How to Make a Singleton sdB Star via Accelerated Stellar Evolution

TL;DR

This paper introduces a new formation channel for singleton hot subdwarf B stars involving the merger of a helium white dwarf with a low-mass star, explaining observed properties unaccounted for by previous models.

Contribution

It proposes a novel merger-based formation pathway for singleton sdBs, addressing gaps in existing binary evolution theories.

Findings

Hierarchical triple mergers can produce long-period sdB binaries.

The merger process explains the slow rotation of singleton sdBs.

Implications for globular cluster and galaxy UV phenomena.

Abstract

Many hot subdwarf B stars (sdBs) are in close binaries, and the favored formation channels for subdwarfs rely on mass transfer in a binary system to strip a core He burning star of its envelope. However, these channels cannot account for sdBs that have been observed in long period binaries nor the narrow mass distribution of isolated (or "singleton") sdBs. We propose a new formation channel involving the merger of a helium white dwarf and a low mass, hydrogen burning star, which addresses these issues. Hierarchical triples whose inner binaries merge and form sdBs by this process could explain the observed long period subdwarf+main sequence binaries. This process would also naturally explain the observed slow rotational speeds of singleton sdBs. We also briefly discuss the implications of this formation channel for extreme horizontal branch morphology in globular clusters and the UV upturn in elliptical galaxies.