Can R CrB stars form from the merger of two helium white dwarfs?

TL;DR

This study explores whether R Coronae Borealis stars can originate from the merger of two helium white dwarfs, using new stellar evolution and nucleosynthesis models to compare with observed star compositions.

Contribution

The paper presents detailed evolution and nucleosynthesis calculations for post-merger helium white dwarf pairs, proposing a new potential formation channel for R CrB stars.

Findings

He-sdO stars can evolve into helium-rich giants after mergers.

Surface chemistries partially match observed R CrB and extreme helium stars.

Lithium can form naturally during the merger process.

Abstract

Due to orbital decay by gravitational-wave radiation, some close-binary helium white dwarfs are expected to merge within a Hubble time. The immediate merger products are believed to be helium-rich sdO stars, essentially helium main-sequence stars. We present new evolution calculations for these post-merger stars beyond the core helium-burning phase. The most massive He-sdO's develop a strong helium-burning shell and evolve to become helium-rich giants. We include nucleosynthesis calculations following the merger of $0.4 \rm M_{\odot}$ helium white-dwarf pairs with metallicities $Z = 0.0001, 0.004, 0.008$ and 0.02. The surface chemistries of the resulting giants are in partial agreement with the observed abundances of R Coronae Borealis and extreme helium stars. Such stars might represent a third, albeit rare, evolution channel for the latter, in addition to the CO+He white dwarf merger and the very-late thermal pulse channels proposed previously. We confirm a recent suggestion that lithium seen in R\,CrB stars could form naturally during the hot phase of a merger in the presence of \iso{3}{He} from the donor white dwarf.