tokens: SNe roughly match the required luminosity and timescale, but we would not expect their progenitors to explode two years later as SNe Ia (Bildsten et al. 2007 ). The fast-declining light curves of SNe Ibn also indicate that very little 56 Ni is produced in the explosion (Foley et al. 2007 ; Pastorello et al. 2008a ), which may challenge white dwarf explosion models. One way of avoiding these issues is to allow PS1-12sk and SN 2006jc to come from different types of stars. SNe Ibn would then be a mixed class, with some low- and some high-mass progenitors. This could potentially explain some of the spectral diversity we see among SNe Ibn (Hosseinzadeh et al. 2017b ). The five spectra that we have of PS1-12sk look almost identical to spectra of SN 2006jc, 12 12 SN 2006jc was discovered after peak, so the phases of these spectra are not well constrained. but perhaps any explosion into helium-rich CSM would look the same. This situation would be analogous to the discovery that some events initially classified as SNe IIn are actually the explosions of white dwarfs with hydrogen-rich CSM (now called SNe Ia-CSM; Dilday et al. 2012 ; Silverman et al. 2013 ). In fact, this may be the reason that events classified as SNe IIn are less correlated with star formation than other classes of core-collapse SNe (Anderson et al. 2012 ; Habergham et al. 2014 ), despite some having luminous progenitor detections. 5. Summary and Conclusions We have presented a very deep limit on the star formation rate at the site of the SN Ibn PS1-12sk, nominally ruling out a massive star origin. We present three alternative proposals for how this SN could have occurred in such a unique environment. 1. The progenitor of PS1-12sk was a massive star ejected or tidally stripped from a nearby ultra-compact dwarf galaxy. 2. The progenitor of PS1-12sk was a rare massive star in a region with a very low star formation rate. 3. The progenitor of PS1-12sk was a low-mass star. Our result only deepens the mystery surrounding PS1-12sk, in that none of these possibilities is fully satisfactory in explaining the origins of SNe Ibn. Given that there is not much more data to collect on this particular event, the resolution will likely come from a statistical study of SN Ibn host galaxies. Although this is currently the only SN Ibn discovered in a non-star-forming environment, we still suffer from a small sample size of only tens of events. Current and future large-scale surveys will be much better suited to find these rare, fast-evolving transients, allowing us to better understand the statistical properties of their host environments. Appendix A Extinction Sanders et al. ( 2013 , p. 4) claimed that “the SED of [PS1-12sk] is not consistent with significant reddening given reasonable assumptions about the photospheric temperature” and neglect extinction throughout their analysis. However, as an additional check, we calculate an upper limit on the host galaxy extinction based on the nondetection of
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