The final fates of close hot subdwarf - white dwarf binaries: mergers involving He/C/O white dwarfs and the formation of unusual giant stars with C/O-dominated envelopes

TL;DR

This paper models the evolution and final outcomes of close hot subdwarf-white dwarf binaries, focusing on mergers that produce unusual giant stars with C/O-dominated envelopes and their implications.

Contribution

It provides new evolutionary calculations for the merger remnants of hot subdwarf-white dwarf binaries, highlighting potential formation of unique giant stars.

Findings

Merger remnants can develop C/O-dominated envelopes with residual He.

These remnants may undergo R Corona Borealis-like shell burning phases.

The study discusses implications for white dwarf cooling processes.

Abstract

Recently, a class of Roche-lobe-filling binary systems consisting of hot subdwarf stars and white dwarfs with sub-hour periods has been discovered. At present, the hot subdwarf is in a shell He burning phase and is transferring some of its remaining thin H envelope to its white dwarf companion. As the evolution of the hot subdwarf continues, it is expected to detach, leaving behind a low mass C/O core white dwarf secondary with a thick He layer. Then, on a timescale of $\sim 10$ Myr, gravitational wave radiation will again bring the systems into contact. If the mass transfer is unstable and results in a merger and a catastrophic thermonuclear explosion is not triggered, it creates a remnant with a C/O-dominated envelope, but one still rich enough in He to support an R Corona Borealis-like shell burning phase. We present evolutionary calculations of this phase and discuss its potential impact on the cooling of the remnant white dwarf.