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 (SN Ibn; cf. SN 2006jc), dominated by intermediate-width (3 × 10³ km s^-1) and time variable He I emission. Our multi-wavelength monitoring establishes the rise time dt ∼ 9-23 days and shows an NUV-NIR spectral energy distribution with temperature ≳ 17 × 10³ K and a peak magnitude of M_z = -18.88 ± 0.02 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17-100 M_⊙) explosion within an He-enriched circumstellar medium. However, unlike previous SNe Ibn, 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 (Σ_Hα ≲ 2 × 10^-3 M_⊙ yr^-1 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 initial mass function in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for SNe Ibn.