Abstract

White dwarfs (WDs) are the endpoint of stellar evolution for 97% of the stars in our universe. WDs are no longer undergoing nuclear fusion so their cooling rate is assumed to be well-known and their cooling ages are often used for dating stellar populations. A majority of WDs are hydrogen-atmosphere (DA) and eventually enter a temperature range where they begin to pulsate, allowing us to use asteroseismology to probe their interiors. Another phenomenon that occurs for all WDs is core crystallization which releases latent heat and causes a delay in the WD cooling process. This could change our determination of stellar population ages by 1 Gyr or more. High-mass WDs (≳1 M⊙) begin to crystallize at a higher temperature than WDs of lower masses. A majority of WDs are around 0.6 M⊙ and begin crystallization at temperatures below the range where DA WDs pulsate, so probing the crystalline interiors through asteroseismology is not possible. Using data from Gaia and the Zwicky Transient Facility, we have identified 38 candidate pulsators with masses high enough to be undergoing core crystallization within the pulsational instability strip. Fourteen of these have been confirmed as pulsators, increasing the number of known ultramassive pulsators from 3 to 17. Pulsators with suitable properties could allow us to constrain the cooling rate and the crystallized mass fraction of the interior.