The 2008 Luminous Optical Transient in the Nearby Galaxy NGC 300
Abstract
A luminous optical transient (OT) that appeared in NGC 300 in early 2008 had a maximum brightness, MV sime -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.
Based in part on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, and from the data archive at STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555; in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA; in part on observations obtained with the 6.5 m Magellan Clay Telescope located at Las Campanas Observatory, Chile; in part on data obtained with the ESO Very Large Telescope under program 281. D5017; and in part on observations obtained with the SMARTS Consortium 1.3 and 1.5 m telescopes located at Cerro Tololo Inter-American Observatory, Chile.- Publication:
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The Astrophysical Journal
- Pub Date:
- April 2009
- DOI:
- 10.1088/0004-637X/695/2/L154
- arXiv:
- arXiv:0901.0198
- Bibcode:
- 2009ApJ...695L.154B
- Keywords:
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- galaxies: individual: NGC 300;
- galaxies: stellar content;
- stars: individual: NGC 300 OT;
- stars: variables: other;
- stars: winds;
- outflows;
- supernovae: general;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Cosmology and Extragalactic Astrophysics;
- Astrophysics - Galaxy Astrophysics
- E-Print:
- Revised version, accepted by ApJ Letters. 7 pages, 4 figures