Program: GN-2018B-Q-126

Luminous Red Novae are a recently recognized class of eruptive events more luminous than a classical nova but less luminous than a supernova. Mergers of close binary stars are convincingly implicated as their progenitors from their archival light curves. Despite the vast scope of today's time-domain sky surveys, no explosive event (nova, supernova, or luminous red nova) has ever been successfully predicted! We have identified a 10.9-hr period contact binary system with an exponentially decaying period that we predict will terminate in a stellar merger and luminous red nova within the next five years. We propose high-cadence R=67,500 Gemini/GRACES optical spectroscopy covering the full range of orbital phases to perform Doppler tomographic mapping of the stellar surfaces. Analysis of the 96 five-minute spectra will allow us to map the locations of spots and determine the binary equipotential fillout factor. Both of these are required to test a model we are developing of the merger mechanism involving mass loss through the outer Lagrange point. We request 2x5.5 hr of Band 1 time for Priority Visitor observations to take advantage of this unique opportunity to discover the mechanism leading to a red nova outburst. CoIs Kobulnicky, Riley, and recent Gemini/GRACES instrument scientist Lundquist would plan to be onsite for five nights of Priority Visitor time to conduct observations.