This project is focused on developing a better understanding of the ignition of matter accreted from stellar companions onto the surfaces of white dwarfs. Until the final explosion of an accreting white dwarf as a Type Ia supernova, the brightest manifestations of mass transfer are the thermonuclear ignitions of accreted shells of matter. With Hydrogen donors, these are Classical Novae; with Helium donors, these are He novae or weak surface explosions. Observing these flashes provides a census of the numbers of such binaries in distant galaxies. Here, theoretical work on surface explosions on accreting white dwarfs will be undertaken, specifically considering a range of new environments, from pure Helium accretion onto a Carbon/Oxygen white dwarf to Hydrogen-rich accretion onto a pure Helium white dwarf. The upcoming advances in the number and quality of optical transient surveys (e.g. Pan-STARRS-1, Pan-STARRS-4, SkyMapper, Large Synoptic Survey Telescope) and the detailed x-ray and optical studies of binaries in the Milky Way makes the time ripe for these theoretical studies. Two specific areas will be investigated.
One: Helium Explosions from AM Canum Venaticorum Binaries. The Helium that accretes onto a Carbon/Oxygen white dwarf in these short orbital period (< 60 minute) binaries undergoes a series of unstable flashes with an exciting range of outcomes, from He Classical Novae to explosions leading to faint Type Ia supernovae. Initial rate estimates for a 100 billion solar mass elliptical galaxy give Helium Novae at a rate of one every 25 years and a radioactive He explosion every 1000 years (< 10% of the Type Ia supernovae rate).
Two: New Regimes of Hydrogen Burning on Accreting White Dwarfs. A large part of the understanding of the origin of Type Ia supernovae is connected to the ability to burn accreted Hydrogen on an accreting white dwarf at the rate accreted. However, when unstable, the actual thermonuclear trigger for the unstable ignition at rapid accretion rates can depend on the metallicity of the accreted material, yielding Classical Novae rates for distant galaxies that depend on both the mass and the metallicity of the hosting stellar population. In addition, a long-standing puzzle has been the role of Helium white dwarfs as accreting objects in mass-transferring binaries. Initial indications are that the masses needed for unstable Hydrogen-burning flashes will be very large on these cold, low gravity objects, potentially providing the ideal environment for outbursts like that seen in M31 as the ''red variable''.
It is expected that this work will motivate new transient surveys of distant galaxies. Moreover, the graduate students involved in the project will be trained in theory, numerical modeling, and data acquisition. The PI will continue to accelerate the inclusion of the Las Cumbres Observatory Global Telescope Network into the international astrophysics community. The development of a K-12 astronomy curriculum for local outreach Docents will enhance their efforts, especially amongst under-represented minorities.
This project led to the discovery of the first eclipsing double white dwarf binary, the theoretical prediction of a new type of faint stellar explosion, and the explanation of super-luminous supernovae as originating from the birth of a rapidly rotating, and highly magnetic neutron star; a magnetar. Part of the PI's efforts were also focused on theoretical interpretations of results from the Palomar Transient Factory, an international collaboration utilizing the 48 inch Schmidt telescope for supernovae and transient discovery. The development and release of a new computational tool for stellar astrophysics, Modules for Experiments in Stellar Astrophysics (MESA), was also supported by this grant, especially the efforts of the PI with the code's prime developer: Bill Paxton. This publicly available code now has over 250 users across the world. The uses of this code range from educational to pure research. The PI was very involved in public outreach, giving many talks to general audiences and schoolchildren. He was also a lecturer at the TRIUMF summer graduate school on ``Nuclear Astrophysics of Exploding Stars'', highlighting the continued impact of nuclear physics on astrophysics and the potential for astrophysics to reveal new realms for nuclear physics. During this proposal period, the PI also worked to expand the Dos Pueblos Engineering Academy, a four year public high school program. The facility expansion now allows for over 100 new freshman per year, increasing the reach of this program.
