Draft version March 7, 2018 A Preprint typeset using LTEX style emulateapj v. 5/2/11 PS1-12SK IS A PECULIAR SUPERNOVA FROM A HE-RICH PROGENITOR SYSTEM IN A BRIGHTEST CLUSTER GALAXY ENVIRONMENT N. E. Sanders,1 A. M. Soderberg,1 R. J. Foley,1 R. Chornock,1 D. Milisavljevic,1 R. Margutti,1 M. R. Drout, 1 M. Moe,1 E. Berger,1 W. R. Brown,1 R. Lunnan,1 S. J. Smartt,2 M. Fraser,2 R. Kotak,2 L. Magill,2 K. W. Smith,2 D. Wright,2 K. Huang,3 Y. Urata,4 J. S.Mulchaey,5 A. Rest,6 D. J. Sand,7 L. Chomiuk,8 A. S. Friedman,1,9 R. P. Kirshner,1 G. H. Marion,1 J. L. Tonry, 10 W. S. Burgett,10 K. C. Chambers,10 K. W. Hodapp,10 R. P. Kudritzki,10 P. A. Price11 Draft version March 7, 2018 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 × 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 SED 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 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 −3 −1 −2 no evidence of star formation at the explosion site to sensitive limits (ΣHα . 2×10 M⊙ yr kpc ). 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. Subject headings: Surveys:Pan-STARRS1 — supernovae: individual (PS1-12sk) 1. INTRODUCTION SN 2006jc (Pastorello et al. 2007), while other exam- Traditionally, hydrogen-poor supernovae (Type I SNe) ples identified in the literature are limited to 2000er have been classified into three sub-classes based on the (Pastorello et al. 2008a), 2002ao (Foley et al. 2007), presence of Si (Type Ia), He (Type Ib), or the ab- 2011hw (Smith et al. 2012), and perhaps SN 2005la sence of both features (Type Ic) in their optical spec- (Pastorello et al. 2008b). tra (see Filippenko 1997 for a review). Since the dis- Several lines of evidence point to a massive star (∼ 17 − 100 M⊙) origin for SNe Ibn. First, the covery of SN 1999cq (Matheson et al. 2000), a new sub- I class of “Type Ibn” SNe have emerged, characterized by He emission is representative of a dense circumstel- − lar medium (CSM), suggesting a progenitor with an He- intermediate-width (FWHM ∼ 3 × 103 km s 1) He I rich envelope and high mass loss rate, such as a Wolf emission. The most well studied of these SNe Ibn is Rayet star (Foley et al. 2007; Pastorello et al. 2008a; [email protected] Smith et al. 2008; Tominaga et al. 2008). Second, a 1 arXiv:1303.1818v2 [astro-ph.CO] 4 Apr 2013 Harvard-Smithsonian Center for Astrophysics, 60 Garden Luminous Blue Variable (LBV)-like eruption was ob- Street, Cambridge, MA 02138 USA 2 Astrophysics Research Centre, School of Maths and served at the location of SN 2006jc ∼ 2 yr before Physics,Queens University, BT7 1NN, Belfast, UK the SN explosion (Pastorello et al. 2007). Third, late- 3 Academia Sinica Institute of Astronomy and Astrophysics, time (∼ 2 months) IR and spectroscopic observations of Taipei 106, Taiwan SN2006jc suggest hot carbon dust formation in the SN 4 Institute of Astronomy, National Central University, Chung- Li 32054, Taiwan ejecta, with total Mej ∼ 5 M⊙ (Di Carlo et al. 2008; 5 Carnegie Observatories, 813 Santa Barbara Street, Mattila et al. 2008; Nozawa et al. 2008; Smith et al. Pasadena, CA 91101, USA 6 2008; Tominaga et al. 2008; Sakon et al. 2009). Fourth, Space Telescope Science Institute, 3700 San Martin Dr., all past SNe Ibn have been found in star-forming Baltimore, MD 21218, USA 7 Texas Tech University, Physics Department, Box 41051, galaxies, consistent with a massive star progenitor. Lubbock, TX 79409-1051 Intermediate-width H emission has been detected in 8 Department of Physics and Astronomy, Michigan State the spectra of some SNe Ibn, with strengths signifi- University, East Lansing, Michigan 48824, USA 9 cantly weaker than the He I lines (Pastorello et al. 2008a; Massachusetts Institute of Technology, 77 Massachusetts Ave., Bldg. E51-173, Cambridge, MA 02138, USA Smith et al. 2008, 2012) . Intermediate-width Hα emis- 10 Institute for Astronomy, University of Hawaii, 2680 Wood- sion suggests a connection between SNe Ibn and IIn – lawn Drive, Honolulu HI 96822 a class whose spectra are dominated by intermediate- 11 Department of Astrophysical Sciences, Princeton Univer- sity, Princeton, NJ 08544, USA width H spectral features and are in some cases as- 2 Sanders et al. sociated with LBV-like progenitors (Pastorello et al. optical design is described in Hodapp et al. (2004), the 2008b; Gal-Yam et al. 2007; Gal-Yam & Leonard 2009; imager is described in Tonry & Onaka (2009), and the Smith et al. 2011; Kochanek et al. 2011; Mauerhan et al. survey design and execution strategy are described in 2012). The close temporal connection between these Chambers (in preparation). The standard reduction, LBV-like events and the SN explosions suggests a mas- astrometric solution, and stacking of the nightly images sive star progenitor, but also challenges models for mas- is done by the Pan-STARRS1 IPP system (Magnier sive star evolution that predict stars should spend the 2006; Magnier et al. 2008). The nightly Medium Deep final ∼ 1 Myr of their lives in a core-He burning Wolf- stacks are transferred to the Harvard FAS Research Rayet phase (Heger et al. 2003; Smith et al. 2012). Computing cluster, where they are processed through Here we present observations of a newly discovered a frame subtraction analysis using the photpipe image SN Ibn found in a host environment with no direct differencing pipeline developed for the SuperMACHO evidence of a young stellar population. The optical and ESSENCE surveys (Rest et al. 2005; Garg et al. transient PS1-12sk was discovered on 2012 March 11 2007; Miknaitis et al. 2007). A summary of details by the Panoramic Survey Telescope & Rapid Response of PS1 operations relevant to SN studies is given in System 1 survey (Pan-STARRS1, abbreviated PS1, Chomiuk et al. (2011), and additional PS1 SN studies Kaiser et al. 2002) at zP1 = 18.66 ± 0.01 mag at position were presented by Gezari et al. (2010); Botticella et al. 08h44m54.86s +42◦58´16.89˝ (J2000), within the galaxy (2010); Chomiuk et al. (2011); Narayan et al. cluster RXC J0844.9+4258. The object was spectroscop- (2011); Berger et al. (2012); Sanders et al. (2012b); ically classified as the first Type Ibn SN discovered by Valenti et al. (2012); Chornock et al. (2013); Pan-STARRS1 after just ∼ 2 years of survey operation. Lunnan et al. (2013). At z = 0.054, PS1-12sk is more distant than any previ- The PS1 observations are obtained through a set of 12 ously discovered SN Ibn. five broadband filters, which we have designated as gP1, We describe our multi-wavelength (radio through X- rP1, iP1, zP1, and yP1(Stubbs et al. 2010). Although the ray) observations of PS1-12sk in Section 2. In Section 3, filter system for PS1 has much in common with that used we discuss the observed properties of PS1-12sk and com- in previous surveys, such as the Sloan Digital Sky Sur- pare to past SNe Ibn. Our multi-wavelength monitoring vey (SDSS: York et al. 2000; Aihara et al. 2011), there of PS1-12sk provides the most detailed information to are important differences. The gP1 filter extends 200 A˚ date on the rise phase and NUV-NIR SED of a Type Ibn redward of gSDSS, and the zP1 filter is cut off at 9200 A.˚ SN. Deep stacks of pre-explosion PS1 imaging and optical SDSS has no corresponding yP1 filter. Further informa- spectroscopy allow us to characterize the host environ- tion on the passband shapes is described in Stubbs et al. ment of PS1-12sk in depth (Section 4), pointing to the (2010). Photometry is in the “natural” PS1 system, massive elliptical brightest cluster galaxy CGCG 208-042 m = 2.5 log(flux) + m′, with a single zero-point adjust- as the most likely host galaxy and placing strong limits ment m′ made in each band to conform to the AB magni- on star formation levels at the explosion site. We infer tude scale (Tonry et al. 2012). We assume a systematic characteristics of the progenitor system from our obser- uncertainty of 1% for our PS1 observations due to the vations of PS1-12sk in Section 5.