Evolutionary Models for R Coronae Borealis Stars

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 ÆSOPUS 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 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 asymptotic giant branch-like mass-loss prescriptions, models with CO WD primaries typically leave the R CrB phase with total masses , 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.