Hydrogen in hot subdwarfs formed by double helium white dwarf mergers

TL;DR

This paper investigates how hydrogen-rich envelopes in helium white dwarf mergers can survive and contribute to the formation of hot subdwarfs, matching observed stellar properties.

Contribution

It provides the first estimates of hydrogen retention in double helium white dwarf mergers and links these remnants to observed hot subdwarfs.

Findings

Hydrogen mass up to about 2 x 10^-3 solar masses can survive the merger.

Most observed hydrogen-rich hot subdwarfs can be explained as merger remnants.

Model atmospheres match the properties of many isolated hot subdwarfs.

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.