Hydrogen Phases on the Surfaces of a Strongly Magnetized Neutron Star

The outermost layers of some neutron stars are likely to be dominated byhydrogen, as a result of fast gravitational settling of heavier elements. Theselayers directly mediate thermal radiation from the stars, and determine thecharacteristics of X-ray/EUV spectra. For a neutron star with surfacetemperature $T\lo 10^6$ K and magnetic field $B\go 10^{12}$ G, various forms ofhydrogen can be present in the envelope, including atom, poly-molecules, andcondensed metal. We study the physical properties of different hydrogen phaseson the surface of a strongly magnetized neutron star for a wide range of fieldstrength $B$ and surface temperature $T$. Depending on the values of $B$ and$T$, the outer envelope can be either in a nondegenerate gaseous phase or in adegenerate metallic phase. For $T\go 10^5$ K and moderately strong magneticfield, $B\lo 10^{13}$ G, the envelope is nondegenerate and the surface materialgradually transforms into a degenerate Coulomb plasma as density increases. Forhigher field strength, $B>> 10^{13}$ G, there exists a first-order phasetransition from the nondegenerate gaseous phase to the condensed metallicphase. The column density of saturated vapor above the metallic hydrogendecreases rapidly as the magnetic field increases or/and temperature decreases.Thus the thermal radiation can directly emerge from the degenerate metallichydrogen surface. The characteristics of surface X-ray/EUV emission fordifferent phases are discussed. A separate study concerning the possibility ofmagnetic field induced nuclear fusion of hydrogen on the neutron star surfaceis also presented.Comment: TeX, 35 pages including 6 postscript figures. To be published in Ap