Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars

Potekhin, Alexander Y., Chabrier, Gilles and Ho, Wynn C.G. (2014) Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars. Astronomy & Astrophysics, 572, A69-[11pp]. (doi:10.1051/0004-6361/201424619).

Abstract

There is observational evidence that central compact objects (CCOs) in supernova remnants have moderately strong magnetic fields B ~ 10^11 G. Meanwhile, available models of partially ionized hydrogen atmospheres of neutron stars with strong magnetic fields are restricted to B > 10^12 G. Extension of the applicability range of the photosphere models to smaller field strengths is complicated by a stronger asymmetry of decentered atomic states and by the importance of excited bound states.

We extend the equation of state and radiative opacities, presented in previous papers for 10^12 G < B < 10^15 G, to weaker fields. We construct analytical fitting formulae for binding energies, sizes, and oscillator strengths for different bound states of a hydrogen atom moving in moderately strong magnetic fields and calculate an extensive database for photoionization cross sections of such atoms. Using these atomic data, in the framework of the chemical picture of plasmas we solve the ionization equilibrium problem and calculate thermodynamic functions and basic opacities of partially ionized hydrogen plasmas at these field strengths. Then plasma polarizabilities are calculated from the Kramers-Kronig relation, and the radiative transfer equation for the coupled normal polarization modes is solved to obtain model spectra.

An equation of state and radiative opacities for a partially ionized hydrogen plasma are obtained at magnetic fields B, temperatures T, and densities rho typical for atmospheres of CCOs and other isolated neutron stars with moderately strong magnetic fields. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 3x10^10 G < B < 10^12 G, 10^5 K < T < 10^7 K, and a wide range of densities. Atmosphere models and spectra are calculated to verify the applicability of the results and to determine the range of magnetic fields and effective temperatures where the incomplete ionization of the hydrogen plasma is important.

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Contributors

Author: Wynn C.G. Ho

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