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

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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 $\sim10^{-2} M_\odot$ of fast ejecta driven by shocks at the onset of common envelope, and later, $\sim0.3 M_\odot$ of further ejecta as the secondary becomes more deeply engulfed within the primary.