The effects of plasma microfield distributions and ionization potential lowering on the opacity and spectra of radiation from photospheres of cooling neutron stars

Abstract

Abstract An approximate method is presented for computing the ionization equilibrium and opacities in the photospheres of cooling neutron stars. The method is based on occupation probability formalism which describes the lowering of the ionization potential due to plasma microfield effect. The approach is simple and reproduces rather accurately the results of a more selfconsistent and sophisticated Hummer–Mihalas theory for a low-temperature and high-density H–He plasma. It is shown that the opacities and spectra of radiation outgoing from the photospheres of cooling neutron stars are insensitive to the type of the microfield distribution, and to the method of including the ionization potential lowering, as long as the neutron star effective temperatures T eff ≳ 105 K.