The 2008 Luminous Optical Transient in the Nearby Galaxy NGC 300

Howard E. Bond, Luigi R. Bedin, Alceste Z. Bonanos, Roberta M. Humphreys, Berto Monard, Jose L. Prieto and Fred Walter

Space Telescope Science Institute, University of Minnesota, Bronberg Observatory, Ohio State university and Stony Brook University

A luminous optical transient (OT) that appeared in NGC~300 in early 2008 had a maximum brightness, $M_Vsimeq-13$, intermediate between classical novae and supernovae. We present ground-based photometry, spectroscopy, and adaptive-optics imaging of the OT, as well as pre-and post-outburst space-based imaging with HST/ and {it Spitzer}. The optical spectrum at maximum showed an F-type supergiant photosphere with superposed emission lines of hydrogen, ion{Ca}{2}, and [ion{Ca}{2}], similar to the spectra of low-luminosity Type~IIn ``supernova impostors'' like SN~2008S, as well as hypergiants like IRC~+10420. The emission lines have a complex, double structure, indicating a bipolar outflow with velocities of $sim$$75,kms$. The energy released in the outburst was $sim$$2 times 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 {it Spitzer/} images reveal a bright mid-IR pre-outburst source. We conclude that the NGC~300~OT was a heavily optically dust-enshrouded luminous star, of roughly $10,M_odot$, which experienced an outburst 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.

Reference: Astrophysical Journal Letters
Status: Manuscript has been submitted