0 Assigned ID: 666 ----- Attempting Submission 1 (Fri Apr 08 18:36:52 GMT 2016) ----- The recent red transient M31 LRN 2015 is thought to be caused by a stellar merger. It was discovered in January 2015 and is located in M31. Remarkably, there is imaging of a pre-outburst source in multiple colors in the Hubble Space Telescope (HST) archive. With the color, magnitude, and distance to the source known, we can use these data to link the binary system to the outburst it produced. Our preliminary calculations show that the pre-outburst source is consistent with a 3 to 5 solar mass star evolving up the giant branch (to about 35 solar radii) before it engulfs its companion in a common envelope (CE) episode. This source therefore offers a unique opportunity to place strong observational constraints on the long-uncertain properties of flows at the onset of common envelope interactions. By comparison of the transient to the pre-outburst modeling we find that the transient is dynamical: it takes place in (of order) one binary dynamical time, and the ejecta velocities are above the local escape velocity. Having modeled the pre-outburst stellar properties, we are able to set up hydrodynamic simulations to study the transient. These simulations will determine which evolutionary channels (and which binary mass ratios) produce ejecta velocity, mass, and entropy distributions consistent with the observed transient. This proposal offers the opportunity to unlock the potential of this serendipitous HST detection of the binary system prior to outburst. The proposed detailed modeling effort will maximize the benefit to CE science and our understanding of this long-uncertain phase of binary evolution. Stellar Physics 0 Regular false false true SUPPORTED