PS1-12sk is a Peculiar Supernova From a He-rich Progenitor System in a
  Brightest Cluster Galaxy Environment
  We report on our discovery and observations of the Pan-STARRS1 supernova (SN)
PS1-12sk, a transient with properties that indicate atypical star formation in
its host galaxy cluster or pose a challenge to popular progenitor system models
for this class of explosion. The optical spectra of PS1-12sk classify it as a
Type Ibn SN (c.f. SN 2006jc), dominated by intermediate-width (3x10^3 km/s) and
time variable He I emission. Our multi-wavelength monitoring establishes the
rise time dt = 9-23 days and shows an NUV-NIR SED with temperature > 17x10^3 K
and a peak rise magnitude of Mz = -18.9 mag. SN Ibn spectroscopic properties
are commonly interpreted as the signature of a massive star (17 - 100 M_sun)
explosion within a He-enriched circumstellar medium. However, unlike previous
Type Ibn supernovae, PS1-12sk is associated with an elliptical brightest
cluster galaxy, CGCG 208-042 (z = 0.054) in cluster RXC J0844.9+4258. The
expected probability of an event like PS1-12sk in such environments is low
given the measured infrequency of core-collapse SNe in red sequence galaxies
compounded by the low volumetric rate of SN Ibn. Furthermore, we find no
evidence of star formation at the explosion site to sensitive limits (Sigma
Halpha < 2x10^-3 M_sun/yr/kpc^2). We therefore discuss white dwarf binary
systems as a possible progenitor channel for SNe Ibn. We conclude that PS1-12sk
represents either a fortuitous and statistically unlikely discovery, evidence
for a top-heavy IMF in galaxy cluster cooling flow filaments, or the first clue
suggesting an alternate progenitor channel for Type Ibn SNe.
