THE 2008 LUMINOUS OPTICAL TRANSIENT IN THE NEARBY GALAXY NGC 300

A luminous optical transient (OT) that appeared in NGC 300 in early 2008 had a maximum brightness, MV ≅ -12 to -13, intermediate between classical novae and supernovae. We present ground-based photometric and spectroscopic monitoring and adaptive-optics imaging of the OT, as well as pre- and postoutburst space-based imaging with the Hubble Space Telescope (HST) and Spitzer. The optical spectrum at maximum showed an F-type supergiant photosphere with superposed emission lines of hydrogen, Ca II, and [Ca II], similar to the spectra of low-luminosity Type IIn 'supernova impostors' like SN 2008S, as well as cool hypergiants like IRC +10420. The emission lines have a complex, double structure, indicating a bipolar outflow with velocities of ∼75 km s-1. The luminous energy released in the eruption was ∼1047 erg, most of it emitted in the first two months. By registering new HST images with deep archival frames, we have precisely located the OT site, and find no detectable optical progenitor brighter than broadband V magnitude 28.5. However, archival Spitzer images reveal a bright, nonvariable mid-infrared (mid-IR) preoutburst source. We conclude that the NGC 300 OT was a heavily dust-enshrouded luminous star, of ∼10-15 M sun, which experienced an eruption that cleared the surrounding dust and initiated a bipolar wind. The progenitor was likely an OH/IR source which had begun to evolve on a blue loop toward higher temperatures, but the precise cause of the outburst remains uncertain.