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arXiv · 0901.0198

The 2008 Luminous Optical Transient in the Nearby Galaxy NGC 300

Howard E. Bond·Luigi R. Bedin·Alceste Z. Bonanos·Roberta M. Humphreys·L. A. G. Berto Monard·Jose L. Prieto·Frederick M. Walter
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Abstract

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.

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Howard E. Bond, Luigi R. Bedin, Alceste Z. Bonanos, Roberta M. Humphreys, L. A. G. Berto Monard, Jose L. Prieto, Frederick M. Walter. 2009-03-24. The 2008 Luminous Optical Transient in the Nearby Galaxy NGC 300. https://doi.org/10.1088/0004-637x%2F695%2F2%2Fl154

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