Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation

The Astrophysical Journal Supplement Series, Volume 243, Issue 1, article id. 10, 44 pp

Bill Paxton, R. Smolec, Josiah Schwab, A. Gautschy, Lars Bildsten, Matteo Cantiello, Aaron Dotter, R. Farmer, Jared A. Goldberg, Adam S. Jermyn, S.M. Kanbur, Pablo Marchant, Anne Thoul, Richard H. D. Townsend, William M. Wolf, Michael Zhang, F.X. Timmes

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). RSP is a new functionality in MESAstar that models the non-linear radial stellar pulsations that characterize RR Lyrae, Cepheids, and other classes of variable stars. We significantly enhance numerical energy conservation capabilities, including during mass changes. For example, this enables calculations through the He flash that conserve energy to better than 0.001 %. To improve the modeling of rotating stars in MESA, we introduce a new approach to modifying the pressure and temperature equations of stellar structure, and a formulation of the projection effects of gravity darkening. A new scheme for tracking convective boundaries yields reliable values of the convective-core mass, and allows the natural emergence of adiabatic semiconvection regions during both core hydrogen- and helium-burning phases. We quantify the parallel performance of MESA on current generation multicore architectures and demonstrate improvements in the computational efficiency of radiative levitation. We report updates to the equation of state and nuclear reaction physics modules. We briefly discuss the current treatment of fallback in core-collapse supernova models and the thermodynamic evolution of supernova explosions. We close by discussing the new MESA Testhub software infrastructure to enhance source-code development.

Mixing via Thermocompositional Convection in Hybrid C/O/Ne White Dwarfs

The Astrophysical Journal, Volume 876, Issue 1, article id. 10, 9 pp.

Josiah Schwab and Pascale Garaud

Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon-oxygen cores and oxygen-neon mantles. As the white dwarf cools, this core-mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core-mantle interface of the hybrid white dwarf. We demonstrate that, while the resulting structure within the convection zone is somewhat different than what is assumed in the stellar evolution calculations, the two approaches yield similar results for the size and growth of the mixed region. These hybrid white dwarfs have been invoked as progenitors of various peculiar thermonuclear supernovae. This lends further support to the idea that if these hybrid white dwarfs form, then they should be fully mixed by the time of explosion. These effects should be included in the progenitor evolution, in order to more accurately characterize the signatures of these events.

The Deaths and Afterlives of Stars

I’m in Baltimore, MD at the Space Telescope Science Institute for the 2019 Spring Symposium.

Hubble Fellows Symposium

I’m in Baltimore, MD at the Space Telescope Science Institute for the 2019 Hubble Fellows Symposium. I’m giving a talk entitled Double White Dwarf Mergers and the Formation of R CrB Stars.

MESA Instrument Paper V

I’m in Santa Barbara, CA finalizing the fifth MESA Instrument paper. Here were all are, hard at work. (Photo taken by Matteo Cantiello.)