Type Ibn Supernovae May not all Come from Massive Stars
  Because core-collapse supernovae are the explosions of massive stars, which
have relatively short lifetimes, they occur almost exclusively in galaxies with
active star formation. On the other hand, the Type Ibn supernova PS1-12sk
exploded in an environment much more typical of thermonuclear (Type Ia)
supernovae: on the outskirts of the brightest elliptical galaxy in a galaxy
cluster. The lack of any obvious star formation at that location presented a
challenge to models of Type Ibn supernovae as the explosions of very massive
Wolf-Rayet stars. Here we present a supplementary search for star formation at
the site of PS1-12sk, now that the supernova has faded, via deep ultraviolet
imaging of the host cluster with the Hubble Space Telescope. We do not detect
any ultraviolet emission within 1 kpc of the supernova location, which allows
us deepen the limit on star formation rate by an order of magnitude compared to
the original study on this event. In light of this new limit, we discuss
whether the progenitors of Type Ibn supernovae can be massive stars and what
reasonable alternatives have been proposed.
