WoU-MMA: The White Dwarf Mass Distribution and the Gravitational Wave Foreground from Double White Dwarfs

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White dwarf (WD) stars are extremely compact objects that are the remnants of stars of similar mass to the Sun, which have exhausted their nuclear fuel, but shine for billions of years from their residual heat. A research collaboration between the University of Oklahoma and the Smithsonian Astrophysical Observatory will study the population statistics of WDs, with an emphasis on extremely low-mass (ELM) double WDs, orbiting each other with periods as short as a few minutes. These studies are very important for astrophysics, since double or binary WDs are expected to be major sources of gravitational waves (GW) detected by future space-based GW observatories such as the planned LISA mission. They are also expected to eventually merge, leading to a stellar explosion or supernova (SN), of Type Ia, an important type of supernova that is used to measure the expansion rate of the universe. The investigators will use data from the Gaia space mission to identify candidate ELM WDs, and then use ground based telescopes to confirm the candidates and determine their characteristics. They will also study the entire population of WDs of all masses within about 300 light years of Earth. This project will provide research opportunities for graduate and undergraduate students, including members of under-represented groups. In conjunction with their research, the principal investigator and his students will provide interactive demonstrations, lectures, question and answer panels, and stargazing events to scout groups, middle and high school students, and K-12 students from Cheyenne-Arapaho and other tribes across Oklahoma, through the Lunar Sooners outreach group. Using parallaxes from the Gaia mission, ELM white dwarfs can be readily identified, since they are over-luminous compared to average mass WDs. These WDs are the signposts of ultra-compact binary systems with periods as short as 12 minutes. Finding such binaries is important for near-term population synthesis studies, for characterizing the expected GW foreground in the mHz frequency range from double WDs, and for significantly increasing the number of LISA verification sources. Well-measured WD binaries will be among the best Electromagnetic plus GW laboratories for understanding the formation of compact objects, dynamical interactions and tides in binaries, and energetic phenomena like Type Ia SNe. The Gaia 100 pc WD sample is essentially complete but contains an unexpected bifurcation in the color/absolute magnitude sequence that is mostly explained by the atmospheric composition. However, the distribution of hydrogen atmosphere WDs requires a population of massive (around 0.8-1 solar mass) WDs, which may be linked to merging WD binaries. This project will obtain an unbiased WD mass distribution and constrain the frequency of massive WDs. This project advances the goals of the NSF Windows on the Universe Big Idea.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.