PS1-12sk is a Peculiar Supernova from a He-rich Progenitor System in a Brightest Cluster Galaxy Environment
Price, P. A.; Berger, E.; Kirshner, R. P.; Rest, A.; Foley, R. J.; Smartt, S. J.; Kudritzki, R. P.; Huang, K.; Chambers, K. C.; Smith, K. W.; Kotak, R.; Mulchaey, J. S.; Chornock, R.; Lunnan, R.; Fraser, M.; Tonry, J. L.; Burgett, W. S.; Sand, D. J.; Soderberg, A. M.; Hodapp, K. W.; Urata, Y.; Wright, D.; Sanders, N. E.; Milisavljevic, D.; Margutti, R.; Drout, M. R.; Moe, M.; Brown, W. R.; Magill, L.; Chomiuk, L.; Friedman, A. S.; Marion, G. H.
United States, United Kingdom, Taiwan
Abstract
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.