2026ApJ...999L..35B

Following the detections of the first extragalactic “luminous red nova” (LRN) M31RV in 1989, and its first Galactic counterpart V4332 Sgr in 1994, there have been many discoveries of similar, or closely related, objects. They are important because they bridge the luminosity gap between the brightest novae and supernovae, a largely unexplored parameter space. The cause of eruptions in LRNe is still unclear, a stellar merger being the most favored mechanism. However, barring V1309 Sco, there has been no direct evidence for a merger in the other objects. Here we present strong evidence that V4332 Sgr was a merger event. High-resolution infrared observations of the CO fundamental band show an unusually small ¹²C/¹³C ratio of 3.5 ± 1. This indicates that a violent event had occurred, whose effects penetrated deep enough to allow the CNO cycle processed ¹³C in the inner H burning shell to be brought to the surface. We rule out planetary ingestion and propose that the eruption was due to a merger between V4332 Sgr and a companion star. It is shown that V4332 Sgr was likely surrounded by an edge-on disk before its eruption. If this disk was a flattened common envelope containing V4332 Sgr and a companion star, then a merger scenario would not be inconsistent. Furthermore, it is also shown that V4332 Sgr had multiple outbursts, previously unreported but an important piece of information, since multiple outbursts are a trait shared by many LRNe.