Evolving R Coronae Borealis Stars with MESA.

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. Two scenarios have been proposed for the origin of an RCB star: the merger of a CO/He white dwarf (WD) binary and a final helium-shell flash. We used the results of WD mergers in 3D hydrodynamics codes to provide the initial conditions for the 1D MESA (Modules for Experiments in Stellar Astrophysics) stellar evolution code including nucleosynthesis. We constructed post-merger spherical models based on realistic merger progenitors and on inputs from our hydrodynamical simulations, and then followed the evolution into the region of the HR diagram where the RCB stars are located. We also investigated nucleosynthesis in the dynamically accreting material of CO/He WD mergers which may provide a suitable environment for significant production of 18O and the very low 16O/18O values observed. Our MESA modeling consists of engineering the star by adding He-WD material to an initial CO-WD model, and then following the post-merger evolution using a nuclear-reaction network to match the observed RCB abundances as it expands and cools to become an RCB star. We follow the later evolution beyond the RCB phase to determine the stars' likely lifetimes. The relative numbers of known RCB and Extreme Helium stars correspond well to the lifetimes predicted from the MESA models.