Lessons from the Onset of a Common Envelope Episode: the Remarkable M31 2015 Luminous Red Nova Outburst

This paper investigates the recent stellar-merger transient M31LRN 2015 in the Andromeda galaxy. We analyze published optical photometry and spectroscopy along with a Hubble Space Telescope detection of the color and magnitude of the pre-outburst source. The transient outburst is consistent with dynamically driven ejecta at the onset of a common envelope episode, which eventually leads to the complete merger of a binary system. The light curve appears to contain two components: first similar to 10(-2) M circle dot of fast ejecta driven by shocks at the onset of common envelope, and later, similar to 0.3M circle dot of further ejecta as the secondary becomes more deeply engulfed within the primary. Just prior to merger, we find that the primary star is a 3-5.5M circle dot subgiant-branch primary star with a radius of 30-40 Re. Its position in the color-magnitude diagram shows that it is growing in radius, consistent with a picture where it engulfs its companion. By matching the properties of the primary star to the transient outburst, we show that the optical transient lasts less than 10 orbits of the original binary, which had a pre-merger period of similar to 10 days. We consider the possible orbital dynamics leading up to the merger, and argue that if the system merged due to the Darwin tidal instability it implies a lower mass main-sequence companion of 0.1-0.6M circle dot. This analysis represents a promising step toward a more detailed understanding of flows in common envelope episodes through direct observational constraints.