Sn 2010mb: Direct Evidence for a Supernova Interacting With a Large Amount of Hydrogen-Free Circumstellar Material

We present our observations of SN 2010mb, a Type Ic supernova (SN) lacking spectroscopic signatures of H and He. SN 2010mb has a slowly declining light curve (LC) (∼600 days) that cannot be powered by 56 Ni/ 56 Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free circumstellar material including a blue quasi-continuum and, uniquely, narrow oxygen emission lines that require high densities (∼10 9 cm −3 ). From the observed spectra and LC, we estimate that the amount of material involved in the interaction was ∼3 M ☉ . Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.