1,989 research outputs found
Free-energy model for fluid helium at high density
We present a semi-analytical free-energy model aimed at characterizing the
thermodynamic properties of dense fluid helium, from the low-density atomic
phase to the high-density fully ionized regime. The model is based on a
free-energy minimization method and includes various different contributions
representative of the correlations between atomic and ionic species and
electrons. This model allows the computation of the thermodynamic properties of
dense helium over an extended range of density and temperature and leads to the
computation of the phase diagram of dense fluid helium, with its various
temperature and pressure ionization contours. One of the predictions of the
model is that pressure ionization occurs abruptly at \rho \simgr 10 g
cm, {\it i.e.} P\simgr 20 Mbar, from atomic helium He to fully ionized
helium He, or at least to a strongly ionized state, without He
stage, except at high enough temperature for temperature ionization to become
dominant. These predictions and this phase diagram provide a guide for future
dynamical experiments or numerical first-principle calculations aimed at
studying the properties of helium at very high density, in particular its
metallization. Indeed, the characterization of the helium phase diagram bears
important consequences for the thermodynamic, magnetic and transport properties
of cool and dense astrophysical objects, among which the solar and the numerous
recently discovered extrasolar giant planets.Comment: Accepted for publication in Phys. Rev.
The Galactic disk mass-budget : II. Brown dwarf mass-function and density
In this paper, we extend the calculations conducted previously in the stellar
regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte
Carlo calculations taking into account the brown dwarf formation rate, spatial
distribution and binary fraction. Comparison with existing surveys seems to
exclude a power-law MF as steep as the one determined in the stellar regime
below 1 \msol and tends to favor a more flatish behaviour. Comparison with
methane-dwarf detections tends to favor an eventually decreasing form like the
lognormal or the more general exponential distributions determined in the
previous paper. We calculate predicting brown dwarf counts in near-infrared
color diagrams and brown dwarf discovery functions. These calculations yield
the presently most accurate determination of the brown dwarf census in the
Galactic disk. The brown dwarf number density is comparable to the stellar one,
pc. The corresponding brown dwarf mass
density, however, represents only about 10% of the stellar contribution, i.e.
\rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density
determined previously yields the density of star-like objects, stars and brown
dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ,
corrected version with correct figure
Dense astrophysical plasmas
We briefly examine the properties of dense plasmas characteristic of the
atmospheres of neutron stars and of the interior of massive white dwarfs. These
astrophysical bodies are natural laboratories to study respectively the problem
of pressure ionization of hydrogen in a strong magnetic field and the
crystallization of the quantum one-component-plasma at finite temperature.Comment: 8 pages, 3 figures, LaTeX using iopart.cls and iopart12.clo
(included). In the special issue "Liquid State Theory: from White Dwarfs to
Colloids" (International Conf. in the honor of Prof. J.-P. Hansen's 60th
birthday, Les Houches, April 1-5, 2002
Atmospheres and radiating surfaces of neutron stars with strong magnetic fields
We review the current status of the theory of thermal emission from the
surface layers of neutron stars with strong magnetic fields G, including formation of the spectrum in a partially ionized
atmosphere and at a condensed surface. In particular, we describe recent
progress in modeling partially ionized atmospheres of central compact objects
in supernova remnants, which may have moderately strong fields G. Special attention is given to polarization of thermal
radiation emitted by a neutron star surface. Finally, we briefly describe
applications of the theory to observations of thermally emitting isolated
neutron stars.Comment: 27 pages, 5 figures, invited review at the conference "The Modern
Physics of Compact Stars 2015" (Yerevan, Armenia, Sept. 30 - Oct. 3, 2015),
edited by R. Avagyan, A. Saharian, and A. Sedrakian. In v.2, a citation
(Ref.114) is correcte
Evolution of low-mass star and brown dwarf eclipsing binaries
We examine the evolution of low-mass star and brown dwarf eclipsing binaries.
These objects are rapid rotators and are believed to shelter large magnetic
fields. We suggest that reduced convective efficiency, due to fast rotation and
large field strengths, and/or to magnetic spot coverage of the radiating
surface significantly affect their evolution, leading to a reduced heat flux
and thus larger radii and cooler effective temperatures than for regular
objects. We have considered such processes in our evolutionary calculations,
using a phenomenological approach. This yields mass-radius and effective
temperature-radius relationships in agreement with the observations. We also
reproduce the effective temperature ratio and the radii of the two components
of the recently discovered puzzling eclipsing brown dwarf system. These
calculations show that fast rotation and/or magnetic activity may significantly
affect the evolution of eclipsing binaries and that the mechanical and thermal
properties of these objects depart from the ones of non-active low-mass
objects. We find that, for internal field strengths compatible with the
observed surface value of a few kiloGauss, convection can be severely
inhibited. The onset of a central radiative zone for rapidly rotating active
low-mass stars might thus occur below the usual \sim 0.35 \msol limit.Comment: to appear in A&A Letter
Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
There is observational evidence that central compact objects (CCOs) in
supernova remnants have moderately strong magnetic fields G.
Meanwhile, available models of partially ionized hydrogen atmospheres of
neutron stars with strong magnetic fields are restricted to
G. We extend the equation of state and radiative opacities, presented in
previous papers for 10^{12}\mbox{ G}\lesssim B \lesssim 10^{15} G, to weaker
fields. An equation of state and radiative opacities for a partially ionized
hydrogen plasma are obtained at magnetic fields , temperatures , and
densities typical for atmospheres of CCOs and other isolated neutron
stars with moderately strong magnetic fields. The first- and second-order
thermodynamic functions, monochromatic radiative opacities, and Rosseland mean
opacities are calculated and tabulated, taking account of partial ionization,
for 3\times10^{10}\mbox{ G}\lesssim B\lesssim 10^{12} G, K K, and a wide range of densities. Atmosphere models and spectra
are calculated to verify the applicability of the results and to determine the
range of magnetic fields and effective temperatures where the incomplete
ionization of the hydrogen plasma is important.Comment: 11 pages, 7 figures, accepted for publication in A&
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