Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gm

Photometric and spectroscopic observations of the faint Supernovae (SNe) 2002kg and 2003gm, and their precursors, in NGC 2403 and NGC 5334, respectively, are presented. The properties of these SNe are discussed in the context of previously proposed scenarios for faint SNe: low-mass progenitors producing underenergetic SNe; SNe with ejecta constrained by a circumstellar medium; and outbursts of massive Luminous Blue Variables (LBVs). The last scenario has been referred to as `Type V SNe', `SN impostors' or `fake SNe'.

The faint SN 2002kg reached a maximum brightness of M_V = -9.6, much fainter than normal Type II SNe. The precursor of SN 2002kg is confirmed to be, as shown in previous work, the LBV NGC 2403-V37. Late-time photometry of SN 2002kg shows it to be only 0.6 mag fainter at 500 d than at the epoch of discovery. Two spectra of SN 2002kg, with an approximately 1-yr interval between observations, show only minor differences. Strong FeII lines are observed in the spectra of SN 2002kg, similar to both the LBV NGC 2363-V1 and the Type IIn SN 1995G. The spectrum of SN 2002kg does show strong resolved [NII] at λλ6549,6583 Å. The identified progenitor of SN 2003gm is a bright yellow star, consistent with a F5-G2 supergiant, similar to the identified progenitor of SN 2004et. SN 2003gm, at the epoch of discovery, was of similar brightness to the possible fake SN 1997bs and the Type IIP SNe 1999br and 2005cs. Photometrically SN 2003gm shows the same decrease in brightness, over the same time period as SN 1997bs. The light curve and the spectral properties of SN 2003gm are also consistent with some intrinsically faint and low-velocity Type II SNe. The early-time spectra of SN 2003gm are dominated by Balmer emission lines, which at the observed resolution, appear similar to SN 2000ch. On the basis of the post-discovery photometric and spectroscopic observations presented here, we suggest that SN 2003gm is a similar event to SN 1997bs, although the SN/LBV nature of both of these objects is debated. At 226 d post-discovery the spectrum of SN 2003gm is strongle contaminated by HII region emission lines, and it cannot be confirmed that the precursor star has disappeared. The presence of strong [NII] lines, near Hα, is suggested as a possible means of identifying objects such as SN 2002kg/NGC 2403-V37 as being LBVs - although not as a general classification criterion of all LBVs masquerading as SNe.