Hydrogen phases on the surface of a strongly magnetized neutron star

The outermost layers of some neutron stars are likely to be dominated by hydrogen, as a result of fast gravitational settling of heavier elements. These layers directly mediate thermal radiation from the stars and determine the characteristics of X-ray/EUV spectra. For a neutron star with surface temperature T less than or similar to 10(6) K and magnetic field B greater than or similar to 10(12) G, various forms of hydrogen can be present in the envelope, including atoms, polyatomic molecules, and condensed metal. We study the physical properties of different hydrogen phases on the surface of a strongly magnetized neutron star for a wide range of field strengths B and surface temperatures T. Depending on the values of B and T, the outer envelope can be either in a nondegenerate gaseous phase or in a degenerate metallic phase. For T greater than or similar to 10(5) K and moderately strong magnetic field, B less than or similar to 10(13) G, the envelope is nondegenerate and the surface material gradually transforms into a degenerate Coulomb plasma as density increases. For higher field strength, B much greater than 10(13) G, there exists a first-order phase transition from the non-degenerate gaseous phase to the condensed metallic phase. The column density of saturated vapor above the metallic hydrogen decreases rapidly as the magnetic field increases or/and temperature decreases. Thus the thermal radiation can directly emerge from the degenerate metallic hydrogen surface. The characteristics of surface X-ray/EUV emission for different phases are discussed. We also study the possibility of magnetic field-induced nuclear fusion on the neutron star surface. Because of the strong compression of molecules and condensed droplets by the magnetic field, the fusion rate can be significantly enhanced even at a low temperature and external pressure. This implies that for nonaccreting neutron stars, there is a limit to the field strength, similar to 10(14) G, above which the surface hydrogen layer is absent.