SN 2012au: A GOLDEN LINK BETWEEN SUPERLUMINOUS SUPERNOVAE AND THEIR LOWER-LUMINOSITY COUNTERPARTS

We present optical and near-infrared observations of SN?2012au, a slow-evolving supernova (SN) with properties that suggest a link between subsets of energetic and H-poor SNe and superluminous SNe. SN?2012au exhibited conspicuous Type-Ib-like He I lines and other absorption features at velocities reaching 2 ? 104?km?s?1 in its early spectra, and a broad light curve that peaked at MB = ?18.1?mag. Models of these data indicate a large explosion kinetic energy of ~1052?erg and 56Ni mass ejection of M Ni 0.3 M ? on par with SN?1998bw. SN?2012au's spectra almost one year after explosion show a blend of persistent Fe II P-Cyg absorptions and nebular emissions originating from two distinct velocity regions. These late-time emissions include strong [Fe II], [Ca II], [O I], Mg I], and Na I lines at velocities 4500?km?s?1, as well as O I and Mg I lines at noticeably smaller velocities 2000?km?s?1. Many of the late-time properties of SN?2012au are similar to the slow-evolving hypernovae SN?1997dq and SN?1997ef, and the superluminous SN?2007bi. Our observations suggest that a single explosion mechanism may unify all of these events that span ?21 MB ?17 mag. The aspherical and possibly jetted explosion was most likely initiated by the core collapse of a massive progenitor star and created substantial high-density, low-velocity Ni-rich material.