V1309 Scorpii
V1309 Scorpii (also known as V1309 Sco) is a contact binary that merged into a single star and went nova in 2008 in a process known as a luminous red nova. It was the first star to provide conclusive evidence that contact binary systems end their evolution in a stellar merger. Its similarities to V838 Monocerotis and V4332 Sagittarii allowed scientists to identify these stars as merged contact binaries as well.
V1309 Scorpii was discovered independently in August, 2008 by three groups: K. Nishiyama and F. Kabashima, Y. Sakuri, and Guoyou Sun and Xing Gao. It was originally identified as a transient object located near the galactic bulge at Right Ascension 17h 57m 32.93s ± 0s.01 and Declination −30° 43′ 10″ ± 0″.1. The astronomers who found it noted that it had been invisible to their 12 mag limit telescope just a few days prior to its discovery, indicating that it had recently gone nova. Before its outburst, its faintness and close proximity to USNO-B1.0 star 0592-0608962 (magnitude B = 16.9 and R = 14.8) just 1.14″ away made it difficult to detect. When discovered, V1309 Scorpii was believed to be nothing more than a classical nova.
Immediately following its outburst, a group of astrophysicists lead by Elena Mason at the European Southern Observatory conducted a study of V1309 Sco's post-outburst spectrum. Originally, the focus of this study was to analyze heavy-metal absorption patterns in a classical nova, but the authors did not realize that this was not a classical nova. In analyzing the spectrum, Mason et al. posited that V1309 Scorpii was surrounded by a slowly expanding gas shell which is denser in the equatorial plane, giving way to a narrow absorption spectrum from this dense region and a broader emission spectrum surrounding it. The incline of this equatorial plane from the observer's line of sight leaves mostly just the polar cap visible. This region would then be approaching the observer as indicated by the overall blueshift of the spectrum. Furthermore, the presence of ejecta from the polar cap at various velocities would account for the observed high velocity wings in the Balmer series. The behavior of the Hα/Hβ ratio, which decreased for a little over a month before shooting up to saturated levels and remaining high months after, was one of many spectral characteristics, also including distinct forbidden lines, that made V1309 Scorpii distinct from classical novae and more similar to red novae.
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