Astron. Astrophys. 323, 415-428 (1997)

4. Conclusions

We have considered thermal structure and evolution of NSs whose envelopes are composed of non-accreted or accreted matter. We have used new, state-of-the-art calculations of EOS and opacities of NS envelopes, described in Sect. 2. In particular, we have recalculated (Sects. 2.3.3, A.1, and A.2) the electron-electron and electron-ion collision frequencies, that determine the electron thermal conductivity, for a wide range of densities and temperatures of a degenerate electron gas and ionic liquid plasmas.

Using this new physics input, we have calculated (Sects. 3.1, 3.2) the temperature profiles in the envelopes of non-accreted and partly accreted NSs and obtained the relationships between the internal and effective temperatures of NSs, and . These relationships are important for simulating the NS cooling; they are fitted by simple analytic expressions in Sect. A.3. They appear to be very sensitive to the presence of accreted matter in the NS envelope; even a small amount of the accreted matter, , can reduce substantially the thermal insulation of the envelopes. For a non-accreting NS, our relationship between and extends the well-known result of GPE to lower temperatures (down to K).

In Sect. 3.3, we have examined briefly the effect of the possible presence of accreted matter on the NS cooling. We show that the accreted matter may increase the surface temperature (photon thermal luminosity) at the neutrino cooling stage, and decrease them at the subsequent photon cooling stage, as compared to the NSs without accreted envelopes. We have shown that these results can be important for a proper interpretation of observed thermal radiation from NSs. In particular, the presence of accreted matter facilitates the explanation of recent observational results concerning the pulsars Vela and Geminga, and RXJ 0002+6246, in the framework of the standard neutrino emission model (without exotic matter, superfluidity, or direct Urca processes).

In this paper, we have neglected effects of magnetic fields on the EOS and the thermal conduction of matter, which can be significant (e.g., Yakovlev & Kaminker 1994). They do deserve further studies using improved EOS and thermal conductivities of magnetized NS envelope (e.g., PY) and improved radiative opacities of magnetized NS atmosphere (e.g., Pavlov & Potekhin 1995), with allowance for the possible presence of light elements in the surface layers.

Finally, it is worth noting that the physics input used in the present calculations can be applied to a variety of other astrophysical problems concerning dense stellar matter, e.g. the thermal structure and bursting activity of X-ray bursters (see, e.g., Miralda-Escudé et al. 1990 and references therein) and the cooling of white dwarfs (Segretain et al. 1994).

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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