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Burying a Binary: Dynamical Mass Loss and a Continuous Optically-Thick Outflow Explain the Candidate Stellar Merger V1309 Scorpii

V1309 Sco was proposed to be a stellar merger and a common envelope transient based on the pre-outburst light curve of a contact eclipsing binary with a rapidly decaying orbital period. Using published data, I show that the period decay timescale P/Pdot of V1309 Sco decreased from ~1000 to ~170 years in less than about 6 years, which implies a very high value of second period derivative. I argue that V1309 Sco experienced an onset of dynamical mass loss through the outer Lagrange point, which eventually obscured the binary. The photosphere of the resulting continuous optically-thick outflow expands as the mass-loss rate increases, explaining the ~200 day rise to optical maximum. The model yields the mass-loss rate of the binary star as a function of time and fits the observed light curve remarkably well. It is also possible to observationally constrain the properties of the surface layers undergoing the dynamical mass loss. V1309 Sco is thus a prototype of a new class of stellar transients distinguished by a slow rise to optical maximum that are driven by dynamical mass loss from a binary. I discuss implications of these findings for stellar transients and other suggested common envelope events.