Modeling White-Dwarf Merger Events: The R Coronae Borealis Stars

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Stellar mergers are mysterious phenomena that pack a broad range of physical processes into a small volume and a fleeting time duration. In a merger, two stars become one as their structures and evolution are violently perturbed. A stellar merger is a natural experiment that can help astronomers answer important questions about stellar evolution. The R Coronae Borealis (RCB) stars are rare, hydrogen-deficient, carbon-rich, supergiant stars surrounded by circumstellar dust. It has been suggested that there is a possible connection between the RCB stars and certain stellar explosions called Type Ia supernovae (SNe), where both classes arise when two white dwarf stars (WDs) merge. A research group at Louisiana State University (LSU) plans to study RCB stars in detail using computer models to simulate the evolution of an RCB star to answer questions about their formation, lifetime, and evolution into a different type of star. By investigating this type of WD merger, the LSU group will also help to better understand the origin of Type Ia SNe, which are important for understanding the expansion of the universe. The researchers will continue to participate in outreach efforts including public observing nights and talks, involving amateur astronomers in research projects and developing innovative distance learning and online astronomy courses that will reach large numbers of the public. They are also training the next generation of astronomy researchers and working to increase diversity in astronomy.The LSU group plans to simulate the evolution of an RCB star using a combination of their new self-gravitating hydrodynamics code and MESA, a 1-D stellar evolution module. Their new code, Octo-tiger/HPX, is a 3-D, finite-volume adaptive mesh refinement (AMR) hydrodynamics code with Newtonian gravity. The group will develop a radiation transport module and incorporate a limited nuclear reaction network. Octo-tiger/HPX is parallelized for execution on multi-core high performance computers, which will allow them to probe new areas of parameter space such as much higher spatial resolution, much larger grids, and multiple dynamical time scales. The results of this work will not only broaden the understanding of stellar merger events that lead to RCB stars, but will also shed light on many other related objects for which mergers and common envelope evolution may play a crucial role, including contact binaries, blue stragglers, hot subdwarfs, red novae, massive star mergers, X-ray binaries, cataclysmic variables, short gamma-ray bursts, and Type Ia supernovae. Beyond the astrophysics domain, the research has promise to pave the way for other fluid simulations requiring a high degree of linear and angular momentum conservation, nuclear simulation, multi-physics simulation, and AMR techniques.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.