The canonical picture of a supernova impostor is a short (~months) –11 MV –14 optical transient from a massive (M * 40 M ☉) 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 SN?1954J, SN?1997bs, SN?1999bw, SN?2000ch, SN?2001ac, SN?2002bu, SN?2002kg, and SN?2003gm, as well as the potential archetype η Carinae. SN?1999bw, SN?2000ch, SN?2001ac, SN?2002bu, and SN?2003gm all show mid-IR emission indicative of dust, and the luminosities of SN?1999bw, SN?2001ac, SN?2002bu, and SN?2003gm are dominated by dust emission. We find only upper limits on dust emission from SN?1954J, SN?1997bs, and SN?2002kg. The properties of these sources are, however, broadly inconsistent with the predictions of the canonical model. Based on their mid-IR properties, there are at least three classes of objects being labeled as "impostors." The first class, containing the luminous blue variable (LBV) SN?2002kg and the non-LBV SN?2000ch, consists of variable stars with little or no dust formation as a consequence of the transient. The second class contains the "classical" impostors SN?1954J, SN?1997bs, and (maybe) SN?2003gm that may be weaker analogs of η Carinae. However, if these sources are stellar eruptions, the visual transient is simply a signal that the star is entering a phase with high mass-loss rates and dust formation that must last far longer than the observed optical transient. The third class consists of the SN?2008S-like transients, SN?1999bw, SN?2001ac, SN?2002bu, and (maybe) SN?2003gm, which are obscured by dust re-forming in a pre-existing wind after it was destroyed by an explosive transient. For all three classes of source, there are no cases where a detected source at late times is significantly fainter than the progenitor star. They are also all transients of relatively low-mass (M * 25 M ☉) stars, rather than high-mass (M * 40 M ☉) stars radiating near the Eddington limit like η Carinae. The durations and energetics of these transients cannot be properly characterized without near/mid-IR observations, and the fragmentary nature of the available data leads to considerable uncertainties in our understanding of the individual sources. Continued monitoring of the sources at both optical and near/mid-IR wavelengths should resolve these ambiguities.
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