Nucleosynthesis during the Merger of White Dwarfs and the Origin of R Coronae Borealis Stars

TL;DR

This study models the merger of helium and carbon-oxygen white dwarfs to explain the unique surface abundances of R Coronae Borealis stars, aligning theoretical predictions with observations.

Contribution

It provides the first nucleosynthesis analysis of white dwarf mergers coupling hydrodynamics with post-processing to explain R Coronae Borealis star compositions.

Findings

Qualitative agreement with observed oxygen and fluorine abundances

Supports merger hypothesis for hydrogen-deficient stars

Highlights importance of merger nucleosynthesis processes

Abstract

Many hydrogen deficient stars are characterised by surface abundance patterns that are hard to reconcile with conventional stellar evolution. Instead, it has been suggested that they may represent the result of a merger episode between a helium and a carbon-oxygen white dwarf. In this Letter, we present a nucleosynthesis study of the merger of a 0.4 M_sol helium white dwarf with a 0.8 M_sol carbon-oxygen white dwarf, by coupling the thermodynamic history of Smoothed Particle Hydrodynamics particles with a post-processing code. The resulting chemical abundance pattern, particularly for oxygen and fluorine, is in qualitative agreement with the observed abundances in R Coronae Borealis stars.