Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars⋆
Centre de Recherche Astrophysique de Lyon, Université de Lyon, Université
Lyon 1, Observatoire de Lyon, École Normale Supérieure de Lyon, CNRS, UMR
46 allée d’Italie,
Lyon Cedex 7,
2 Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
3 Central Astronomical Observatory at Pulkovo, Pulkovskoe Shosse 65, 196140 Saint Petersburg, Russia
4 Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
5 School of Physics, University of Exeter, Exeter, UK, EX4 4QL, UK
6 Mathematical Sciences and STAG Research Centre, University of Southampton, Southampton SO17 1BJ, UK
Received: 16 July 2014
Accepted: 26 September 2014
Context. There is observational evidence that central compact objects (CCOs) in supernova remnants have moderately strong magnetic fields B ~ 1011 G. Meanwhile, available models of partially ionized hydrogen atmospheres of neutron stars with strong magnetic fields are restricted to B ≳ 1012 G. Extension of the applicability range of the photosphere models to lower field strengths is complicated by a stronger asymmetry of decentered atomic states and by the importance of excited bound states.
Aims. We extend the equation of state and radiative opacities, as presented in previous papers for 1012G ≲ B ≲ 1015 G, to weaker fields.
Methods. We constructed 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 solved the ionization equilibrium problem and calculated thermodynamic functions and basic opacities of partially ionized hydrogen plasmas at these field strengths. Then plasma polarizabilities were calculated from the Kramers-Kronig relation, and the radiative transfer equation for the coupled normal polarization modes was solved to obtain model spectra.
Results. An equation of state and radiative opacities for a partially ionized hydrogen plasma are obtained at magnetic fields B, temperatures T, and densities ρ 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, considering partial ionization, for 3 × 1010G ≲ B ≲ 1012 G, 105 K ≲ T ≲ 107 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.
Key words: magnetic fields / plasmas / stars: atmospheres / stars: neutron
Tables of thermodynamic functions, atomic number fractions, opacities, and plasma polarizability coefficients are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A69
© ESO, 2014