Current Challenges in the Physics of White Dwarfs: the Origins of the Ultramassive White Dwarf population

White dwarfs are the dense, Earth-sized remains of stars like our Sun. Some of these white dwarfs are unusually massive, and their origins are still not fully understood. One possibility is that they form when two stars merge into one. This project will study the most massive white dwarfs in our galaxy to understand how they form, what they are made of, and how they evolve. By using data from Gaia and ground-based telescopes, the team will identify white dwarf merger remnants, measure their masses, interiors, and magnetic properties, and explore their role in producing supernovae - powerful explosions used to measure distances in the universe. This program will provide training and research opportunities for graduate and undergraduate students in astrophysics and observational techniques, and it will continue a successful science outreach program that engages K–12 students and local communities in Oklahoma. This award investigates the origin and evolution of ultramassive white dwarfs (M > 1.1 solar masses) in the solar neighborhood, building on a completed spectroscopic survey of the 100 pc Gaia sample. The team will carry out five complementary objectives: (1) characterize the mass and temperature distribution of the most massive white dwarfs and constrain their merger fraction; (2) conduct an all-sky survey of warm DQ/DAQ white dwarfs using combined GALEX and Gaia photometry to identify likely merger remnants; (3) identify and analyze massive pulsating white dwarfs using asteroseismology to probe their internal structure; (4) constrain the origin of magnetic ultramassive white dwarfs via time-series spectroscopy to measure rapid rotation signatures; and (5) survey the coolest DQ white dwarfs to determine their fate and trace their connection to merger products and crystallization. Observations will be conducted using APO 3.5m, MMT/Magellan 6.5m, Gemini 8m, and GTC 10m telescopes. The work involves a combination of spectroscopic and high-cadence photometric follow-up and theoretical modeling through atmospheric analysis and asteroseismic interpretation. 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.