SN 2021csp -- the explosion of a stripped envelope star within a H and He-poor circumstellar medium

Abstract

We present observations of SN 2021csp, a unique supernova (SN) which displays evidence for interaction with H- and He- poor circumstellar material (CSM) at early times. Using high-cadence spectroscopy taken over the first week after explosion, we show that the spectra of SN 2021csp are dominated by C III lines with a velocity of 1800 km s${}^{-1}$. We associate this emission with CSM lost by the progenitor prior to explosion. Subsequently, the SN displays narrow He lines before metamorphosing into a broad-lined Type Ic SN. We model the bolometric light curve of SN 2021csp, and show that it is consistent with the energetic ($4\times 10^{51}$ erg) explosion of a stripped star, producing 0.4 M${}_{\odot }$ of 56Ni within a $\sim$1 M${}_{\odot }$ shell of CSM extending out to 400 R${}_{\odot }$.

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