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

Published in MNRAS. 13 pages, 5 figures

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• Discovery of a highly magnetic He-sdO star from a double-degenerate binary merger
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• A Grid of Synthetic Spectra for Subdwarfs: Non-LTE line-blanketed atmosphere models
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• New Level Resolved Ground and Excited State Pb III, IV, V & VI Photoionization Cross Sections for Heavy Metal Subdwarf Modeling
• Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields