On the formation of single and binary helium-rich sdO stars

TL;DR

This paper proposes a new formation channel for helium-rich sdO stars involving post-sdB stars reigniting helium burning after accreting matter from helium white dwarf companions, explaining their observed properties.

Contribution

It introduces a novel evolutionary pathway for He-rich sdO stars through binary interactions and mergers, supported by stellar evolution calculations.

Findings

Properties match observed He-rich sdO stars

Luminosity is about ten times higher than canonical sdB stars

Binary systems with double hot subdwarfs may result from common-envelope phases

Abstract

We propose a formation channel for the previously unexplained helium-rich subdwarf O (He-rich sdO) stars in which post-subdwarf B (sdB) stars (i.e. hybrid COHe white dwarfs) reignite helium burning in a shell after gaining matter from their helium white-dwarf (WD) companions. Such short-period binaries containing post-sdB WDs and helium WDs are predicted by one of the major binary formation channels for sdB stars. In the majority of cases, mass transfer is expected to lead to a dynamically unstable merger event, leaving a single-star remnant. Calculations of the evolution of these stars show that their properties are consistent with the observed He-rich sdO stars. The luminosity of these stars is about an order of magnitude higher than that of canonical sdB stars. We also suggest that binary systems such as PG 1544+488 (Ahmad et al. 2004) and HE 0301-3039 (Lisker et al. 2004), which each contain two hot subdwarfs, could be the outcome of a double-core common-envelope phase. Since this favours intermediate-mass progenitors, this may also explain why the subdwarfs in these systems are He-rich.