHz. We discuss the origin of this variation and propose scaling relations between synchrotron luminosity and gas density. We have also predicted the time dependence of the radio spectral index and of the spectrum itself, for both the ``active'' (SBS 0335-052) and ``passive'' (I Zw 18) cases. These models enable us to roughly age date and classify radio spectra of star-forming galaxies into active/passive classes. Implications for high-z galaxy evolution are also discussed.
Deep surveys have recently discovered galaxies at the tail end of the epoch of reionization. In the near future, these discoveries will be complemented by a new generation of low-frequency radio observatories that will map the distribution of neutral hydrogen in the intergalactic medium through its redshifted 21cm emission. In this paper we calculate the expected cross-correlation between the distribution of galaxies and intergalactic 21cm emission at high redshifts. We demonstrate using a simple model that overdense regions are expected to be ionized early as a result of their biased galaxy formation. This early phase leads to an anti-correlation between the 21cm emission and the overdensities in galaxies, matter, and neutral hydrogen. Existing Ly-alpha surveys probe galaxies that are highly clustered in overdense regions. By comparing 21cm emission from regions near observed galaxies to those away from observed galaxies, future observations will be able to test this generic prediction and calibrate the ionizing luminosity of high-redshift galaxies.
Wide-field (~8' x 8') and deep near-infrared (JHKs bands) polarization images of the Orion nebulae (IRNe) around young stellar objects (YSOs), both massive and low-mass. We found the IRNe around both IRc2 and BN to be very extensive, suggesting that there might be two extended (>0.7 pc) bipolar/monopolar IRNe in these sources. We discovered at least 13 smaller-scale (~0.01-0.1 pc) IRNe around less-massive YSOs including the famous source theta^2 Ori C. We also suggest the presence of many unresolved (<690 AU) systems around low-mass YSOs and young brown dwarfs showing possible intrinsic polarizations. Wide-field infrared polarimetry is thus demonstrated to be a powerful technique in revealing IRNe and hence potential disk/outflow systems among high-mass to substellar YSOs.
We report improved transformation equations between the $u'g'r'i'z'$ and $UBVR_CI_C$ photometric systems. Although the details of the transformations depend on luminosity class, we find a typical rms scatter on the order of 0.001 magnitude if the sample is limited to main sequence stars. Furthermore, we find an accurate transformation requires complex, multi-color dependencies for the bluer bandpasses. Results for giant stars will be reported in a subsequent paper.
Cataclysmic Variables (CV) are close binary systems, in which the primary, the more massive star, is a white dwarf. CVs usually exhibit a number of periodicities, most of which are now understood. However, recently, a new phenomenon was discovered that does not fit the standard picture. Two objects have been discovered to show periods that are much longer than orbital, and have no relation to it, either in light curves or in radial velocity (RV) variations measured from spectroscopy. Here, we show that the precession of the fast rotating magnetically accreting white dwarf can successfully explain these phenomena. The theory of compact objects predicts certain relations between the spin and precession periods, and our finding provides a good test for the theory and establishes a qualitative model to be explored both theoretically and observationally. Detection of precession can become a powerful tool in searching for the internal properties of compact stars, which would be otherwise inaccessible for us.
We select far-infrared (FIR-60 microns) and far-ultraviolet (FUV-1530 A) samples of nearby galaxies in order to discuss the biases encountered by monochromatic surveys (FIR or FUV). Very different volumes are sampled by each selection and much care is taken to apply volume corrections to all the analyses. The distributions of the bolometric luminosity of young stars are compared for both samples: they are found to be consistent with each other for galaxies of intermediate luminosities but some differences are found for high (>5 10^{10} L_sun) luminosities. The shallowness of the IRAS survey prevents us from securing comparison at low luminosities (<2 10^9 L_sun). The ratio of the total infrared (TIR) luminosity to the FUV luminosity is found to increase with the bolometric luminosity in a similar way for both samples up to 5 10^{10} L_sun. Brighter galaxies are found to have a different behavior according to their selection: the L_TIR/L_FUV ratio of the FUV-selected galaxies brighter than 5 10^{10} L_sun reaches a plateau whereas L_TIR/L_FUV continues to increase with the luminosity of bright galaxies selected in FIR. The volume-averaged specific star formation rate (SFR per unit galaxy stellar mass, SSFR) is found to decrease toward massive galaxies within each selection. The SSFR is found to be larger than that measured for optical and NIR-selected sample over the whole mass range for the FIR selection, and for masses larger than 10^{10} M_sun for the FUV selection. Luminous and massive galaxies selected in FIR appear as active as galaxies with similar characteristics detected at z ~ 0.7.
Traditional thermal evolution models of giant planets employ arbitrary initial conditions selected more for computational expediency than physical accuracy. Since the initial conditions are eventually forgotten by the evolving planet, this approach is valid for mature planets, if not young ones. To explore the evolution at young ages of jovian mass planets we have employed model planets created by one implementation of the core accretion mechanism as initial conditions for evolutionary calculations. We find that young jovian planets are smaller, cooler, and several to 100 times less luminous than predicted by earlier models. Furthermore the time interval during which the young jupiters are fainter than expected depends on the mass of planet. Jupiter mass planets (1 M_J) align with the conventional model luminosity in as little at 20 million years, but 10 M_J planets can take up to 1 billion years to match commonly cited luminosities, given our implementation of the core accretion mechanism. If our assumptions, especially including our treatment of the accretion shock, are correct, then young jovian planets are substantially fainter at young ages than currently believed. These results have important consequences both for detection strategies and for assigning masses to young jovian planets based on observed luminosities.
We consider nonlinear biasing models of galaxies with particular attention to a correlation between linear and quadratic biasing coefficients, b_1 and b_2. We first derive perturbative expressions for b_1 and b_2 in halo and peak biasing models. Then we compute power spectra and bispectra of dark matter particles and halos using N-body simulation data and of volume-limited subsamples of Sloan Digital Sky Survey (SDSS) galaxies, and determine their b_1 and b_2. We find that the values of those coefficients at linear regimes (k<0.2h/Mpc) are fairly insensitive to the redshift-space distortion and the survey volume shape. The resulting normalized amplitudes of bispectra, Q, for equilateral triangles, are insensitive to the values of b_1 implying that b_2 indeed correlates with b_1. The present results explain the previous finding of Kayo et al. (2004) for the hierarchical relation of three-point correlation functions of SDSS galaxies. While the relations between b_1 and b_2 are quantitatively different for specific biasing models, their approximately similar correlations indicate a fairly generic outcome of the biasing due to the gravity in primordial Gaussian density fields.
Massive black holes (MBHs), with masses in the range 10^3-10^8 Msolar, which merge with a companion black hole of similar mass are expected to be the most powerful source of gravitational radiation in the frequency range probed by LISA. MBH binaries can be detected by LISA up to z~5-15. Gravitational waves from MBH mergers can serve as a powerful tool to study the early evolution of the MBH population, and possibly the role played by MBHs in the joint evolution with their hosts. I review scenarios for the co-evolution of MBHs and cosmological structures, where MBH seeds form in pre-galactic structures. These black holes evolve then in a hierarchical fashion, following the merger hierarchy of their hosts. Accretion of gas, traced by quasar activity, plays a fundamental role in determining the two parameters defining a black hole, mass and spin. Gravitational waves, together with observations in electromagnetic bands, can help constrain the evolution of both MBH mass and spin.
The nature of the progenitors of type Ia supernovae is still under controversial debate. KPD 1930+2752 is one of the best SN Ia progenitor candidates known today. The object is a double degenerate system consisting of a subluminous B star (sdB) and a massive white dwarf (WD). Maxted et al. (2000) conclude that the system mass exceeds the Chandrasekhar mass. This conclusion, however, rests on the assumption that the sdB mass is 0.5 Mo. However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 Mo to more than 0.7 Mo. It is therefore important to measure the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit the inclination of the system can be constrained if the sdB radius and the projected rotational velocity can be measured with high precision. An analysis of the ellipsoidal variations in the light curve allows to tighten the constraints derived from spectroscopy. We derive the mass-radius relation for the sdB star from a quantitative spectral analysis of 150 low-resolution spectra. The projected rotational velocity is determined for the first time from 200 high-resolution spectra. In addition a reanalysis of the published light curve is performed. The atmospheric and orbital parameters are measured with unprecedented accuracy. In particular the projected rotational velocity is determined. A neutron star companion can be ruled out and the mass of the sdB is limited between 0.45 Mo and 0.52 Mo. The total mass of the system ranges from 1.36 Mo to 1.48 Mo and hence is likely to exceed the Chandrasekhar mass. So KPD 1930+2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate.
The Baikal Neutrino Telescope has been operating in its NT200 configuration
since April, 1998. The telescope has been upgraded in April, 2005, to the 10
Mton scale detector NT200+. It's main physics goal is the detection of signals
from high energy neutrino cascades. NT200+ reaches a 3-year sensitivity of 2
\times 10^{-7}cm^{-2}s^{-1}sr^{-1}GeV for an all-flavor diffuse cosmic E^{-2}
neutrino flux for energies 10^2 TeV \div 10^5 TeV.
Desgin and sensitivity of NT200+ are described. NT200+ is forming the basic
building block of a future km3-scale (Gigaton-Volume) Baikal Telescope.
Research and development work on that next stage detector has started.
To describe the evaporation status of the extrasolar planets, we propose to
consider an energy diagram in which the potential energy of the planets is
plotted versus the energy received by the upper atmosphere. Here we present a
basic method to estimate these quantities. For the potential energy, we include
the modification of the gravity field by the tidal forces from the parent
stars. This description allows a quick estimate of both the escape rate of the
atmospheric gas and the lifetime of a planet against the evaporation process.
In the energy diagram, we find an evaporation-forbidden region in which a
gaseous planet would evaporate in less than 5 billion years. With their
observed characteristics, all extrasolar planets are found outside this
evaporation-forbidden region. The escape rates are estimated to be in the range
10^5 g/s to 10^{12} g/s, with few cases above 10^{11} g/s. The estimated escape
rate for HD209458b is found to be consistent with the lower limit of 10^{10}
g/s obtained from interpretation of the HI Lyman-alpha observations.
Finally, this diagram suggests possibilities for the nature of the recently
discovered Neptune-mass planets. We find that GJ436b, 55Cnc_e and HD69830b
cannot be low mass gaseous planets. With density necessarily above 0.5g/cm3 to
survive evaporation, these planets must contain a large fraction of
solid/liquid material. Concerning GJ876d, we find that it must have a density
larger than ~3g/cm3 to survive the strong EUV energy flux from its nearby
parent star. GJ876d must contain a large fraction of massive elements.
Blue straggler stars present as secure members in the Galactic open clusters form a major challenge to the conventional picture of evolutionary population synthesis based on the stellar evolution theory of single stars, as illustrated in our previous work. Expansion of our sample in the current work to include younger age clusters provides a larger data base to expose the question raised for the simple stellar population model. The working sample now includes 97 Galactic open clusters of ages ranging from 0.1 to several Gyrs. The contributions of blue straggler stars to the integrated light of the host clusters are calculated on an individual cluster base. A data base of observational constrained simple stellar population model is made which has a larger age coverage than our previous work. It is shown in this work that the general existence of blue stragglers in star clusters of our sample dramatically altered the predictions of convectional stellar population model in terms of spectral energy distribution. The integrated spectral energy distributions of the synthetic spectra of the clusters are enhanced towards shorter wavelengths, therefore the results of the present work will cast new lights in understanding the properties of stellar populations.
We present a detailed analysis of the soft X-ray spectrum of the Seyfert 1 galaxy Markarian 509 taken with the XMM-Newton Reflection Grating Spectrometer. An underlying power-law continuum and three warm absorber phases provide a good fit to the data along with a number of broad and narrow emission lines. Our three warm absorber phases each have different ionization parameters and column densities. We identify a low ionization, log xi=0.89, high outflow velocity phase producing an Fe M-shell UTA feature along with absorption from O VI and N VI. There is an intermediate phase, log xi=2.14, showing absorption from H-like carbon and nitrogen and He-like neon and oxygen. The third, high ionization, log xi=3.26, low outflow velocity phase contains absorption from O VIII, Ne X and highly ionized iron. All phases are blueshifted with respect to the systemic velocity with flow velocities ranging from -60km/s to -510km/s. The observed broad emission features have an RMS velocity of ~8000km/s for the C VI and O VII lines.
One explanation for the disparity between Cold Dark Matter (CDM) predictions of galaxy numbers and observations could be that there are numerous dark galaxies in the Universe. These galaxies may still contain baryons, but no stars, and may be detectable in the 21cm line of atomic hydrogen. The results of surveys for such objects, and simulations that do/do not predict their existence, are controversial. In this paper we use an analytical model of galaxy formation, consistent with CDM, to firstly show that dark galaxies are certainly a prediction of the model. Secondly, we show that objects like VIRGOHI21, a dark galaxy candidate recently discovered by us, while rare are predicted by the model. Thirdly, we show that previous 'blind' HI surveys have placed few constraints on the existence of dark galaxies. This is because they have either lacked the sensitivity and/or velocity resolution or have not had the required detailed optical follow up. We look forward to new 21cm blind surveys (ALFALFA and AGES) using the Arecibo multi-beam instrument which should find large numbers of dark galaxies if they exist.
Several studies have investigated the fractal and multifractal nature of magnetic features in the solar photosphere and its variation with the solar magnetic activity cycle. Here we extend those studies by examining the fractal geometry of bright magnetic features at higher atmospheric levels, specifically in the solar chromosphere. We analyze structures identified in CaIIK images obtained with the Precision Solar Photometric Telescopes (PSPTs) at Osservatorio Astronomico di Roma (OAR) and Mauna Loa Solar Observatory (MLSO). Fractal dimension estimates depend on the estimator employed, the quality of the images, and the structure identification techniques used. We examine both real and simulated data and employ two different perimeter-area estimators in order to understand the sensitivity of the deduced fractal properties to pixelization and image quality. The fractal dimension of bright 'magnetic' features in CaIIK images ranges between values of 1.2 and 1.7 for small and large structures respectively. This size dependency largely reflects the importance of image pixelization in the measurement of small objects. The fractal dimension of chromospheric features does not show any clear systematic variation with time over the period examined, the descending phase of solar cycle 23. These conclusions, and the analysis of both real and synthetic images on which they are based, are important in the interpretation of previously reported results.
We use catalogues of superclusters of galaxies from the 2dF Galaxy Redshift Survey to study the properties of galaxies in superclusters. We compare the properties of galaxies in high and low density regions of rich superclusters, in poor superclusters and in the field, as well as in groups, and of isolated galaxies in superclusters of various richness. We show that in rich superclusters the values of the luminosity density smoothed on a scale of 8 \Mpc are higher than in poor superclusters: the median density in rich superclusters is $\delta \approx 7.5$, in poor superclusters $\delta \approx 6.0$. Rich superclusters contain high density cores with densities $\delta > 10$ while in poor superclusters such high density cores are absent. The properties of galaxies in rich and poor superclusters and in the field are different: the fraction of early type, passive galaxies in rich superclusters is slightly larger than in poor superclusters, and is the smallest among the field galaxies. Most importantly, in high density cores of rich superclusters ($\delta > 10$) there is an excess of early type, passive galaxies in groups and clusters, as well as among those which do not belong to groups or clusters. The main galaxies of superclusters have a rather limited range of absolute magnitudes. The main galaxies of rich superclusters have larger luminosities than those of poor superclusters and of groups in the field. Our results show that both the local (group/cluster) environments and global (supercluster) environments influence galaxy morphologies and their star formation activity.
We report here the results of 2 observations performed simultaneously with INTEGRAL, RXTE, the Ryle and Nancay radio telescopes. These observations belong to the so-called $\nu$ and $\lambda$ classes of variability during which a high level of correlated X-ray and radio variability is observed. We study the connection between the accretion processes seen in the X-rays, and the ejections seen in radio. By observing an ejection during class $\lambda$, we generalise the fact that the discrete ejections in GRS 1915+105 occur after sequences of soft X-ray dips/spikes. We then identify the most likely trigger of the ejection through a spectral approach to our INTEGRAL data. We show that each ejection is very probably the result of the ejection of a Comptonising medium responsible for the hard X-ray emission seen above 15 keV.
We present a new cluster-finding algorithm based on a combination of the Voronoi Tessellation and Friends-Of-Friends methods. The algorithm utilises probability distribution functions derived from a photometric redshift analysis. We test our algorithm on a set of simulated cluster-catalogues and have published elsewhere its employment on UKIDSS Ultra Deep Survey infrared J and K data combined with 3.6 micro-m and 4.5 micro-m Spitzer bands and optical BVRi'z' imaging from the Subaru Telescope. This pilot study has detected clusters over 0.5 square degrees in the Subaru XMM-Newton Deep Field. The resulting cluster catalogue contains 13 clusters at redshifts 0.61 <= z <= 1.39 with luminosities 10 L* <~ L_tot <~ 50 L*.
A comparison is made between mean-field models and direct numerical simulations of rotating magnetoconvection and the geodynamo. The mean-field coefficients are calculated with the fluid velocity taken from the direct numerical simulations. The magnetic fields resulting from mean-field models are then compared with the mean magnetic field from the direct numerical simulations.
High-precision radial-velocity techniques, which enabled the detection of extrasolar planets are now sensitive to relativistic effects in the data of spectroscopic binary stars (SBs). We show how these effects can be used to derive the absolute masses of the components of eclipsing single-lined SBs and double-lined SBs from Doppler measurements alone. High-precision stellar spectroscopy can thus substantially increase the number of measured sellar masses, thereby improving the mass-radius and mass-magnitude calibrations.
In preparation for James Webb Space Telescope (JWST), a set of cross calibration programs with HST and Spitzer for suitable primary photometric standards and astrometric fields were developed. NICMOS/HST and IRAC/Spitzer photometry observations of new solar analog standards in NGC 6791 and Melotte 66 were executed. These new photometric standards will provide ~ 5% photometric precision at V~19 from the near-IR to the mid-IR wavelength range for efficient on-orbit calibration and measuring of photometric stability of the JWST complement. For the astrometric calibration, a field in the LMC has been selected. This field falls within the JWST continuous viewing zone, within 5 degrees from the ecliptic poles, and has the stellar density necessary to achieve accuracies better than 1 mas with HST/ACS. These independent observations will play a key role in meeting the mission requirements and will allow a fast commissioning of the observatory.
We outline a scenario in which dark energy is associated with the particle horizon. An energy density related to the area of the particle horizon is found to behave in accordance with current observational constraints. The equation of state drops as the particle horizon traverses a closed universe. Depending on the constant of proportionality, either the ensuing inflationary period prevents the particle horizon from vanishing, or it may lead to a sequence of "Big Rips".
Purely hydrodynamic numerical experiments into the evolution of astrophysical discs typically include some sort of viscosity in order to cause accretion. In this paper, we demonstrate an alternative method of implementing viscous forces, with extremely good angular momentum conservation properties. The method is based on altering the cell fluxes, rather than incorporating a viscous force. We test this method on the classical `ring spreading' problem, and demonstrate angular momentum conservation at the $10^{-8}$ level.
The black-hole X-ray binary transient GRO J1655-40 underwent an outburst beginning in early 2005. We present the results of our multi-wavelength observational campaign to study the early outburst spectral and temporal evolution, which combines data from X-ray (RXTE, INTEGRAL), radio (VLA) and optical (ROTSE, SMARTS) instruments. During the reported period the source left quiescence and went through four major accreting black hole states: low-hard, hard intermediate, soft intermediate and high-soft. We investigated dipping behavior in the RXTE band and compare our results to the 1996-1997 case, when the source was predominantly in the high-soft state, finding significant differences. We consider the evolution of the low frequency quasi-periodic oscillations and find that the frequency strongly correlates with the spectral characteristics, before shutting off prior to the transition to the high-soft state. We model the broad-band high-energy spectrum in the context of empirical models, as well as more physically motivated thermal and bulk-motion Comptonization and Compton reflection models. RXTE and INTEGRAL data together support a statistically significant high energy cut-off in the energy spectrum at 100~200 keV during the low-hard state. The RXTE data alone also show it very significantly during the transition, but cannot see one in the high-soft state spectra. We consider radio, optical and X-ray connections in the context of possible synchrotron and synchrotron self-Compton origins of X-ray emission in low-hard and intermediate states. In this outburst of GRO J1655-40, the radio flux does not rise strongly with the X-ray flux.
We present the results of a spectral study of the soft X-ray emission
(0.2-2.5 keV) from low-latitude (`disk') regions of Jupiter. The data were
obtained during two observing campaigns with XMM-Newton in April and November
2003. While the level of the emission remained approximately the same between
April and the first half of the November observation, the second part of the
latter shows an enhancement by about 40% in the 0.2-2.5 keV flux. A very
similar, and apparently correlated increase, in time and scale, was observed in
the solar X-ray and EUV flux.
The months of October and November 2003 saw a period of particularly intense
solar activity, which appears reflected in the behaviour of the soft X-rays
from Jupiter's disk. The X-ray spectra, from the XMM-Newton EPIC CCD cameras,
are all well fitted by a coronal model with temperatures in the range 0.4-0.5
keV, with additional line emission from Mg XI (1.35 keV) and Si XIII (1.86
keV): these are characteristic lines of solar X-ray spectra at maximum activity
and during flares.
The XMM-Newton observations lend further support to the theory that Jupiter's
disk X-ray emission is controlled by the Sun, and may be produced in large part
by scattering, elastic and fluorescent, of solar X-rays in the upper atmosphere
of the planet.
Polaris, the nearest and brightest classical Cepheid, is a member of at least
a triple system. It has a wide ($18''$) physical companion, the F-type dwarf
Polaris B. Polaris itself is a single-lined spectroscopic binary with an
orbital period of 30 years (Kamper, 1996, JRASC, 90, 140). By combining {\it
Hipparcos} measurements of the instantaneous proper motion with long-term
measurements and the Kamper radial-velocity orbit, Wielen et al. (2000, A&A,
360, 399) have predicted the astrometric orbit of the close companion. Using
the {\it Hubble Space Telescope} and the Advanced Camera for Surveys'
High-Resolution Channel with an ultraviolet (F220W) filter, we have now
directly detected the close companion. Based on the Wielen et al. orbit, the
{\it Hipparcos} parallax, and our measurement of the separation ($0.176''$
$\pm$ $0.002''$), we find a preliminary mass of 5.0 $\pm$ 1.5 M$_{\odot}$ for
the Cepheid and 1.38 $\pm$ 0.61 M$_{\odot}$ for the close companion. These
values will be refined by additional {\it HST} observations scheduled for the
next 3 years.
We have also obtained a {\it Chandra} ACIS-I image of the Polaris field. Two
distant companions C and D are not X-rays sources and hence are not young
enough to be physical companions of the Cepheid. There is one additional
stellar X-ray source in the field, located $253''$ from Polaris A, which is a
possible companion. Further investigation of such a distant companion is
valuable to confirm the full extent of the system.
We present a statistical analysis of the photometric properties and spatial distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using an improved image stacking technique, we measure the cross-correlation between MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer the light-weighted impact parameter distribution of MgII absorbers. Such a quantity is well described by a power-law with an index that strongly depends on W_0, ranging from ~-1 for W_0<~1\AA to ~-2 for W_0>~ 1.5\AA. At redshift 0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global luminosity-weighted colors are typical of present-day intermediate type galaxies. However, while the light of weaker absorbers originates mostly from red passive galaxies, stronger systems display the colors of blue star-forming galaxies. Based on these observations, we argue that the origin of strong MgII absorber systems might be better explained by models of metal-enriched gas outflows from star-forming/bursting galaxies. Our analysis does not show any redshift dependence for both impact parameter and rest-frame colors up to z=1. However, we do observe a brightening of the absorbers related light at high redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon typical of a given evolutionary phase that more massive galaxies experience earlier than less massive ones, in a downsizing fashion. (abridged)
We have performed optical polarimetric observations of the SN2006aj associated to the gamma-ray burst (GRB) of February 18, 2006, GRB060218 that provide information on its expansion geometry. The data were acquired in the R-band with the 0.7m telescope of Crimea, 2.5m Nordic Optical Telescope and the 2.2m of Calar Alto. We report the detection of linear polarization between 3 and 39 days after the gamma-ray event (t-t_0). This represents the first polarization detection of a Ic supernova (SN) associated to an X-ray flash. Our data exhibit a degree of linear polarization (P) around P~4% at t-t_0 ~ 3-5 days, followed by a constant polarization phase with P~1.4% at 13.7 < t-t_0 < 39 days. Our data suggest a decay in P, and more interestingly, show a position angle (\theta) rotation of ~100 degrees comparing data taken before and after the R-band lightcurve peak. The reported polarization measurements can be explained by the evolution of an asymmetric SN expansion. We discuss on several ingredients that could account for the observed \theta rotation.
We combine the results from several HST investigations of the central structure of early-type galaxies to generate a large sample of parameterized surface photometry. The studies included were those that used the "Nuker law" to characterize the inner light distributions of the galaxies. The sample comprises WFPC1 and WFPC2 V band observations published earlier by our group, R band WFPC2 photometry of Rest et al., NICMOS H band photometry by Ravindranath et al. and Quillen et al., and the BCG WFPC2 I band photometry of Laine et al. The distribution of the logarithmic slopes of the central profiles strongly affirms that the central structure of elliptical galaxies with Mv < -19 is bimodal, based on both parametric and non-parametric analysis. At the HST resolution limit, most galaxies are either power-law systems, which have steep cusps in surface brightness, or core systems, which have shallow cusps interior to a steeper envelope brightness distribution. A rapid transition between the two forms occurs over the luminosity range -22 < Mv < -20, with cores dominating at the highest luminosities, and power-laws at the lowest. There are a few "intermediate" systems that have both cusp slopes and total luminosities that fall within the core/power-law transition, but they are rare and do not fill in the overall bimodal distribution of cusp slopes. These results are inconsistent with the Ferrarese et al. Virgo Cluster Survey (VCS) analysis. However, using galaxies common to the VCS samples, we demonstrate that the VCS models of the cusps are either a poor match to the observations or consist of forms fitted to the galaxy envelopes and extrapolated inward to the HST resolution limit.
Nearby dwarf galaxies exhibit tight correlations between their global stellar and dynamical properties, such as circular velocity, mass-to-light ratio, stellar mass, surface brightness, and metallicity. Such correlations have often been attributed to gas or metal-rich outflows driven by supernova energy feedback to the interstellar medium. We use high-resolution cosmological simulations of high-z galaxies with and without energy feedback, as well as analytic chemical evolution modeling, to investigate whether the observed correlations can arise without supernova-driven outflows. We find that the simulated dwarf galaxies exhibit correlations similar to those observed as early as z~10 even in simulations in which feedback is explicitly turned off. We also show that the correlations can be well reproduced by our analytic chemical evolution model that accounts for gas inflow but does not include outflows, and a star formation rate obeying the Kennicutt law with a critical density threshold. We argue that correlations in simulated galaxies arise due to the increasingly inefficient conversion of gas into stars in low-mass dwarf galaxies rather than mass loss in winds. We also show that the decrease of the observed effective yield in low-mass objects, often used as an indicator of gas and metal outflows, can be reasonably reproduced in our simulations without outflows. We show that this trend can arise if a significant fraction of metals in small galaxies is spread to the outer regions of the gas disk outside the stellar extent via mixing. In this case the effective yield can be significantly underestimated if only metals within the stellar radius are taken into account.
We present a maximum likelihood method for fitting two-dimensional model
distributions to stellar data in colour-magnitude space. This allows one to
include (for example) binary stars in an isochronal population. The method also
allows one to derive formal uncertainties for fitted parameters, and assess the
likelihood that a good fit has been found. We use the method to derive an age
of 38.5 +3.5/-6.5 Myrs and a true distance modulus of 7.79 +0.11/-0.05 mags
from the V vs V-I diagram of NGC2547 (the uncertainties are 67 percent
confidence limits, and the parameters are insensitive to the assumed binary
fraction). These values are consistent with those previously determined from
low-mass isochronal fitting, and are the first measurements to have
statistically meaningful uncertainties. The age is also consistent with the
lithium depletion age of NGC2547, and the HIPPARCOS distance to the cluster is
consistent with our value.
The method appears to be quite general and could be applied to any
N-dimensional dataset, with uncertainties in each dimension. However, it is
particularly useful when the data are sparse, in the sense that both the
typical uncertainties for a datapoint and the size of structure in the function
being fitted are small compared with the typical distance between datapoints.
In this case binning the data will lose resolution, whilst the method presented
here preserves it.
Software implementing the methods described in this paper is available from
this http URL
We present the design of, and a first analysis of data from, the atmospheric seeing monitor at the Australia Telescope Compact Array (ATCA). The seeing monitor has been operational almost continuously since May 2004 and every 10min delivers a measurement of the atmospheric phase stability at the observatory. Its measurements can be used by observers to help deciding whether it is worth carrying out observations at millimetre wavelengths or whether a longer-wavelength backup project should be observed. We present a statistical analysis of the data recorded since September 2004 to characterize the annual variations in atmospheric path length fluctuations. Our analysis shows that in terms of phase stability, nights in spring, summer, and autumn are as good as, or better than, days in winter. We also find that the data imply that the turbulence in the lower few hundred metres of the atmosphere is predominantly responsible for the atmospheric seeing.
We model the electromagnetic emission signatures of massive black hole binaries (MBHBs) with an associated gas component. The method comprises numerical simulations of relativistic binaries and gas coupled with calculations of the physical properties of the emitting gas. We calculate the accretion powered UV/X-ray and Halpha light curves and the Halpha emission line profiles. The simulations have been carried out with a modified version of the parallel tree SPH code Gadget. The heating, cooling, and radiative processes for the solar metallicity gas have been calculated with the photoionization code Cloudy. We investigate gravitationally bound, sub-parsec binaries which have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a coplanar binary and gas disk, suggest that the outbursts in the X-ray light curve are pronounced during pericentric passages and can serve as a fingerprint for this type of binaries if periodic outbursts are a long lived signature of the binary. The Halpha emission-line profiles also offer strong indications of a binary presence and may be used as a criterion for selection of MBHB candidates for further monitoring from existing archival data. The orbital period and mass ratio of a binary could be determined from the Halpha light curves and profiles of carefully monitored candidates. Although systems with the orbital periods studied here are not within the frequency band of the Laser Interferometer Space Antenna (LISA), their discovery is important for understanding of the merger rates of MBHBs and the evolution of such binaries through the last parsec and towards the detectable gravitational wave window.
Here we present an exact method for orbit determinations of an astrometric binary for which the primary star can be seen but the companion is unseen like a black hole or an extra-solar planet. Since the laws of the motion of celestial objects were discovered by Kepler in the seventeenth century, many attempts have been made to solve a fundamental problem of how to determine the orbital elements (and the mass) of a binary by measuring its positions projected onto a celestial sphere. It has been long believed that some approximate/iterative or numerical technique must be employed to determine the orbital elements. In the nineteenth century, the problem was solved for resolved double stars, for which the relative vector from the primary star to the secondary is observable. For astrometric binaries, however, for which this relative vector is not observable, an exact solution was very recently found by Asada, Akasaka and Kasai by assuming no observational errors. This paper extends the solution considering realistic observational data. The generalized solution is still simply expressed only by elementary functions.
We discuss uncertainties and possible sources of errors associated with the determination of the diffuse Galactic gamma-ray emission using the EGRET data. Most of the issues will be relevant also in the GLAST era. The focus here is on issues that impact evaluation of dark matter annihilation signals against the diffuse gamma-ray emission of the Milky Way.
We present an exact solution of the equations for orbit determination of a two body system in a hyperbolic or parabolic motion. In solving this problem, we extend the method employed by Asada, Akasaka and Kasai (AAK) for a binary system in an elliptic orbit. The solutions applicable to each of elliptic, hyperbolic and parabolic orbits are obtained by the new approach, and they are all expressed in an explicit form, remarkably, only in terms of elementary functions. We show also that the solutions for an open orbit are recovered by making a suitable transformation of the AAK solution for an elliptic case.
There has been tremendous progress in observing oscillations in solar-type stars. In a few short years we have moved from ambiguous detections to firm measurements. We review the recent results, most of which have come from high-precision Doppler measurements. We also review briefly the results on giant and supergiant stars and the prospects for the future.
Self-similar shock solutions in spherically symmetric polytropic gas flows are constructed and analyzed in contexts of proto-star formation processes. Among other possible solutions, we model a similarity shock across the sonic critical curve with an inner free-fall core collapse and a simultaneous outer expansion of the extended envelope; the separation or stagnation surface between these two flow zones travels outwards in a self-similar manner at a variable speed. One readily obtains accretion shock solutions. Semi-complete self-similar solutions across the sonic critical curve either once or twice without shocks can also be constructed. Features of star formation clouds of our polytropic model include the mass density scaling in the outer flow zone $\rho\propto r^{-2/(2-\gamma)}$, the temperature scalings of the inner flow zone $T\propto r^{-3(\gamma-1)/2}$ and of the outer flow zone $T\propto r^{-2(\gamma-1)/(2-\gamma)}$, and the variable central mass accretion rate $\dot{M}=k^{3/2}t^{(3-3\gamma)}m_0/G$ where $\gamma$ is the polytropic index, $k$ is a constant, $m_0$ is the core mass, and $G$ is the gravitational constant. Spectral line profiles characteristic of the `envelope expansion with core collapse' (EECC) shock solutions are expected. Random magnetic field permeated in a partially ionized cloud can be incorporated into this theoretical polytropic model framework. We discuss briefly our results in context of the oft-observed starless B335 cloud system as an example.
We conducted a radial velocity survey of the Cygnus OB2 Association over a 6 year (1999 - 2005) time interval to search for massive close binaries. During this time we obtained 1139 spectra on 146 OB stars to measure mean systemic radial velocities and radial velocity variations. We spectroscopically identify 73 new OB stars for the first time, the majority of which are likely to be Association members. Spectroscopic evidence is also presented for a B3Iae classification and temperature class variation (B3 - B8) on the order of 1 year for Cygnus OB2 No. 12. Calculations of the intial mass function with the current spectroscopic sample yield Gamma = -2.2 +/- 0.1. Of the 120 stars with the most reliable data, 36 are probable and 9 are possible single-lined spectroscopic binaries. We also identify 3 new and 8 candidate double-lined spectroscopic binaries. These data imply a lower limit on the massive binary fraction of 30% - 42%. The calculated velocity dispersion for Cygnus OB2 is 2.44 +/- km/s, which is typical of open clusters. No runaway OB stars were found.
None of N-body gravitating systems have been considered to emit periodic gravitational waves because of their chaotic orbits when N=3 (or more). Nevertheless, we show that triple stars are capable of generating periodic waves, thereby demonstrating that the true number of sources detectable by large-scale interferometers such as LIGO, VIRGO, GEO600 and TAMA300 is larger than previously thought. A tooth-shaped waveform generated by this novel source is different from that of a binary system. Therefore, it will be possible to distinguish it in future observations.
We present high-resolution echelle spectroscopy, obtained with the UVES
spectrograph on ESO/VLT, of two luminous star clusters in the metal-poor blue
compact galaxy ESO 338-IG04 at a distance of 37.5 Mpc. Cross-correlating with
template stars, we obtain line-of-sight velocity dispersions of 33 and 17 km/s.
By combining with size estimates from Hubble Space Telescope images we infer
dynamical masses of 1.3x10^7 and 4.0x10^6 solar masses for the two clusters,
making them among the most massive known. The less massive cluster is the
faintest cluster for which a dynamical mass has yet been obtained. In both
clusters we detect Balmer absorption lines which we use to estimate their ages.
From the younger (~6 Myr) and more massive cluster, we detect He II 4686
emission of intermediate width, indicating the presence of very massive
O-stars. Moreover, analysis of the [O III] 5007 and H-alpha emission lines from
the region near the younger cluster indicates that it is associated with a
bubble expanding at ~40 km/s. We also see from the Na I D absorption lines
indications of neutral gas flows towards the younger cluster. We compare the
dynamical masses with those derived from photometry and discuss implications
for the stellar initial mass function (IMF) in each cluster. Both clusters are
compatible with rather normal IMFs which will favour their long-term survival
and evolution into massive bona fide globular clusters.
Recent experiments suggest that polarized photons may couple significantly to pseudoscalar particles such as axions. We study the possible observational signatures of axion-photon coupling for radiation from magnetic stars, with particular focus on neutron stars. We present general methods for calculating the axion-photon conversion probability during propagation through a varying magnetized vacuum as well as across an inhomogeneous atmosphere. Partial axion-photon conversion may take place in the vacuum region outside the star. Strong axion-photon mixing occurs due to a resonance in the atmosphere, and depending on the axion coupling strength and other parameters, significant axion-photon conversion can take place at the resonance. Such conversions may produce observable effects on the radiation spectra and polarization signals from the star. We also apply our results to axion-photon propagation in the Sun and in magnetic white dwarfs.
Aims: Hete J1900.1-2455 is the seventh known X-ray transient accreting millisecond pulsar and has been in outburst for more than one year. We compared the data on Hete J1900.1-2455 with other similar objects and made an attempt at deriving constraints on the physical processes responsible for a spectral formation. Methods: The broad-band spectrum of the persistent emission in the 2-300 keV energy band and the timing properties were studied using simultaneous INTEGRAL and the publicly available RXTE data obtained in October 2005. The properties of the X-ray bursts observed from Hete J1900.1-2455 were also investigated. Results: The spectrum is well described by a two-component model consisting of a blackbody-like soft X-ray emission at 0.8 keV temperature and a thermal Comptonized spectrum with the electron temperature of 30 keV and Thomson optical depth tau_T ~ 2 for the slab geometry. The source is detected by INTEGRAL up to 200 keV at the luminosity of 5E36 erg/s (assuming a distance of 5 kpc) in the 0.1-200 keV energy band. We have also detected one type I X-ray burst which shows the photospheric radius expansion. The burst occurred at an inferred persistent emission level of ~ 3-4% of the Eddington luminosity. Using the data for all X-ray bursts observed to date from Hete J1900.1-2455, the burst recurrence time is estimated to be about 2 days. No pulsations have been detected either in the RXTE or in the INTEGRAL data which puts interesting constraints on theories of the magnetic field evolution in neutron star low-mass X-ray binaries.
We present a complete particle physics model that explains three major problems of modern cosmology: inflation, dark matter and dark energy, and also gives a mechanism for leptogenesis. The model has a new gauge group $SU(2)_Z$ that grows strong at a scale $\Lambda\sim 10^{-3}$ eV. We focus on the inflationary aspects of the model. Inflation occurs with a Coleman-Weinberg potential at a low scale, down to $\sim 6\times 10^5\gev$, being compatible with observational data.
The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range in particular has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study 6 X-class and 1 M-class flares in the energy range between 100 keV and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2% and 54%, with errors ranging from 10% to 26% in 1 sigma level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme properties.
The existence of extrasolar planets with short orbital periods suggests that planetary migration induced by tidal interaction with the protoplanetary disk is important. Cores and terrestrial planets may undergo migration as they form. In this paper we investigate the evolution of a population of cores with initial masses in the range 0.1-1 earth mass embedded in a disk. Mutual interactions lead to orbit crossing and mergers, so that the cores grow during their evolution. Interaction with the disk leads to orbital migration, which results in the cores capturing each other in mean motion resonances. As the cores migrate inside the disk inner edge, scatterings and mergers of planets on unstable orbits together with orbital circularization causes strict commensurability to be lost. Near commensurability however is usually maintained. All the simulations end with a population of typically between two and five planets, with masses depending on the initial mass. These results indicate that if hot super-Earths or Neptunes form by mergers of inwardly migrating cores, then such planets are most likely not isolated. We would expect to always find at least one, more likely a few, companions on close and often near-commensurable orbits. To test this hypothesis, it would be of interest to look for planets of a few to about 10 earth masses in systems where hot super-Earths or Neptunes have already been found.
The equation of state of dark energy is investigated to determine how much it may deviate from the equation of state of the cosmological constant (CC). Two aspects of the problem are studied: the "expansion" around the vacuum equation of state and the problem of the crossing of the cosmological constant boundary.
We have carried out a deep (21.5<R<23.9) systematic survey of galaxies selected from SWIRE using GMOS to target X-ray sources, AGN and galaxies with photometric redshifts greater than 1. We have obtained a total of 198 redshifts, of which 132 have secure redshifts in the range 0.12<z<1.21. This sample, combined with the 111 redshifts targeted with WIYN in the same field and the 1 square degree survey of ELAIS-N1 with ACIS-I, will allow us to calibrate our photometric redshift techniques; characterise the star formation history of the universe between 0.5<z<1.2 where galaxy formation activity peaks; and explore the connection between AGN and star-formation activity.
Our aim is to constrain the properties of dark matter halos inhabiting high density environments, such as is the case in massive galaxy clusters. We use galaxy-galaxy lensing techniques that utilize a maximum likelihood method to constrain the parameters of the lenses. It has been demonstrated that such a technique provides strong constraints on the parameters that characterize a galaxy halo, as well as on the aperture mass of these halos. In this analysis, we only use weak shear data and do not include strong lensing constraints. We present the results of a study of galaxy-galaxy lensing in a homogeneous sample of massive X-ray luminous clusters at z~0.2. These have been observed in three bands with the CFH12k instrument. We find dark matter halos in these clusters to be compact compared to those inferred around isolated field galaxies of equivalent luminosity at this redshift: the half mass radius is found to be smaller than 50 kpc, with a mean total mass of order 0.2 10^{12} M_sun. This is in good agreement with previous galaxy-galaxy lensing results and with numerical simulations, in particular with the tidal stripping scenario. We thus provide a strong confirmation of tidal truncation from a homogeneous sample of galaxy clusters. Moreover, it is the first time that cluster galaxies are probed successfully using galaxy-galaxy lensing techniques from ground based data.
The SPIRE Photometer Simulator reproduces the entire Herschel-SPIRE system in a modular IDL program. Almost every aspect of the operation of SPIRE can be investigated in a systematic way to ensure that observations are performed in the most efficient way possible when Herschel flies. This paper describes some of the work done with the Simulator to help prepare for large observing programmes such as deep extra-galactic, high-redshift surveys.
It has long been realized that dark matter halos formed in cosmological N-body simulations are characterized by density profiles $\rho(r)$ that, when suitably scaled, have similar shapes. Additionally, combining the density and velocity dispersion profiles $\sigma(r)$, each of which have decidedly nonpower-law shapes, leads to quantity \psd that is a power-law in radius over 3 orders of magnitude in radius. Halos' velocity anisotropy profiles $\beta(r)$ vary from isotropic near the centers of halos to quite radially anisotropic near the virial radius. Finally, there appears to be a nearly linear correlation between $\beta$ and the logarithmic density slope $\gamma$ for a wide variety of halos. This work is part of a continuing investigation of the above interrelationships and their origins using analytical and semi-analytical techniques. Our finding suggest that the nearly linear $\beta$--$\gamma$ relationship is not just another expression of scale-free \psd behavior. We also note that simultaneously reproducing density and anisotropy profiles like those found in simulations requires $\beta(r)$ and $\gamma(r)$ to have similar shapes, leading to nearly linear $\beta$--$\gamma$ correlations. This work suggests that the $\beta$--$\gamma$ and power-law \psd relations have distinct physical origins.
The nature of the Hyades stream, or Hyades moving group, is a long-standing question of Galactic Astronomy. Is this stream caused by non-axisymmetric perturbations of the galactic potential, such as transient or quasi-stationary spiral waves, or by the on-going evaporation of the Hyades cluster? A simple observational test has been designed to answer that question. Using the Geneva-Copenhagen survey of F and G dwarfs, we compare the mass distribution and metallicity of the stream to those of field disk stars. If the Hyades stream is composed of stars trapped at resonance, its mass distribution should obey the present-day mass function (PDMF) of the disk, and its metallicity should reflect its origin in the inner regions of the Galaxy. On the other hand, if it is an evaporated cluster, we expect a different mass distribution, depending on the inital mass function (IMF) of the cluster, and on the proportion of evaporated stars as a function of mass. We find that extreme conditions have to be adopted for the selective evaporation and IMF of the cluster to make the observed mass ditribution of the stream only roughly consistent (at a one-sigma level) with the coeval evaporated cluster scenario. The observed mass distribution is in much better agreement with the PDMF of the field. We also note that the peculiar metallicity of the stream is inconsistent with that of a field population from the solar neighbourhood trapped in the primordial cluster during its formation process and subsequently evaporated. These observations thus favour a resonant origin for the Hyades stream.
Using high spatial resolution Hubble Space Telescope/Advanced Camera for Surveys archival imaging observations of Arp 116, centred on the elliptical galaxy NGC 4649, we explore the novel technique of pixel-by-pixel analysis of the galaxy's colour-magnitude diagramme to search for any evidence of recent enhanced star formation due to the apparent tidal interaction with its spiral companion, NGC 4647. From a detailed analysis of the system's geometry, and based on additional circumstantial evidence from extant multi-wavelength observations, we conclude that, while there may be grounds for the tidal-interaction assumption for this system, any interaction has thus far been of insufficient strength to trigger an enhanced level of recent star formation in the elliptical component, although close inspection of our colour images shows a faint excess of bluer pixels (a ~0.20 mag bluer "loop") in the elliptical galaxy on the side of the spiral companion. Given that there appears to be a moderate reservoir of available gas for ongoing star formation (although at low column density), this suggests that we are currently witnessing the onset of the tidal interaction between NGC 4647 and NGC 4649. In addition, the triggering of new star formation in NGC 4649 may be significantly impeded due to the much lower mass of the spiral component.
We have developed a technique of isolating elongated structures in galactic images, such as spiral arms, using anisotropic wavelets and apply this to maps of the CO, infrared and radio continuum emission of the grand-design spiral galaxy M51. Systematic shifts between the ridges of CO, infrared and radio continuum emission that are several $\kpc$ long are identified, as well as large variations in pitch angle along spiral arms, of a few tens of degrees. We find two types of arms of polarized radio emission: one has a ridge close to the ridge of CO, with similar pitch angles for the CO and polarization spirals and the regular magnetic field; the other does not always coincide with the CO arm and its pitch angle differs from the orientation of its regular magnetic field. The offsets between ridges of regular magnetic field, dense gas and warm dust are compatible with the sequence expected from spiral density wave triggered star formation, with a delay of a few tens of millions of years between gas entering the shock and the formation of giant molecular clouds and a similar interval between the formation of the clouds and the emergence of young star clusters. At the position of the CO arms the orientation of the regular magnetic field is the same as the pitch angle of the spiral arm, but away from the gaseous arms the orientation of the regular field varies significantly. Spiral shock compression can explain the generation of one type of arm of strong polarized radio emission but a different mechanism is probably responsible for a second type of polarization arm. (Shortened abstract.)
We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our previous results of s-process nucleosynthesis during the core He-burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell-carbon burning. We show that the recent compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of the efficiency of the s-process during core He-burning. In particular, this rate leads to the lowest overproduction factor of Kr80 found to date during core He-burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of C12 attained at the end of core helium burning, which in turn is mainly determined by the C12(alpha,gamma)O16 reaction. The still present uncertainty in the rate for this reaction implies that the s-process in massive stars is also subject to this uncertainty. We identify some isotopes like Zn70 and Rb87 as the signatures of the s-process during shell carbon burning in massive stars. In determining the relative contribution of our s-only stellar yields to the solar abundances, we find it is important to take into account the neutron exposure of shell carbon burning. When we analyze our yields with a Salpeter Initial Mass Function, we find that massive stars contribute at least 40% to s-only nuclei with mass A <= 87. For s-only nuclei with mass A >90, massive stars contribute on average ~7%, except for Gd152, Os187, and Hg198 which are ~14%, \~13%, and ~11%, respectively.
Theory predicts and observations confirm that low-mass stars (like the Sun) in their early life grow by accreting gas from the surrounding material. But for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful stellar radiation is expected to inhibit accretion and thus limit the growth of their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way for them to form. The problem may be solved by non-spherical accretion, which allows some of the stellar photons to escape along the symmetry axis where the density is lower. The recent detection of rotating disks and toroids around very young massive stars has lent support to the idea that high-mass (> 8 M_sun) stars could form in this way. Here we report observations of an ammonia line towards a high-mass star forming region. We conclude from the data that the gas is falling inwards towards a very young star of ~20 M_sun, in line with theoretical predictions of non-spherical accretion.
We present basic observational data on the 6C** sample. This is a new sample
of radio sources drawn from the 151 MHz 6C survey, which was filtered with
radio criteria chosen to optimize the chances of finding radio galaxies at z >
4. The filtering criteria are a steep-spectral index and a small angular size.
The final sample consists of 68 sources from a region of sky covering 0.421 sr.
We present VLA radio maps, and the results of K-band imaging and optical
spectroscopy.
Near-infrared counterparts are identified for 66 of the 68 sources, down to a
3-sigma limiting magnitude of K ~ 22 mag in a 3-arcsec aperture. Eight of these
identifications are spatially compact, implying an unresolved nuclear source.
The K-magnitude distribution peaks at a median K=18.7 mag, and is found to be
statistically indistinguishable from that of the similarly selected 6C* sample,
implying that the redshift distribution could extend to z > 4.
Redshifts determined from spectroscopy are available for 22 (32 per cent) of
the sources, over the range of 0.2 < z < 3.3 . We measure 15 of these, whereas
the other 7 were previously known. Six sources are at z > 2.5. Four sources
show broad emission lines in their spectra and are classified as quasars. Three
of these show also an unresolved K-band identification. Eleven sources fail to
show any distinctive emission and/or absorption features in their spectra. We
suggest that these could be (i) in the so-called `redshift desert' region of
1.2 < z < 1.8, or (ii) at a greater redshift, but feature weak emission line
spectra.
Understanding the acceleration of the universe and its cause is one of the key problems in physics and cosmology today, and is best studied using a variety of mutually complementary approaches. Daly and Djorgovski (2003, 2004) proposed a model independent approach to determine the expansion and acceleration history of the universe and a number of important physical parameters of the dark energy as functions of redshift directly from the data. Here, we apply the method to explicitly determine the first and second derivatives of the coordinate distance with respect to redshift and combine them to solve for the acceleration of the universe and the kinetic and potential energy density of the dark energy as functions of redshift. A data set of 228 supernova and 20 radio galaxy measurements with redshifts from zero to 1.79 is used for this study. The values we obtain are shown to be consistent with the values expected in a standard Lambda Cold Dark Matter model.