Evolution of Helium Star - White Dwarf Binaries Leading up to Thermonuclear Supernovae
Physical and Biological Sciences
Physics (Astrophysics) Department Senior Thesis Requirement
My thesis attempts to understand how certain types of stars known as white dwarfs explode as some of the brightest events in the universe called supernovae. Type Ia supernovae (and thermonuclear supernovae in general) are believed to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs). Type Ia supernovae have played a crucial role in the discovery of the accelerating Universe, yet despite their cosmological importance, how these CO WDs reach explosion conditions remains debated. This project aims to understand which binary star systems, i.e. progenitors, may contribute to thermonuclear supernovae (TN SNe) through the ''helium donor scenario''.
In the helium donor channel, a CO WD gains mass from a helium star and explodes through igniting carbon in its center when it approaches the critical Chandrasekhar mass. In this work we model the mass transfer episode from the helium star to the CO WD, and investigate the combinations of initial orbital period and He star and WD masses (i.e., the parameter space) that can produce likely TN SN candidates from the helium donor channel. We add to the contribution by previous works in two meaningful ways. First, we account for the possibility of an off-center carbon ignition in the CO WD. The off-center ignition leads to the formation of an oxygen-neon white dwarf which likely ultimately forms a neutron star instead of a thermonuclear supernova. Thus, by resolving the stellar structure of the CO WD to account for the off-center ignitions, we are able to refine the parameter space by previous studies. Second, we investigate the effect of a slow WD wind which implies a specific angular momentum loss from the binary that is larger than typically assumed. We find that additional angular momentum loss does not significantly alter the region of parameter space over which systems evolve toward thermonuclear supernovae. Our determination of the correspondence between initial binary parameters and the final outcome is important in informing the contribution of the helium donor channel to thermonuclear supernovae.
