A Luminous Red Nova in M31 and Its Progenitor System

We present observations of M31LRN 2015 (MASTER OT J004207.99+405501.1), discovered in M31 in 2015 January, and identified as a rare and enigmatic luminous red nova (LRN). Spectroscopic and photometric observations obtained by the Liverpool Telescope showed the LRN becoming extremely red as it faded from its peak. Early spectra showed strong H α emission that weakened over time as a number of absorption features appeared, including Na i D and Ba ii . At later times strong TiO absorption bands were also seen. A search of archival Hubble Space Telescope data revealed a luminous red source to be the likely progenitor system, with pre-outburst H α emission also detected in ground-based data. The outburst of M31LRN 2015 shows many similarities, both spectroscopically and photometrically, with that of V838 Mon, the best studied LRN. We finally discuss the possible progenitor scenarios.