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 subarcsecond extension around the star is indicative of a past outburst, and from the nebula's size (0.5 pc diameter) we estimate that the outburst took place roughly 2000 years 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 H-R 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 85 M-circle dot star. The star is likely in a transition from an O star to a late-type WN Wolf-Rayet star.