An evolutionary channel for CO-rich and pulsating He-rich subdwarfs

TL;DR

This paper proposes a merger scenario involving white dwarfs to explain the formation and pulsations of CO-rich, He-rich subdwarfs and related stars, supported by models and simulations matching observed properties.

Contribution

It introduces a novel merger-based evolutionary pathway for CO-sdOs, He-sdBVs, and related stars, linking their formation, composition, and pulsation characteristics.

Findings

Merger models reproduce observed surface parameters of CO-sdOs.

Simulations show pulsation periods similar to He-sdBVs.

The scenario plausibly explains the origin of multiple rare star types.

Abstract

Recently a new class of hot subluminous stars strongly enriched in C and O have been discovered (CO-sdOs). These stars show abundances very similar to those observed in PG1159 stars but at lower temperatures. Moreover, it has been recently suggested that C and O enrichment might be the key ingredient driving the pulsations in He-rich hot subdwarf stars (He-sdBVs). Here we argue that these two types of rare stars can be explained by a variant of one of the main channels forming hot subdwarf stars. The scenario involves the formation and merging of a He-core white dwarf and a less massive CO-core white dwarf. We have constructed a simple merger models and computed their subsequent evolution. The merger products are in agreement with the surface parameters and composition of CO-sdOs. In addition, we have performed simulations including the effects of element diffusion and the excitation of pulsations. These simulations show that less massive merger products can form stellar structures that have surface parameters, abundances, and pulsation periods similar to those displayed by He-sdBVs. We conclude that the proposed scenario, or some variant of it, offers a very plausible explanation for the formation of CO-sdOs, pulsating He-sdBs and low-luminosity PG1159 stars.