Lithium in red novae and their remnants

Abstract

Context. Galactic red novae are thought to be produced in stellar mergers between non-compact stars, such as main-sequence stars and cool giants. It is hoped that they will help explain the physical processes involved in common-envelope evolution and stellar binary collisions. Aims. We investigate the presence of lithium in the three best observed Galactic red nova remnants. Explaining the origin of lithium may point to a mixing mechanism present before, during, or after the merger. Methods. We compared the lithium line at 6707.81 Å to a feature of Ca I] at 6572.78 Å to derive relative abundances in circumstellar gas. We next calculated absolute abundances, assuming the solar calcium-to-lithium abundance ratio. Results. Our analysis yields the following lithium abundances: A(Li) = 2.3 for V838 Mon, A(Li) = 2.5 for CK Vul, and A(Li) = 1.8 for V1309 Sco. Conclusions. The super-solar lithium abundances in red novae may suggest that at least some merger products activate mixing mechanisms capable of producing lithium, but other interpretations are possible. In particular, the merger product of V838 Mon may be enhanced in lithium by the matter of one of the progenitor stars. Early observations of lithium in V838 Mon and V1309 Sco hint that Li-producing mechanisms, if present, activate early, perhaps already in the common-envelope phase. Determining the origin of lithium in red novae requires further investigation and a larger sample of measured stars.