* Astronomy

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

 We report on our discovery and multi-wavelength follow-up observations of the Pan-STARRS1 supernova (SN) PS1-12sk, a transient with uncommon circumstellar 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 are dominated by intermediate-width He I emission at z = 0.054, reminiscent of the spectra of the archetypal Type Ibn SN 2006jc. Dense Pan-STARRS1 multi-band photometry provides the best constraints to date for the rise time of a SN Ibn and a peak magnitude of M_z = -18.9 mag. SN Ibn spectroscopic properties are commonly interpreted as the signature of a massive star (17 - 100 solar masses) explosion within a He-enriched circumstellar medium. However, unlike previous Type Ibn supernovae, PS1-12sk is associated with a galaxy cluster. The elliptical brightest cluster galaxy is the most likely host galaxy. We find no evidence for star formation at the explosion site to sensitive limits (< 0.002 solar masses/yr/kpc²). We consider two progenitor system scenarios for PS1-12sk: a massive star explosion or a white dwarf binary system. If PS1-12sk represents the explosions of a massive star then it serves as direct evidence of vigorous star formation within the galaxy cluster, associated with a cooling flow or a tidally-stripped dwarf galaxy. The probability of this is low given the infrequency of core-collapse SNe in red sequence galaxies compounded by the low volumetric rate of SN Ibn. Progenitor models involving a white dwarf explosion and He-rich environment are challenged by the high mass loss rate of material ejected 2-5 years prior to the event. PS1-12sk represents either a statistically unlikely discovery, evidence for a top-heavy IMF in galaxy cluster cooling flows, or the first clue suggesting an alternate progenitor channel for Type Ibn SNe.

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