A luminous optical transient ( OT ) that appeared in NGC 300 in early 2008 had a maximum brightness , M _ { V } \simeq - 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 \sim 75 { \ > km\ > s ^ { -1 } } .
The luminous energy released in the eruption was \sim 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 \sim 10– 15 M _ { \odot } , 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 .