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, M_V ~ -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
post-outburst space-based imaging with 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. The luminous energy released in
the eruption was ~10^47 ergs, most of it emitted in the first 2 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 broad-band
V 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 a
heavily dust-enshrouded luminous star, of ~10-15 Msun, 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.
