Modeling R Coronae Borealis Stars: Effects of He-Burning Shell Temperature and Metallicity

Abstract

The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars which produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be formed primarily through white dwarf mergers. In this paper, we use MESA to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 - 5.4 *10^8 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 *10^8 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10% solar metallicity models, with the exception of carbon and lithium which are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. Additionally, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.

arXiv comment13 pages, 12 figures, accepted to MNRAS on August 15, 2020

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• Census of R Coronae Borealis stars I: Infrared light curves from Palomar Gattini IR
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• A Spectral Classification System for Hydrogen-deficient Carbon Stars
• R Coronae Borealis and dustless Hydrogen-deficient Carbon stars have different oxygen isotope ratios
• The carbon star DY Persei may be a cool R Coronae Borealis variable
• Revised surface abundances of R Coronae Borealis stars
• Simulating Stellar Merger using HPX/Kokkos on A64FX on Supercomputer Fugaku
• Peculiar Hydrogen-deficient Carbon Stars: Strontium-Rich Stars and the s-Process
• Hydrodynamic simulations of WD-WD mergers and the origin of RCB stars
• The final fates of close hot subdwarf - white dwarf binaries: mergers involving He/C/O white dwarfs and the formation of unusual giant stars with C/O-dominated envelopes