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

Abstract

Recently, a class of Roche-lobe-filling binary systems consisting of hot subdwarf stars and white dwarfs (WDs) 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 WD companion. As the evolution of the hot subdwarf continues, it is expected to detach, leaving behind a low-mass C/O-core WD secondary with a thick He layer. Then, on a timescale of ∼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 WD.