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 M and 0.16 M, V1309 Sco showed an initial period of P = 1.4 d 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 10 yr after the outburst, based on near-IR variability and colour data from the ESO surveys VISTA Variables in the Vía Láctea (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 - K s) = 1.40 in 2010 to (J - K S) = 0.42 in 2015 and a possible low-amplitude periodic signal with P = 0.49 d in the post-outburst data are consistent with a 'blue straggler' star, predicted to be formed from a stellar merger.