The Discovery of a P Cygni Analog in M31

We present spectroscopy and discuss the photometric history of a previously obscure star in M31. The spectrum of the star is an extremely close match to that of P Cygni, one of the archetypes of Luminous Blue Variables (LBVs). The star has not shown much variability over the past 40 years (< 0.2 mag), although small-scale (0.05 mag) variations over a year appear to be real. Nevertheless, the presence of a sub-arcsecond extension around the star is indicative of a past outburst, and from the nebula’s size (0.5 pc diameter) we estimate the outburst took place roughly 2000 yrs ago. P Cygni itself exhibits a similar photometric behavior, and has a similar nebula (0.2 pc diameter). We argue that this may be more typical behavior for LBVs than commonly assumed. The star’s location in the HR diagram offers substantial support for stellar evolutionary models that include the effects of rotation, as the star is just at a juncture in the evolutionary track of a 85M⊙ star. The star is likely in a transition from an O star to a late-type WN Wolf-Rayet. Subject headings: stars: early-type—stars: evolution–stars: mass loss—supergiants