Alexander Y. Potekhin

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.

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 for studying 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.

Thermal relaxation in young neutron stars

ABSTRA C T The internal properties of the neutron star crust can be probed by observing the epoch of thermal relaxation. After the supernova explosion, powerful neutrino emission quickly cools the stellar core, while the crust stays hot. The cooling wave then propagates through the crust, as a result of its finite thermal conductivity. When the cooling wave reaches the surface (age 10‐100 yrU, the effective temperature drops sharply from 250 eV to 30 or 100 eV, depending on the cooling model. The crust relaxation time is sensitive to the (poorly known) microscopic properties of matter of subnuclear density, such as the heat capacity, thermal conductivity, and superfluidity of free neutrons. We calculate the cooling models with the new values of the electron thermal conductivity in the inner crust, based on a realistic treatment of the shapes of atomic nuclei. Superfluid effects may shorten the relaxation time by a factor of 4. The comparison of theoretical cooling curves with observations provides a potentially powerful method of studying the properties of the neutron superfluid and highly unusual atomic nuclei in the inner crust.

Dense plasmas in astrophysics: from giant planets to neutron stars

We briefly examine the properties of the dense plasmas characteristic of the interior of giant planets and of the atmospheres of neutron stars. Special attention is devoted to the equation of state of hydrogen and helium at high density and to the effect of magnetic fields on the properties of dense matter.

Analytical representations of unified equations of state of neutron-star matter

Analytical representations are derived for two equations of state (EOSs) of neutron-star matter: FPS and SLy. Each of these EOSs is unified, that is, it describes the crust and the core of a neutron star using the same physical model. Two versions of the EOS parametrization are considered. In the first one, pressure and mass density are given as functions of the baryon density. In the second version, pressure, mass density, and baryon density are given as functions of the pseudo-enthalpy, which makes this representation particularly useful for 2-D calculations of stationary rotating configurations of neutron stars.

Equation of state of fully ionized electron-ion plasmas

The analytic equation of state of nonideal Coulomb plasmas consisting of pointlike ions immersed in a polarizable electron background [G. Chabrier and A. Y. Potekhin, Phys. Rev. E 58, 4941 (1998)] is improved, and its applicability range is considerably extended. First, the fit of the electron screening contribution in the free energy of the Coulomb liquid is refined at high densities where the electrons are relativistic. Second, we calculate the screening contribution for the Coulomb solid (bcc and fcc) and derive an analytic fitting expression. Third, we propose a simple approximation to the internal and free energy of the liquid one-component plasma of ions, accurate within the numerical errors of the most recent Monte Carlo simulations. We obtain an updated value of the coupling parameter at the solid-liquid phase transition for the one-component plasma: Gamma(m)=175.0+/-0.4(1sigma).

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 (INSs), and consider- able 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-fitting parameters are an interstellar column density NH ≈ 1 × 10 20 cm −2 and an emitting area with R ∞ ≈ 17 km (assuming a distance to RX J1856.5−3754 of 140 pc), temperature T ∞ ≈ 4.3 × 10 5 K, gravitational redshift zg ∼ 0.22, atmospheric hydrogen column yH ≈ 1gc m −2 , and magnetic field B ≈ (3-4) × 10 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 NS surface as well as general relativistic effects, to determine pulsations; we find that 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 pro- ducing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.

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 the 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 the interpretation of observations of the isolated neutron stars hottest for their age, such as RX J0822� 43 and PSR B1055� 52. Subject headings: dense matter — magnetic fields — stars: individual (PSR B1055� 52, RX J0822� 4300) — stars: neutron

Neutron star cooling after deep crustal heating in the X-ray transient KS 1731-260

We simulate the cooling of the neutron star in the X-ray transient KS 1731−260 after the source returned to quiescence in 2001 from a long (≳12.5 yr) outburst state. We show that the cooling can be explained assuming that the crust underwent deep heating during the outburst stage. In our best theoretical scenario the neutron star has no enhanced neutrino emission in the core, and its crust is thin, superfluid, and has the normal thermal conductivity. The thermal afterburst crust–core relaxation in the star may not be over.

Cooling rates of neutron stars and the young neutron star in the Cassiopeia A supernova remnant

We explore the thermal state of the neutron star in the Cassio peia A supernova remnant using the recent result of Ho & Heinke (2009) that the thermal radiation of this star is well-described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling sta ge from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles ‐ stars with non-superfluid nucleon cores cooling slowly via t he modified Urca process. We find that the internal temperature of standard candles is a we ll-defined function of the stellar compactness parameter x = rg/R, irrespective of the equation of state of neutron star matte r (R and rg are circumferential and gravitational radii, respectivel y). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters: fl, the neutrino cooling effi ciency with respect to the standard candle; the compactness x; and the amount of light elements in the heat blanketing envelope. For an ordin ary (iron) heat blanketing envelope or a low-mass (. 10 −13 M⊙) carbon envelope, we find the e ffi ciency fl∼ 1 (standard cooling) for x . 0.5 and fl ∼ 0.02 (slower cooling) for a maximum compactness x ≈ 0.7. A heat blanket containing the maximum mass (∼ 10 −8 M⊙) of light elements increases fl by a factor of 50. We also examine the (unlikely) possibility that the st ar is still thermally non-relaxed.