Thermal Structure and Cooling of Superfluid Neutron Stars with Accreted Magnetized Envelopes

Abstract

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 J0822À43 and PSR B1055À52.

Neutron Stars—Cooling and Transport

“…5). Because of the 2D treatment and the allowance for neutrino emission, the new fit supersedes the previous one (Potekhin et al 2003), whenever B > 10 12 G or T b 10 8 K. We stress that its use is restricted by non-accreted (i.e., composed of heavy chemical elements) envelopes in the range of 10 6.5 K T b 10 10 K and B p 10 15 G, which is covered by the underlying numerical data. For envelopes with B 10 12 G (either non-accreted or accreted), the previous fit can be used, however the surface temperature T s (but not the flux at the inner boundary, F bsee item 4 below) should be limited for hot stars according to Eq.…”

“…For weakly magnetized, isolated cooling neutron stars, ρ b is usually set at 10 10 g cm −3 (Gudmundsson et al 1983), but in general it varies from 10 8 g cm −3 for neutron stars with rapid variations of thermal emission ) to ρ drip for relatively hot and strongly magnetized neutron stars (Potekhin et al 2003).…”

“…This effect was studied in detail by for non-magnetic envelopes and by Potekhin et al (2003) for strongly magnetized envelopes. The effect is related to the Z-dependence of the collision frequencies ν ei .…”

“…This conclusion confirmed the earlier conjectures of Hernquist (1985) and Schaaf (1990). In the 2000s, detailed studies of the T b -T s relation in strong magnetic fields were performed for iron envelopes and accreted envelopes composed of light elements (Potekhin et al 2003), as well as for the large-scale (dipole) and small-scale (stochastic) surface magnetic fields . These studies confirmed the conclusions of Shibanov and Yakovlev (1996), but showed that in superstrong fields B 10 14 G the quantum enhancement of the conductivity and the corresponding increase of T s at the places where B B B is nearly radial overpowers the decrease in the regions of nearly tangential field lines, so that T eff at a given T b increases.…”

Thermonuclear fusion in dense stars

“…In the domain where t burn > 14 Gyr the lines are dotted. For the carbon case we also plot the fit (Potekhin et al 2003) to the results of Sahrling & Chabrier (1998) (the short-dashed line). thermonuclear one.…”