993 research outputs found
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
Radiative properties of highly magnetized isolated neutron star surfaces and approximate treatment of absorption features in their spectra
In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs)
absorption features with equivalent widths (EWs) of 50 -- 200 eV are observed.
We theoretically investigate different models to explain absorption features
and compare their properties with the observations. We consider various
theoretical models for the magnetized neutron star surface: naked condensed
iron surfaces and partially ionized hydrogen model atmospheres, including
semi-infinite and thin atmospheres above a condensed surface. The properties of
the absorption features (especially equivalent widths) and the angular
distributions of the emergent radiation are described for all models. A code
for computing light curves and integral emergent spectra of magnetized neutron
stars is developed. We assume a dipole surface magnetic field distribution with
a possible toroidal component and corresponding temperature distribution. A
model with two uniform hot spots at the magnetic poles can also be employed.
Light curves and spectra of highly magnetized neutron stars with parameters
typical for XDINSs are computed using different surface temperature
distributions and various local surface models. Spectra of magnetized model
atmospheres are approximated by diluted blackbody spectra with one or two
Gaussian lines having parameters, which allow us to describe the model
absorption features. To explain the prominent absorption features in the soft
X-ray spectra of XDINSs a thin atmosphere above the condensed surface can be
invoked, whereas a strong toroidal magnetic field component on the XDINS
surfaces can be excluded.Comment: 54 pages, 17 figures, accepted for publication in A&
Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models
We review the theory of electron-conduction opacity, a fundamental ingredient
in the computation of low-mass stellar models; shortcomings and limitations of
the existing calculations used in stellar evolution are discussed. We then
present new determinations of the electron-conduction opacity in stellar
conditions for an arbitrary chemical composition, that improve over previous
works and, most importantly, cover the whole parameter space relevant to
stellar evolution models (i.e., both the regime of partial and high electron
degeneracy). A detailed comparison with the currently used tabulations is also
performed. The impact of our new opacities on the evolution of low-mass stars
is assessed by computing stellar models along both the H- and He-burning
evolutionary phases, as well as Main Sequence models of very low-mass stars and
white dwarf cooling tracks.Comment: 11 pages, 6 figures, ApJ in pres
Thermal structure and cooling of superfluid neutron stars with accreted magnetized envelopes
We study the thermal structure of neutron stars with magnetized envelopes
composed of accreted material, using updated thermal conductivities of plasmas
in quantizing magnetic fields, as well as equation of state and radiative
opacities for partially ionized hydrogen in strong magnetic fields. The
relation between the internal and local surface temperatures is calculated and
fitted by an analytic function of the internal temperature, magnetic field
strength, angle between the field lines and the normal to the surface, surface
gravity, and the mass of the accreted material. The luminosity of a neutron
star with a dipole magnetic field is calculated for various values of the
accreted mass, internal temperature, and magnetic field strength. Using these
results, we simulate cooling of superfluid neutron stars with magnetized
accreted envelopes. We consider slow and fast cooling regimes, paying special
attention to very slow cooling of low-mass superfluid neutron stars. In the
latter case, the cooling is strongly affected by the combined effect of
magnetized accreted envelopes and neutron superfluidity in the stellar crust.
Our results are important for interpretation of observations of isolated
neutron stars hottest for their age, such as RX J0822-43 and PSR B1055-52.Comment: 15 pages, 12 figures, 2 tables. Corrected title only (v2
Magnetic Hydrogen Atmosphere Models and the Neutron Star RX J1856.5-3754
RX J1856.5-3754 is one of the brightest nearby isolated neutron stars, and
considerable observational resources have been devoted to it. However, current
models are unable to satisfactorily explain the data. We show that our latest
models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a
condensed surface can fit the entire spectrum, from X-rays to optical, of RX
J1856.5-3754, within the uncertainties. In our simplest model, the best-fit
parameters are an interstellar column density N_H \approx 1x10^20 cm^-2 and an
emitting area with R^infty \approx 17 km (assuming a distance to RX
J1856.5-3754 of 140 pc), temperature T^infty \approx 4.3x10^5 K, gravitational
redshift z_g \sim 0.22, atmospheric hydrogen column y_H \approx 1 g cm^-2, and
magnetic field B \approx (3-4)x10^12 G; the values for the temperature and
magnetic field indicate an effective average over the surface. We also
calculate a more realistic model, which accounts for magnetic field and
temperature variations over the neutron star surface as well as general
relativistic effects, to determine pulsations; we find there exist viewing
geometries that produce pulsations near the currently observed limits. The
origin of the thin atmospheres required to fit the data is an important
question, and we briefly discuss mechanisms for producing these atmospheres.
Our model thus represents the most self-consistent picture to date for
explaining all the observations of RX J1856.5-3754.Comment: 11 pages, 8 figures; MNRAS, accepte
Diagnosing magnetars with transient cooling
Transient X-ray emission, with an approximate t^{-0.7} decay, was observed
from SGR 1900+14 over 40 days following the the giant flare of 27 Aug 1998. We
calculate in detail the diffusion of heat to the surface of a neutron star
through an intense 10^{14}-10^{15} G magnetic field, following the release of
magnetic energy in its outer layers. We show that the power law index, the
fraction of burst energy in the afterglow, and the return to persistent
emission can all be understood if the star is composed of normal baryonic
material.Comment: 9 pages, 1 eps figur
Atmospheres and Spectra of Strongly Magnetized Neutron Stars -- III. Partially Ionized Hydrogen Models
We construct partially ionized hydrogen atmosphere models for magnetized
neutron stars in radiative equilibrium with surface fields B=10^12-5 \times
10^14 G and effective temperatures T_eff \sim a few \times 10^5-10^6 K. These
models are based on the latest equation of state and opacity results for
magnetized, partially ionized hydrogen plasmas that take into account various
magnetic and dense medium effects. The atmospheres directly determine the
characteristics of thermal emission from isolated neutron stars. For the models
with B=10^12-10^13 G, the spectral features due to neutral atoms lie at extreme
UV and very soft X-ray energy bands and therefore are difficult to observe.
However, the continuum flux is also different from the fully ionized case,
especially at lower energies. For the superstrong field models (B\ga 10^14 G),
we show that the vacuum polarization effect not only suppresses the proton
cyclotron line as shown previously, but also suppresses spectral features due
to bound species; therefore spectral lines or features in thermal radiation are
more difficult to observe when the neutron star magnetic field is \ga 10^14 G.Comment: 12 pages, 10 figures; ApJ, accepted (v599: Dec 20, 2003
- …