23 Sep 2020
This research was featured in AAS Nova.
The Astrophysical Journal Letters, Volume 901, Issue 1, id. L18, 5 pp.
Josiah Schwab
Observations demonstrate that the surface abundance of ${}^{7}\mathrm{Li}$ in low-mass stars changes dramatically between the tip of the red giant branch and the red clump. This naturally suggests an association with the helium core flash, which occurs between these two stages. Using stellar evolution models and a simple, ad hoc mixing prescription, we demonstrate that the ${}^{7}\mathrm{Li}$ enhancement can be explained by a brief chemical mixing event that occurs at the time of the first, strongest He subflash. The amount of ${}^{7}\mathrm{Be}$ already present above the H-burning shell just before the flash, once it mixes into the cooler envelope and undergoes an electron capture converting it to ${}^{7}\mathrm{Li}$ , is sufficient to explain the observed abundance at the red clump. We suggest that the excitation of internal gravity waves by the vigorous turbulent convection during the flash may provide a physical mechanism that can induce such mixing.
27 Feb 2020
The Astrophysical Journal, Volume 891, Issue 1, id.5, 10 pp.
Josiah Schwab, R. Farmer, and F.X. Timmes
The collapse of degenerate oxygen-neon cores (i.e., electron-capture supernovae or accretion-induced collapse) proceeds through a phase in which a deflagration wave ("flame") forms at or near the center and propagates through the star. In models, the assumed speed of this flame influences whether this process leads to an explosion or to the formation of a neutron star. We calculate the laminar flame speeds in degenerate oxygen-neon mixtures with compositions motivated by detailed stellar evolution models. These mixtures include trace amounts of carbon and have a lower electron fraction than those considered in previous work. We find that trace carbon has little effect on the flame speeds, but that material with electron fraction Ye ≈ 0.48-0.49 has laminar flame speeds that are ≈ 2 times faster than those at Ye = 0.5. We provide tabulated flame speeds and a corresponding fitting function so that the impact of this difference can be assessed via full star hydrodynamical simulations of the collapse process.
25 Oct 2019
The Astrophysical Journal, Volume 885, Issue 1, article id. 27, 11 pp.
Josiah Schwab
We use Modules for Experiments in Stellar Astrophysics (MESA) to construct stellar evolution models that reach a hydrogen-deficient, carbon-rich giant phase like the R Coronae Borealis (R CrB) stars. These models use opacities from OPAL and ÆSOPUS that cover the conditions in the cool, H-deficient, CNO-enhanced envelopes of these stars. We compare models that begin from homogeneous He stars with models constructed to reproduce the remnant structure shortly after the merger of a He and a CO white dwarf (WD). We emphasize that models originating from merger scenarios have a thermal reconfiguration phase that can last up to ≈ 1 kyr post-merger, suggesting some galactic objects should be in this phase. We illustrate the important role of mass loss in setting the lifetimes of the R CrB stars. Using asymptotic giant branch-like mass-loss prescriptions, models with CO WD primaries ≲ 0.7 M⊙ typically leave the R CrB phase with total masses ≈ 0.6-0.7 M⊙ , roughly independent of their total mass immediately post-merger. This implies that the descendants of the R CrB stars may have a relatively narrow range in mass and luminosity as extreme He stars and a relatively narrow range in mass as single WDs.
