Neutron Star Group

Equation of State and Opacities for Hydrogen Atmospheres of Neutron Stars with Strong Magnetic Fields

Most of the typical neutron stars have surface magnetic fields B about 10¹²-10¹³ Gauss, and some of the neutron stars (magnetars) have B about 10¹⁴-10¹⁵ Gauss. Such strong fields profoundly affect the outermost stellar layers.

First, the bottom density of a photosphere increases due to reduction of radiative opacities. Since the same magnetic field that reduces the opacities also suppresses the electron degeneracy, a typical neutron-star atmosphere remains non-degenerate in spite of this density increase. Consequently, at a given density, the pressure in the magnetized atmosphere is considerably lower than in non-magnetic one.

Second, the increase of the atomic binding energies tends to lower the ionization degree. Hence, there can be a significant amount of bound species in a highly magnetized atmosphere, even if it was negligibly small at the same temperature in the zero-field case.

Third, the quantum-mechanical effects of atomic and ionic thermal motion across the field influence the ionization equilibrium and equation of state. The motion lowers binding energies but increases the statistical weight of the atoms. The net effect on the ionization degree results from a balance of these two factors.

Here we present thermodynamic functions and Rosseland mean opacities for partially ionized hydrogen atmospheres in strong magnetic fields. The tables cover range of magnetic fields B from 7 10¹¹ to 10¹⁵ Gauss, temperature T from 2 10⁵ to 10⁷ K (for ordinary neutron stars) or from 5 10⁵ to 4 10⁷ K (for magnetars), and astrophysical density parameter R from 4 10^-8 to 2.5 10³ [R=(rho)/T₆³, where (rho) is density in grams per cubic centimeter and T₆=T/10⁶ K].

The data presented here have been obtained as a result of joint research by two theoretical astrophysics groups: at CRAL (Ecole Normale Superieure de Lyon) and at the Depatrment of Theoretical Astrophysics of the Ioffe Physico-Technical Institute (St. Petersburg).

Here are the basic references to the thermodynamic model [1], the EOS and opacity calculation for ordinary neutron stars [2], and similar calculation for magnetars [3]:

1. A.Y. Potekhin, G. Chabrier, Yu.A. Shibanov (1999). Partially ionized hydrogen plasma in strong magnetic fields, Phys. Rev. E 60, 2193-2208 [gzipped PostScript file (926 K)] [astro-ph/9907006]
2. A.Y. Potekhin, G. Chabrier (2003). Equation of state and opacities for hydrogen atmospheres of neutron stars with strong magnetic fields, Astrophys. J. 585, 955-974 [astro-ph/0212062]
3. A.Y. Potekhin, G. Chabrier (2004). Equation of state and opacities for hydrogen atmospheres of magnetars, Astrophys. J., 600, 317 [gzipped PostScript file (289 K)] [astro-ph/0309310]

• A.Y. Potekhin, D. Lai, G. Chabrier, W.C.G. Ho, M. van Adelsberg (2006). Nonideal strongly magnetized plasmas of neutron stars and their electromagnetic radiation, J. Phys. A: Math. Gen. (accepted) [PDF file (158 K)] [astro-ph/0512088]
• A.Y. Potekhin, D. Lai, G. Chabrier, W.C.G. Ho (2005). Partially ionized atmospheres of neutron stars with strong magnetic fields Adv. Space Res. 35, 1158-1161 [PDF file (130 K) [astro-ph/0501467]
• G. Chabrier, F. Douchin, A.Y. Potekhin (2002). Dense astrophysical plasmas, J. Phys.: Condens. Matter 14, 9133-9139 [gzipped PostScript file (166 K)] [physics/0211089]
• A.Y. Potekhin, G. Chabrier, D.G. Yakovlev (2001). Coulomb plasmas in outer envelopes of neutron stars, Contrib. Plasma Phys. 41, 231-234 [gzipped PostScript file (79 K)] [astro-ph/0012316]
• A. Potekhin, G. Chabrier, Yu. Shibanov (2001). Partially ionized layers of accreted envelopes of weakly and strongly magnetized neutron stars, Pulsar Astronomy - 2000 and beyond (Proc. IAU Coll. 177), ASP Conf. Ser., 202, ed. M. Kramer, N. Wex, R. Wielebinski (San Francisco, 2001), 619-620 [gzipped PostScript file (50 K)]
• A.Y. Potekhin, G. Chabrier, Yu.A. Shibanov, J. Ventura (1999). Ionization equilibrium and equation of state of hydrogen plasmas in strong magnetic fields, Contrib. Plasma Phys. 39, 101-104 [gzipped PostScript file (59 K)] [physics/9811051]
• A.Y. Potekhin, Yu.A. Shibanov, J. Ventura (1998). Equation of state and opacities for hydrogen atmospheres of strongly magnetized cooling neutron stars, Neutron Stars and Pulsars, ed. N. Shibazaki, N. Kawai, S. Shibata, T. Kifune (Universal Academy Press, Tokyo), 161-164 [gzipped PostScript file (59 K)] [astro-ph/9803046]

• A.Y. Potekhin, D. Lai, G. Chabrier, W.C.G. Ho (2004). Electromagnetic polarization in partially ionized plasmas with strong magnetic fields and neutron star atmosphere models. Astrophys. J. 612, 1034-1043 [PDF file (456 K)] [astro-ph/0405383]
• W.C.G. Ho, D. Lai, A.Y. Potekhin, G. Chabrier (2003). Atmospheres and spectra of strongly magnetized neutron stars -- III. Partially ionized hydrogen models Astrophys. J. 599, 1293 [gzipped PS file (327 K)] [astro-ph/0309261]

Page design and maintenance by Alexander Potekhin. Remarks are always welcome. Page created on May 30, 2002. last updated on May 20, 2007.