The asymptotic evolution of the stellar merger V1309 Sco: a Blue Straggler in the making?

Abstract

Stellar mergers are estimated to be common events in the Galaxy. The best studied stellar merger case to date is V1309 Sco (= Nova Scorpii 2008) which was originally misclassified as a Nova event. Later identified as the merger of the components of a cool overcontact binary system with 1.52 Msun and 0.16 Msun, V1309 Sco showed an initial period of P = 1.4 days before the merger. Post-outburst evolution demonstrated that V1309 Sco was unlike the typical Classical Novae and Symbiotic Recurrent Novae with significant dust production around it, and indicated that the system may become a post-AGB (or pre-PN) soon. Here we present a study of V1309 Sco about ten years after the outburst, based on near-IR variability and colour data from the ESO surveys VISTA Variables in the V\'{i}a L\'actea (VVV) and VVV eXtended (VVVX). We find that reasonable equilibrium in this stellar merger is being reached and that the star has settled into a nearly constant magnitude. A dramatic change in its near-IR colours from (J-Ks) = 1.40 in 2010 to (J-Ks) = 0.42 in 2015 and a possible low amplitude periodic signal with P = 0.49 days in the post-outburst data are consistent with a "blue straggler" star, predicted to be formed from a stellar merger.