The Landscape of Stellar Mergers with Time-Domain Surveys

Stellar mergers result in a wide range of outcomes, from luminous cosmic transients to peculiar variable stars. Mergers provide valuable insights into a broad range of astrophysical phenomena, from stellar evolution to gravitational waves to the origins of the universe's heaviest elements. In this thesis, I systematically explore the outcomes of merging white dwarfs, merging neutron stars, and merging massive stars, using robotic survey telescopes to fill gaps in our understanding of the diverse merger landscape. In Part I of this thesis, I present the first infrared census of dusty variable stars formed from low-mass white dwarf mergers. This population offers new insights into the binary white dwarfs that will be detected by the upcoming Laser Interferometer Space Antenna (LISA). I also present the least luminous thermonuclear supernova discovered to date, which possibly originated in the merger of two massive white dwarfs. In Part II of this thesis, I present the data processing pipeline of the novel Wide-field Infrared Transient Explorer (WINTER) surveyor at Palomar Observatory, designed for infrared followup of gravitational wave events from neutron star mergers. I present results from WINTER’s first search for an infrared counterpart to a neutron star merger recently detected by the International Gravitational Wave Network. In Part III of this thesis, I present the first systematic study of extragalactic transient eruptions from massive stellar mergers and estimate their volumetric rate and luminosity function. I also present the first infrared observations of such mergers with the James Webb Space Telescope, which suggest that stellar mergers could be significant contributors to the cosmic dust budget. Additionally, I present a slow-evolving infrared transient identified by WINTER that originated in a merger involving a giant star primary, revealing a new class of events that have been overlooked by previous optical surveys. Together, these studies set the stage for more comprehensive explorations of the merger landscape in the future, with i) the Vera Rubin Observatory to study large populations of low luminosity transients from massive stellar mergers and white-dwarf mergers, and ii) the upcoming suite of ground and space-based infrared surveys to discover the dustiest stellar mergers and quantify their contributions to the cosmic dust budget.