General Relativistic Radiative Transfer

We present a general method to calculate radiative transfer including scattering in the continuum as well as in lines in spherically symmetric systems that are influenced by the effects of general relativity (GR). We utilize a comoving wavelength ansatz that allows to resolve spectral lines throughout the atmosphere. The used numerical solution is an operator splitting (OS) technique that uses a characteristic formal solution. The bending of photon paths and the wavelength shifts due to the effects of GR are fully taken into account, as is the treatment of image generation in a curved spacetime. We describe the algorithm we use and demonstrate the effects of GR on the radiative transport of a two level atom line in a neutron star like atmosphere for various combinations of continuous and line scattering coefficients. In addition, we present grey continuum models and discuss the effects of different scattering albedos on the emergent spectra and the determination of effective temperatures and radii of neutron star atmospheres

NLTE analysis of spectra: OBA stars

Methods of calculation of NLTE model atmosphere are discussed. The NLTE trace element procedure is compared with the full NLTE model atmosphere calculation. Differences between LTE and NLTE atmosphere modeling are evaluated. The ways of model atom construction are discussed. Finally, modelling of expanding atmospheres of hot stars with winds is briefly reviewed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type Stars, E. Niemczura et al. eds., Springer, in pres

Pair plasma relaxation time scales

By numerically solving the relativistic Boltzmann equations, we compute the time scale for relaxation to thermal equilibrium for an optically thick electron-positron plasma with baryon loading. We focus on the time scales of electromagnetic interactions. The collisional integrals are obtained directly from the corresponding QED matrix elements. Thermalization time scales are computed for a wide range of values of both the total energy density (over 10 orders of magnitude) and of the baryonic loading parameter (over 6 orders of magnitude). This also allows us to study such interesting limiting cases as the almost purely electron-positron plasma or electron-proton plasma as well as intermediate cases. These results appear to be important both for laboratory experiments aimed at generating optically thick pair plasmas as well as for astrophysical models in which electron-positron pair plasmas play a relevant role.Comment: Phys. Rev. E, in pres

Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models. I. Theory and practical implementation

A photometric calibration of Kurucz static model atmospheres is used to obtain the following parameters of RR Lyrae stars: variation of stellar angular radius $\vartheta$, effective temperature $T_{\rm e}$, gravity $g_{\rm e}$ as a function of phase, interstellar reddening $E(B-V)$ towards the star and atmospheric metallicity $M$. Photometric and hydrodynamic conditions are given to find the phases of pulsation when the quasi-static atmosphere approximation (QSAA) can be applied. The QSAA is generalized to a non-uniformly moving spherical atmosphere, and the distance $d$, mass ${\cal M}$ and atmospheric motion are derived from the laws of mass and momentum conservation. To demonstrate the efficiency of the method, the $UBV(RI)_C$ photometry of SU Dra was used to derive the following parameters: $[M]=-1.60\pm .10$~dex, $E(B-V)=0.015\pm .010$, $d=663\pm 67$~pc, ${\cal M}=(0.68\pm .03){\cal M}_\odot$, equilibrium luminosity $L_{\rm eq}=45.9\pm 9.3L_\odot$ and $T_{\rm eq}=6813\pm 20$~K.Comment: 8 pages, 3 figure

Importance of cooling in triggering the collapse of hypermassive neutron stars

The inspiral and merger of a binary neutron star (NSNS) can lead to the formation of a hypermassive neutron star (HMNS). As the HMNS loses thermal pressure due to neutrino cooling and/or centrifugal support due to gravitational wave (GW) emission, and/or magnetic breaking of differential rotation it will collapse to a black hole. To assess the importance of shock-induced thermal pressure and cooling, we adopt an idealized equation of state and perform NSNS simulations in full GR through late inspiral, merger, and HMNS formation, accounting for cooling. We show that thermal pressure contributes significantly to the support of the HMNS against collapse and that thermal cooling accelerates its "delayed" collapse. Our simulations demonstrate explicitly that cooling can induce the catastrophic collapse of a hot hypermassive neutron star formed following the merger of binary neutron stars. Thus, cooling physics is important to include in NSNS merger calculations to accurately determine the lifetime of the HMNS remnant and to extract information about the NS equation of state, cooling mechanisms, bar instabilities and B-fields from the GWs emitted during the transient phase prior to BH formation.Comment: 13 pages, 7 figures, matches published versio

General Relativistic versus Newtonian: a universality in radiation hydrodynamics

We compare Newtonian and general relativistic descriptions of the stationary accretion of self-gravitating fluids onto compact bodies. Spherical symmetry and thin gas approximation are assumed. Luminosity depends, amongst other factors, on the temperature and the contribution of gas to the total mass, in both -- general relativistic ($L_{GR}$) and Newtonian ($L_N$) -- models. We discover a remarkable universal behaviour for transonic flows: the ratio of respective luminosities $L_{GR}/L_N$ is independent of the fractional mass of the gas and depends on asymptotic temperature. It is close to 1 in the regime of low asymptotic temperatures and can grow by one order of magnitude for high temperatures. These conclusions are valid for a wide range of polytropic equations of state.Comment: 8 pages, 4 figure

Non-LTE Abundances of Magnesium, Aluminum and Sulfur in OB Stars Near the Solar Circle

Non-LTE abundances of magnesium, aluminum and sulfur are derived for a sample of 23 low-v \sin i stars belonging to six northern OB associations of the Galactic disk within 1 kpc of the Sun. The abundances are obtained from the fitting of synthetic line profiles to high resolution spectra. A comparison of our results with HII region abundances indicates good agreement for sulfur while the cepheid abundances are higher. The derived abundances of Mg show good overlap with the cepheid results. The aluminum abundances for OB stars are significantly below the cepheid values. But, the OB star results show a dependence with effective temperature and need further investigation. The high Al abundances in the cepheids could be the result of mixing. A discussion of the oxygen abundance in objects near the solar circle suggests that the current mean galactic oxygen abundance in this region is 8.6-8.7 and in agreement with the recently revised oxygen abundance in the solar photosphere. Meaningful comparisons of the absolute S, Al and Mg abundances in OB stars with the Sun must await a reinvestigation of these elements, as well as the meteoritic reference element Si, with 3D hydrodynamical model atmospheres for the Sun. No abundance gradients are found within the limited range in galactocentric distances in the present study. Such variations would be expected only if there were large metallicity gradients in the disk.Comment: 3 figures, accepted for publication in A&A, needs aa.st

Detection of Anisotropies in the Gravitational-Wave Stochastic Background

By correlating the signals from a pair of gravitational-wave detectors, one can undertake sensitive searches for a stochastic background of gravitational radiation. If the stochastic background is anisotropic, then this correlated signal varies harmonically with the earth's rotation. We calculate how the harmonics of this varying signal are related to the multipole moments which characterize the anisotropy, and give a formula for the signal-to-noise ratio of a given harmonic. The specific case of the two LIGO (Laser Interferometric Gravitational Observatory) detectors, which will begin operation around the year 2000, is analyzed in detail. We consider two possible examples of anisotropy. If the gravitational-wave stochastic background contains a dipole intensity anisotropy whose origin (like that of the Cosmic Background Radiation) is motion of our local system, then that anisotropy will be observable by the advanced LIGO detector (with 90% confidence in one year of observation) if \Omega_{gw} > 5.3 \times 10^{-8} h_{100}^{-2}. We also study the signal produced by stochastic sources distributed in the same way as the luminous matter in the galactic disk, and in the same way as the galactic halo. The anisotropy due to sources distributed as the galactic disk or as the galactic halo will be observable by the advanced LIGO detector (with 90% confidence in one year of observation) if \Omega_{gw} > 1.8 \times 10^{-10} h_{100}^{-2} or \Omega_{gw} > 6.7 \times 10^{-8} h_{100}^{-2}, respectively.Comment: 25 pages, Latex with RevTeX and epsfig, now includes S/N ratio calculations, expected response from anisotropy due to local motion & sources in galax

A photon transport problem with a time-dependent point source

We consider a time-dependent problem of photon transport in an interstellar cloud with a point photon source modeled by a Dirac δ functional. The existence of a unique distributional solution to this problem is established by using the theory of continuous semigroups of operators on locally convex spaces coupled with a constructive approach for producing spaces of generalized functions

Similarity Properties and Scaling Laws of Radiation Hydrodynamic Flows in Laboratory Astrophysics

The spectacular recent development of modern high-energy density laboratory facilities which concentrate more and more energy in millimetric volumes allows the astrophysical community to reproduce and to explore, in millimeter-scale targets and during very short times, astrophysical phenomena where radiation and matter are strongly coupled. The astrophysical relevance of these experiments can be checked from the similarity properties and especially scaling laws establishment, which constitutes the keystone of laboratory astrophysics. From the radiating optically thin regime to the so-called optically thick radiative pressure regime, we present in this paper, for the first time, a complete analysis of the main radiating regimes that we encountered in laboratory astrophysics with the same formalism based on the Lie-group theory. The use of the Lie group method appears as systematic which allows to construct easily and orderly the scaling laws of a given problem. This powerful tool permits to unify the recent major advances on scaling laws and to identify new similarity concepts that we discuss in this paper and which opens important applications for the present and the future laboratory astrophysics experiments. All these results enable to demonstrate theoretically that astrophysical phenomena in such radiating regimes can be explored experimentally thanks to powerful facilities. Consequently the results presented here are a fundamental tool for the high-energy density laboratory astrophysics community in order to quantify the astrophysics relevance and justify laser experiments. Moreover, relying on the Lie-group theory, this paper constitutes the starting point of any analysis of the self-similar dynamics of radiating fluids.Comment: Astrophys. J. accepte