Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions

The Astrophysical Journal Supplement Series, Volume 220, Issue 1, article id. 15, 44 pp.

Bill Paxton, Pablo Marchant, Josiah Schwab, Evan B. Bauer, Lars Bildsten, Matteo Cantiello, Luc Dessart, R. Farmer, H. Hu, N. Langer, R.H.D. Townsend, Dean M. Townsley, F.X. Timmes

We substantially update the capabilities of the open-source software instrument Modules for Experiments in Stellar Astrophysics (MESA). MESA can now simultaneously evolve an interacting pair of differentially rotating stars undergoing transfer and loss of mass and angular momentum, greatly enhancing the prior ability to model binary evolution. New MESA capabilities in fully coupled calculation of nuclear networks with hundreds of isotopes now allow MESA to accurately simulate advanced burning stages needed to construct supernova progenitor models. Implicit hydrodynamics with shocks can now be treated with MESA, enabling modeling of the entire massive star lifecycle, from pre-main sequence evolution to the onset of core collapse and nucleosynthesis from the resulting explosion. Coupling of the GYRE non-adiabatic pulsation instrument with MESA allows for new explorations of the instability strips for massive stars while also accelerating the astrophysical use of asteroseismology data. We improve treatment of mass accretion, giving more accurate and robust near-surface profiles. A new MESA capability to calculate weak reaction rates "on-the-fly" from input nuclear data allows better simulation of accretion induced collapse of massive white dwarfs and the fate of some massive stars. We discuss the ongoing challenge of chemical diffusion in the strongly coupled plasma regime, and exhibit improvements in MESA that now allow for the simulation of radiative levitation of heavy elements in hot stars. We close by noting that the MESA software infrastructure provides bit-for-bit consistency for all results across all the supported platforms, a profound enabling capability for accelerating MESA's development.

Thermal Runaway During the Evolution of ONeMg Cores towards Accretion-Induced Collapse

Monthly Notices of the Royal Astronomical Society, Volume 453, Issue 2, p.1910-1927

Josiah Schwab, Eliot Quataert, Lars Bildsten

We study the evolution of degenerate electron cores primarily composed of the carbon burning products oxygen, neon, and magnesium (hereafter ONeMg cores) that are undergoing compression. Electron capture reactions on A=20 and A=24 isotopes reduce the electron fraction and heat the core. We develop and use a new capability of the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code that provides a highly accurate implementation of these key reactions. These new accurate rates and the ability of MESA to perform extremely small spatial zoning demonstrates a thermal runaway in the core triggered by the temperature and density sensitivity of the Ne-20 electron capture reactions. Both analytics and numerics show that this thermal runaway does not trigger core convection, but rather leads to a centrally concentrated (r < km) thermal runaway that will subsequently launch an oxygen deflagration wave from the center of the star. We use MESA to perform a parameter study that quantifies the influence of the magnesium mass fraction, the central temperature, the compression rate, and uncertainties in the electron capture reaction rates on the ONeMg core evolution. This allows us to establish a lower limit on the central density at which the oxygen deflagration wave initiates of 8.5e9 g/cc. Based on previous work and order-of-magnitude calculations, we expect objects which ignite oxygen at or above these densities to collapse and form a neutron star. Calculations such as these are an important step in producing more realistic progenitor models for studies of the signature of accretion-induced collapse.

MESA Summer School 2015

I’m in Santa Barbara, CA at the MESA Summer School. I’m lecturing on how to use MESA’s run_star_extras.f functionality.

You can read a press release about it!

Seventh Meeting on Hot Subdwarfs and Related Objects

I’m in Oxford, UK at sdb07 giving a talk on The Long-Term Outcomes of White Dwarf Mergers.

Summer Institute for Preparing Future Faculty

I just completed the Summer Institute for Preparing Future Faculty, a program offered by the Graduate Student Instructor Teaching and Resource Center.