Convection, Oscillations, Outbursts, and Limit Cycle Behavior of the Coolest Pulsating White Dwarfs

This project will study the puzzling behavior of a type of star called a white dwarf. These are dense remnants of stars like our Sun. Some white dwarfs pulse with light, getting brighter and dimmer over time. But as they cool, these pulsations stop abruptly - and scientists don’t fully understand why. Recently, space telescopes like Kepler and TESS have discovered strange behavior in some of these stars, such as sudden outbursts and random changes in brightness. This project will use computer models to understand what causes these phenomena and why pulsations cease as white dwarfs cool. Alongside the research, the project will train two graduate students and involve undergraduates in hands-on astrophysics. Some of these students will explore real telescope data in the classroom and contribute to research projects. A study will track how these experiences affect students’ views of science and their academic paths.

The investigator will conduct a comprehensive theoretical investigation into the cessation of pulsations and the origin of outbursts in cool DA (hydrogen-atmosphere) white dwarfs near the empirical red edge of the instability strip. The project combines large grids of evolutionary models generated using MESA, WDEC, and ISUEVO, with linear adiabatic and nonadiabatic pulsation calculations via GYRE and GNR1-based codes. The work will evaluate mode growth rates, explore sensitivity to convection prescriptions, and analyze amplitude coherence as a function of stellar mass and effective temperature. A key innovation of the project is the inclusion of nonlinear three-mode resonant coupling calculations to test whether these interactions can suppress observable pulsations or trigger episodic outbursts. The modeling will explore amplitude saturation, limit-cycle behavior, and energy deposition mechanisms that could reproduce observed features such as broadened power spectra and burst-like luminosity increases. The project also includes integration of white dwarf variability research into undergraduate astronomy coursework and a longitudinal study of student learning outcomes.

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.

Award Number: 2511209
Principal Investigator: Steven Kawaler
Funds Obligated: $269,197
State: IA