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Unmasking the Supernova Impostors

(ABRIDGED) The canonical picture of a supernova impostor is a -11 < M_V < -14 optical transient from a massive (M > 40Msun) star during which the star ejects a dense shell of material. Dust formed in the ejecta then obscures the star. In this picture, the geometric expansion of the shell leads to clear predictions for the evolution of the optical depths and hence the evolution of the optical through mid-IR emissions. Here we review the theory of this standard model and then examine the impostors SN1954J, SN1997bs, SN1999bw, SN2000ch, SN2001ac, SN2002bu, SN2002kg and SN2003gm, as well as the potential archetype eta Carinae. SN1999bw, SN2000ch, SN2001ac, SN2002bu and SN2003gm all show mid-IR emission indicative of dust, and the luminosities of SN1999bw, SN2001ac, SN2002bu and SN2003gm are dominated by dust emission. The properties of these sources are broadly inconsistent with the predictions of the canonical model. There are probably two classes of sources. In one class (eta Carinae, SN1954J, SN1997bs, and (maybe) SN2003gm), the optical transient is a signal that the star is entering a phase with very high mass loss rates that must last far longer than the visual transient. The second class (SN1999bw, SN2001ac, SN2002bu and (maybe) SN2003gm) has the different physics of SN2008S and the 2008 NGC300 transient, where they are obscured by dust re-forming in a pre-existing wind after it was destroyed by an explosive transient. There are no cases where the source at late times is significantly fainter than the progenitor star. All these dusty transients are occurring in relatively low mass (M < 25Msun) stars rather than high mass (M > 40Msun) stars radiating near the Eddington limit like eta Carinae. The durations and energetics of these transients cannot be properly characterized without near/mid-IR observations.