The Type Icn SN 2021csp: Implications for the Progenitors of Fast/Luminous Transients and the Fates of Wolf-Rayet Stars

We present observations of SN 2021csp, the second example of a newly-identified type of supernova (Type Icn) hallmarked by strong, narrow, P-Cygni carbon features. The SN appears as a fast and luminous blue transient at early times early in its evolution, reaching a peak absolute magnitude of -20 within 3 days due to strong interaction between fast SN ejecta (v ~ 30000 km/s) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow line features disappear from the spectrum 10-20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ~60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic supernovae. An intriguing possibility is that SN 2021csp and other fast-luminous transients are powered by central engines and fueled by fallback accretion following the collapse of the core of a massive star to a black hole. Events of this nature may be common, but are detectable only in rare circumstances when the progenitor star is surrounded by dense circumstellar matter at the time of explosion.