R Coronae Borealis Star Evolution: Simulating 3D Merger Events to 1D Stellar Evolution Including Large Scale Nucleosynthesis

TL;DR

This study models R Coronae Borealis stars by simulating white dwarf mergers in 3D and mapping to 1D stellar evolution, providing insights into surface abundances, nucleosynthesis, and isotopic ratios, with implications for understanding these rare stars.

Contribution

It introduces a combined 3D hydrodynamics and 1D stellar evolution approach to model RCB stars, including detailed nucleosynthesis and surface abundance predictions.

Findings

Models match observed surface abundances and isotopic ratios.

Partial He-burning products are visible on the surface.

Lack of s-process enhancement due to hydrogen deficiency.

Abstract

R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich variable supergiants thought to be the result of dynamically unstable white dwarf mergers. We attempt to model RCBs through all the relevant timescales by simulating a merger event in Octo-tiger, a 3D adaptive mesh refinement (AMR) hydrodynamics code and mapping the post-merger object into MESA, a 1D stellar evolution code. We then post-process the nucleosynthesis on a much larger nuclear reaction network to study the enhancement of s-process elements. We present models that match observations or previous studies in most surface abundances, isotopic ratios, early evolution and lifetimes. We also observe similar mixing behavior as previous modeling attempts which result in the partial He-burning products visible on the surface in observations. However, we do note that our sub-solar models lack any enhancement in s-process elements, which we attribute to a lack of hydrogen in the envelope. We also find that the Oxygen-16/Oxygen-18 isotopic ratio is very sensitive to initial hydrogen abundance and increases outside of the acceptable range with a hydrogen mass fraction greater than $10^{-4}$.