# # Program information file # PROGRAM_ID 2023B053 PROGRAM_TITLE Luminous mid-infrared transients in M31: A NEOWISE census of binary coalescence across the stellar lifecycle PROGRAM_INV1 Kishalay De PROGRAM_INV2 Morgan MacLeod PROGRAM_INV3 Jacob Jencson PROGRAM_INV4 Viraj Karambelkar PROGRAM_INV5 Ryan Lau PROGRAM_SCICAT extra-galactic PROGRAM_ABSTRACT_BEG Stellar mergers are ubiquitous and important phases in binary stars. Altering the stellar mass spectrum and providing gateways to compact binary systems, stellar coalescence features centrally in all of the three pillars of NASA astrophysics. Observationally manifesting as outbursts called the red novae, these events exhibit hallmark signatures of cooling ejecta and dust formation via their short-lived optical and extremely long-lived mid-infrared [MIR] eruptions. However, there is clear growing evidence of a missing piece in our understanding. While lower mass mergers should dominate the observed demographics of nearby events and commonly produce compact binaries relevant to the high energy universe, the known optical sample of events is heavily skewed towards rarer massive binaries. Recent theoretical suggestions indicate that these events are possibly dust obscured by mass loss leading up to the merger, and therefore optically invisible, and possibly even observed previously as mysterious infrared transients. With the field now entering a revolution in MIR time domain astronomy, thanks to archival searches in NASA's NEOWISE mission data, the hunt for this population is now on the horizon. Utilizing its well known distance and exquisite archival coverage, the proposing team has identified a sample of luminous MIR transients in M31 as candidate red novae, including ones with no optical emission -- indicative of the missing low mass population. The WISE sensitivity is adequate to identify events to the faintest end of the merger luminosity function in M31. The team requests 8 hours of imaging and prism spectroscopy with SpeX to characterize the most promising stellar merger candidates, aimed at revealing the demographics of these spectacular events in the nearest Milky Way analog, providing indispensable context for several areas of astrophysics highlighted in the decadal survey. PROGRAM_ABSTRACT_END