Evolutionary models for R Coronae Borealis stars

TL;DR

This study models the evolution of R Coronae Borealis stars using stellar evolution simulations, comparing different formation scenarios, and highlighting the impact of mass loss and merger phases on their properties and lifetimes.

Contribution

It introduces detailed stellar evolution models for R CrB stars from merger and homogeneous origins, emphasizing the merger phase and mass loss effects.

Findings

Merger-origin models have a thermal reconfiguration phase lasting up to 1 kyr.

Mass loss influences the lifetime and final mass of R CrB stars.

Descendants of R CrB stars likely have narrow mass and luminosity ranges.

Abstract

We use Modules for Experiments in Stellar Astrophysics (MESA) to construct stellar evolution models that reach a hydrogen-deficient, carbon-rich giant phase like the R Coronae Borealis (R CrB) stars. These models use opacities from OPAL and AESOPUS that cover the conditions in the cool, H-deficient, CNO-enhanced envelopes of these stars. We compare models that begin from homogeneous He stars with models constructed to reproduce the remnant structure shortly after the merger of a He and a CO white dwarf (WD). We emphasize that models originating from merger scenarios have a thermal reconfiguration phase that can last up to $\approx$ 1 kyr post merger, suggesting some galactic objects should be in this phase. We illustrate the important role of mass loss in setting the lifetimes of the R CrB stars. Using AGB-like mass loss prescriptions, models with CO WD primaries $\lesssim 0.7\,M_\odot$ typically leave the R CrB phase with total masses $\approx 0.6-0.7\,M_\odot$, roughly independent of their total mass immediately post-merger. This implies that the descendants of the R CrB stars may have a relatively narrow range in mass and luminosity as extreme He stars and a relatively narrow range in mass as single WDs.