KITP Conference: White Dwarfs from Physics to Astrophysics

At the KITP conference White Dwarfs from Physics to Astrophysics, I gave a talk entitled Ashes to Ashes: White Dwarfs from Double White Dwarf Mergers.

UFRGS Astronomy Seminar

I am “visiting” Universidade Federal do Rio Grande do Sul to give the Astronomy Seminar. A recording of my talk is available on YouTube.

Evolutionary Models for the Remnant of the Merger of Two Carbon-Oxygen Core White Dwarfs

The Astrophysical Journal, Volume 906, Issue 1, id.53, 14 pp.

Josiah Schwab

We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range 1-2 M, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a ∼10 kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass-loss rate during this phase influences the final remnant mass and fate (WD or NS). We find that the initial CO core composition of the WD is converted to oxygen-neon (ONe) in remnants with final masses ≳1.05 M. This implies that the CO core/ONe core transition in single WDs formed via mergers occurs at a similar mass as in WDs descended from single stars and thus that WD-WD mergers do not naturally provide a route to producing ultramassive CO-core WDs. As the remnant contracts toward a compact configuration, it experiences a "bottleneck" that sets the characteristic total angular momentum that can be retained. This limit predicts that single WDs formed from WD-WD mergers have rotational periods of ≍10-20 minutes on the WD cooling track. Similarly, it predicts remnants that collapse can form NSs with rotational periods ∼10 ms.

Northwestern CIERA Seminar

I am “visiting” Northwestern University to give the CIERA Astro Seminar. A recording of my talk is available on YouTube.

A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars

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.