PS1-12sk IS A PECULIAR SUPERNOVA FROM A He-RICH PROGENITOR SYSTEM IN A BRIGHTEST CLUSTER GALAXY ENVIRONMENT

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 (c.f. SN 2006jc), dominated by intermediate-width (3 × 10 3 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 SED with temperature 17 × 10 3 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 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 (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 IMF in galaxy cluster cooling flow filaments, or the first clue suggesting an alternate progenitor channel for Type Ibn SNe.

iPTF13beo: the double-peaked light curve of a Type Ibn supernova discovered shortly after explosion

“…Type Ibn SNe are characterized by the presence of narrow or intermediate-width (∼2000-3000 km s −1 ) He I emission lines, which are attributed to the SN ejecta interacting with He-rich CSM (Pastorello et al 2008a). Several lines of evidence suggest that Type Ibn SN progenitors are massive stars characterized by high massloss rates and stripped He-rich envelopes, such as WolfRayet stars (Foley et al 2007;Pastorello et al 2007Pastorello et al , 2008a; Sanders et al 2013). A luminous outburst coincident with the location of Type Ibn SN 2006jc, occurring two years before the SN explosion, was reported by Nakano et al (2006).…”

Massive stars exploding in a He-rich circumstellar medium – V. Observations of the slow-evolving SN Ibn OGLE-2012-SN-006

“…We note, however, that a degenerate progenitor system scenario had been previously proposed by Sanders et al (2013) for another Type Ibn SN, PS1-12sk. The unusual location of that object in the outskirts of an elliptical galaxy, suggested a progenitor scenario which could have been different from the canonical core-collapse of a massive, strippedenvelope star normally assumed for SNe Ibn, including the possibility of a white dwarf explosion in a He-rich environment (Sanders et al 2013).…”

“…The unusual location of that object in the outskirts of an elliptical galaxy, suggested a progenitor scenario which could have been different from the canonical core-collapse of a massive, strippedenvelope star normally assumed for SNe Ibn, including the possibility of a white dwarf explosion in a He-rich environment (Sanders et al 2013). However, to our knowledge, this still remains a unique case in the Type Ibn SN zoo, since all other objects exploded in spiral galaxies (Pastorello et al 2015b).…”

“…However, to our knowledge, this still remains a unique case in the Type Ibn SN zoo, since all other objects exploded in spiral galaxies (Pastorello et al 2015b). In addition, as admitted by Sanders et al (2013), some residual star formation at the SN location and/or the association with a nearby, low luminosity dwarf galaxy cannot be ruled out, and -consequently-the progenitor may still be a massive star.…”

“…Pastorello et al (2008a), on the basis of a very small sample of objects, proposed that SNe Ibn might form a relatively homogeneous group of mildly interacting strippedenvelope SNe. However, recent discoveries suggest that SNe Ibn span a much wider range of properties than initially believed (Smith et al 2012;Gorbikov et al 2014;Sanders et al 2013;Pastorello et al 2015a,b). In this con-text, because of its slow evolving light curve (see Section 2.1), OGLE-2012-SN-006 can be considered as an unprecedented addition to the Type Ibn SN zoo.…”