Abundance analyses of helium-rich subluminous B stars

TL;DR

This study analyzes the surface compositions of six helium-rich subluminous B stars to understand their origins, revealing diverse elemental abundances and suggesting merger scenarios as a possible formation mechanism.

Contribution

It provides detailed abundance analyses of six He-sdB stars using model atmospheres, exploring their chemical diversity and potential formation pathways.

Findings

Most stars are moderately metal-poor compared to the Sun.

Four stars are nitrogen-rich, with some also being carbon- and neon-rich.

Data suggest merger origins, such as white dwarf mergers, for the N-rich He-sdBs.

Abstract

The connection between helium-rich hot subdwarfs of spectral types O and B (He-sdB) has been relatively unexplored since the latter were found in significant numbers in the 1980's. In order to explore this connection further, we have analysed the surface composition of six He-sdB stars, including LB 1766, LB 3229, SB 21 (= Ton-S 137 = BPS 29503-0009), BPS 22940-0009, BPS 29496-0010, and BPS 22956-0094. Opacity-sampled line-blanketed model atmospheres have been used to derive atmospheric properties and elemental abundances. All the stars are moderately metal-poor compared with the Sun ([Fe/H] ~ -0.5). Four stars are nitrogen-rich, two of these are carbon-rich, and at least four appear to be neon-rich. The data are insufficient to rule out binarity in any of the sample. The surface composition and locus of the N-rich He-sdBs are currently best explained by the merger of two helium white dwarfs, or possibly by the merger of a helium white dwarf with a post-sdB white dwarf. C-rich He-sdBs require further investigation.