Abstract

The final fates of stars and their planetary systems are in most cases embodied by white dwarfs. These are the compact remnants of stellar evolution for all low- to intermediate-mass stars in our Galaxy, including the Sun. The relative simplicity of white dwarfs makes them valuable tools in a variety of astrophysical contexts, from placing limits on the age of our Galaxy, to measuring the bulk abundances of exoplanets via the presence of actively accreted metals in their atmospheres. White dwarfs can also exhibit photometric variability for a variety of reasons, both as isolated stars and in binary systems, providing insights into white dwarf structure and circumstellar environments. In this dissertation we present novel techniques to identify variable white dwarfs in modern time-domain photometric surveys. We then focus on identifying and characterizing two types of variable white dwarfs: those that pulsate and those being transited by circumstellar planetary debris. Using the unprecedented number of white dwarfs observed by Gaia and the Zwicky Transient Facility (ZTF), we define a set of variability metrics based on these publicly available catalogues to efficiently identify variable white dwarfs across the Gaia color magnitude diagram. We then present a dedicated search for pulsating He-atmosphere white dwarfs (DBVs) using McDonald Observatory follow-up observations, in an attempt to better define the observed extent of the helium-atmosphere white dwarf instability strip. We increase the number of DBVs from 26 to 45 using Sloan Digital Sky Survey spectroscopy to initially identify candidate DBVs. We find the observed blue and red edges of the DB instability strip are still hotter than their theoretical counterparts by about 1500K and 600K, respectively. Using our set of Gaia and ZTF variability metrics, we also identify and characterize the second and third cases of white dwarfs with recurring planetary debris transits, with debris orbital periods differing by more than two orders of magnitude. These objects, in combination with the first such system, probe a unique parameter space constraining earlier stages of the planetary debris disruption and migration process.