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Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars

Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars
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.
0004-6361
A69-[11pp]
Potekhin, Alexander Y.
b9551650-f6ed-4c4f-b53c-d019cccc49fa
Chabrier, Gilles
95efa689-95ef-4fc0-8e14-9381ea796384
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45
Potekhin, Alexander Y.
b9551650-f6ed-4c4f-b53c-d019cccc49fa
Chabrier, Gilles
95efa689-95ef-4fc0-8e14-9381ea796384
Ho, Wynn C.G.
d78d4c52-8f92-4846-876f-e04a8f803a45

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).

Record type: Article

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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More information

Published date: 1 December 2014
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Organisations: Applied Mathematics

Identifiers

Local EPrints ID: 369424
URI: http://eprints.soton.ac.uk/id/eprint/369424
ISSN: 0004-6361
PURE UUID: 7ad63e0b-0910-4bc2-b2e7-f2d5b8d73c64
ORCID for Wynn C.G. Ho: ORCID iD orcid.org/0000-0002-6089-6836

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Date deposited: 01 Oct 2014 12:50
Last modified: 14 Mar 2024 18:03

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Contributors

Author: Alexander Y. Potekhin
Author: Gilles Chabrier
Author: Wynn C.G. Ho ORCID iD

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