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 SN lacking spectroscopic
signatures of H and He. SN 2010mb has a slowly-declining light curve
($\sim600\,$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 CSM including a blue quasi-continuum and, uniquely, narrow
oxygen emission lines that require high densities ($\sim10^9$cm$^{-3}$). From
the observed spectra and light curve we estimate that the amount of material
involved in the interaction was $\sim3$M$_{\odot}$. Our observations are in
agreement with models of pulsational pair-instability SNe described in the
literature.
