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 M_B = -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 ≲ M_B ≲ -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.