Hydrogen in hot subdwarfs formed by double helium white dwarf mergers

Abstract

Isolated hot subdwarfs might be formed by the merging of two helium-core\nwhite dwarfs. Before merging, helium-core white dwarfs have hydrogen-rich\nenvelopes and some of this hydrogen may survive the merger. We calculate the\nmass of hydrogen that is present at the start of such mergers and, with the\nassumption that hydrogen is mixed throughout the disrupted white dwarf in the\nmerger process, estimate how much can survive. We find a hydrogen mass of up to\nabout $2 \\times 10^{-3}\\,\\mathrm{M}_{\\odot}$ in merger remnants. We make model\nmerger remnants that include the hydrogen mass appropriate to their total mass\nand compare their atmospheric parameters with a sample of apparently isolated\nhot subdwarfs, hydrogen-rich sdBs. The majority of these stars can be explained\nas the remnants of double helium white dwarf mergers.\n