The Photometric and Spectral Evolution of the 2008 Luminous Optical Transient in NGC 300
Walter, Frederick M.; Bond, Howard E.; Humphreys, Roberta M.; Davidson, Kris; Bonanos, Alceste Z.; Bedin, Luigi R.; Prieto, José L.; Berto Monard, L. A. G.
United States, Italy, Greece, South Africa
Abstract
The 2008 optical transient in NGC 300 is one of a growing class of intermediate-luminosity transients that brighten several orders of magnitude from a previously optically obscured state. The origin of their eruptions is not understood. Our multi-wavelength photometry and spectroscopy from maximum light to more than a year later provide a record of its post-eruption behavior. We describe its changing spectral energy distribution, the evolution of its absorption- and emission-line spectrum, the development of a bipolar outflow, and the rapid transition from a dense wind to an optically thin ionized wind. In addition to strong, narrow hydrogen lines, the F-type absorption-line spectrum of the transient is characterized by strong Ca II and [Ca II] emission. The very broad wings of the Ca II triplet and the asymmetric [Ca II] emission lines are due to strong Thomson scattering in the expanding ejecta. Post-maximum, the hydrogen and Ca II lines developed double-peaked emission profiles that we attribute to a bipolar outflow. Between approximately 60 and 100 days after maximum, the F-type absorption spectrum, formed in its dense wind, weakened and the wind became transparent to ionizing radiation. We discuss the probable evolutionary state of the transient and similar objects such as SN 2008S and conclude that they were most likely post-red supergiants or post-asymptotic giant branch stars on a blue loop to warmer temperatures when the eruption occurred. These objects are not luminous blue variables.
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 observations obtained with the SMARTS Consortium 1.3 and 1.5 m telescopes located at Cerro Tololo Inter-American Observatory, Chile, and in part on observations with VLT/UT1/ISAAC under ESO DDT proposal: 283.D-5019.