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A luminous optical transient (OT) that appeared in NGC 300 in early 2008 hada maximum brightness, M_V ~ -12 to -13, intermediate between classical novaeand supernovae. We present ground-based photometric and spectroscopicmonitoring and adaptive-optics imaging of the OT, as well as pre- andpost-outburst space-based imaging with HST and Spitzer. The optical spectrum atmaximum showed an F-type supergiant photosphere with superposed emission linesof hydrogen, Ca II, and [Ca II], similar to the spectra of low-luminosity TypeIIn "supernova impostors" like SN 2008S, as well as cool hypergiants like IRC+10420. The emission lines have a complex, double structure, indicating abipolar outflow with velocities of ~75 km/s. The luminous energy released inthe eruption was ~10^47 ergs, most of it emitted in the first 2 months. Byregistering new HST images with deep archival frames, we have precisely locatedthe OT site, and find no detectable optical progenitor brighter than broad-bandV magnitude 28.5. However, archival Spitzer images reveal a bright,non-variable mid-IR pre-outburst source. We conclude that the NGC 300 OT was aheavily dust-enshrouded luminous star, of ~10-15 Msun, which experienced aneruption that cleared the surrounding dust and initiated a bipolar wind. Theprogenitor was likely an OH/IR source which had begun to evolve on a blue looptoward higher temperatures, but the precise cause of the outburst remainsuncertain.

The 2008 Luminous Optical Transient in the Nearby Galaxy NGC 300