Slow-speed supernovae from the Palomar transient factory : two channels

White, Christopher J. and Kasliwal, Mansi M. and Nugent, Peter E. and Gal-Yam, Avishay and Howell, D. Andrew and Sullivan, Mark and Goobar, Ariel and Piro, Anthony L. and Bloom, Joshua S. and Kulkarni, Shrinivas R. and Laher, Russ R. and Masci, Frank and Ofek, Eran O. and Surace, Jason and Ben-Ami, Sagi and Cao, Yi and Cenko, S. Bradley and Hook, Isobel M. and Jonsson, Jakob and Matheson, Thomas and Sternberg, Assaf and Quimby, Robert M. and Yaron, Ofer (2015) Slow-speed supernovae from the Palomar transient factory : two channels. The Astrophysical Journal, 799 (1): 52. ISSN 0004-637X

Abstract

Since the discovery of the unusual prototype SN 2002cx, the eponymous class of Type I (hydrogen-poor) supernovae with low ejecta speeds has grown to include approximately two dozen members identified from several heterogeneous surveys, in some cases ambiguously. Here we present the results of a systematic study of 1077 Type I supernovae discovered by the Palomar Transient Factory, leading to nine new members of this peculiar class. Moreover, we find there are two distinct subclasses based on their spectroscopic, photometric, and host galaxy properties: "SN 2002cx-like" supernovae tend to be in later-type or more irregular hosts, have more varied and generally dimmer luminosities, have longer rise times, and lack a Ti II trough when compared to "SN 2002es-like" supernovae. None of our objects show helium, and we counter a previous claim of two such events. We also find that the occurrence rate of these transients relative to Type Ia supernovae is 5.6(-3.8)(+22) % (90% confidence), lower compared to earlier estimates. Combining our objects with the literature sample, we propose that these subclasses have two distinct physical origins.