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 Mz = –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 (ΣHα 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.
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