A Luminous Red Nova in M31 and its Progenitor System

TL;DR

This paper reports on the discovery and detailed observations of a luminous red nova in M31, analyzing its spectral evolution, progenitor system, and comparing it with similar known LRNs to understand its nature.

Contribution

It provides the first detailed spectroscopic and photometric analysis of M31LRN 2015, identifying its progenitor and comparing its properties with other LRNs, especially V838 Mon.

Findings

The nova became extremely red as it faded.

Spectra showed evolving features including Halpha, Na I D, Ba II, and TiO bands.

A luminous red progenitor was identified in archival HST data.

Abstract

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 M(V) = -9.4 +/- 0.2 peak. Early spectra showed strong Halpha 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 Halpha 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.