The SALVATION Project: A Red Nova in M31 and Other Highlights

The SALVATION (Spectroscopic Analysis of Luminous Variables and Transients in our Neighbor) project, which started in the fall of 2019, uses Lick Observatory’s Shane 3-m telescope and Kast double spectrograph in cadence and target-of-opportunity modes to carry out visible (~4000–9000 Angstrom) spectroscopic follow up of variable stars and transients identified by the ANTARES community alert broker from the real-time data stream of the Zwicky Transient Factory. Most of these sources are in the Andromeda galaxy (M31) or along the line-of-sight to it; during times of the night when M31 is inaccessible, we obtain spectra of transients along other lines of sight. We are carrying out routine spectroscopic monitoring of ~50 luminous variable stars, and have obtained spectra of several tens of transients to date, many of which have been reported in rapid response via Astronomer’s Telegrams.

In fall of 2021, we spectroscopically classified a transient, AT2021tdu, identified from the community alert broker ANTARES as a red nova in M31. Red novae and their luminous counterparts are thought to be the result of stellar mergers and occupy a particularly sparse portion of the transient gap (the gap between classical novae and supernovae) with only a few published examples, three of which are recorded in M31, making this the discovery of a rare object. Due to the initial preference from observers of finding the most luminous and common stellar explosions, groups of transients between these parameters, including red novae like AT2021tdu, have been left unobserved. The Nancy Grace Roman Space Telescope will grant near infrared multi-band imaging to analyze the rarely documented IR emissions that are characteristic of red novae. We are currently collecting photometric and spectroscopic data of AT2021tdu to report more information on red novae and improve their characterization.