Luminous blue variable eruptions and related transients: Diversity of progenitors and outburst properties

We present new light curves and optical spectra for a number of extragalactic optical transients or 'supernova impostors' related to giant eruptions of luminous blue variables (LBVs), and we provide a comparative discussion of LBV-like giant eruptions known thus far. New data include photometry and spectroscopy of supernovae (SNe) 1999bw, 2000ch, 2001ac, 2002bu, 2006bv and 2010dn. SN 2010dn appears to be a carbon copy of SN 2008S and NGC 300-OT, whereas SN 2002bu shows spectral evolution from a normal LBV at early times to a twin of these cooler transients at late times. SN 2008S, NGC 300-OT and SN 2010dn appear to be special cases of a broader eruptive phenomenon where the progenitor star was enshrouded by dust, perhaps from a previous unseen eruptive episode. Evidence suggests that their progenitors have initial masses in the range 10-20M_⊙, extending the range of masses susceptible to the violent eruptive phenomenon below the canonical LBV mass range. Examining the full sample, SN impostors are characterized by strong photometric variability on a range of time-scales from a day to decades, potentially suffering multiple eruptions of the same source. The upper end of the luminosity distribution overlaps with the least-luminous core-collapse SNe, but in most cases a distinction can be made based on spectra. The low end of the luminosity distribution is far less well defined, and a distinction between LBV giant eruptions, S Doradus phases of LBVs, novae and possible eruptions of intermediate-mass stars is not entirely clear. We discuss observational clues concerning stellar winds or shocks as the relevant mass-loss mechanism, and we evaluate possible ideas for the physical mechanisms of outbursts, but there is still a great need for theoretical work on this problem. Although known examples of these eruptions are sufficient to illustrate their remarkably wide diversity in the peak absolute magnitude, duration, progenitor stars, outburst spectra and other observable properties, their statistical distribution is an area that will benefit greatly from current and upcoming transient surveys. Based on the distribution of these eruptive properties, we propose that the prototypical object SN 1961V was not a member of this class of impostors after all, but was instead a true core-collapse Type IIn SN that was preceded by a giant LBV eruption.