SN 2017hcc and SN 2023usc—A Comparative Spectroscopic Study of Type IIn Supernovae

Abstract We report on a spectroscopic study of the bright, nearby type IIn supernovae SN 2017hcc and SN 2023usc using data obtained from the Himalayan Chandra Telescope. SN 2017hcc is a well-studied event, and our sampling covers seven epochs, starting from +14 days post-explosion, and continuing into the nebular stage, at +411 days. The type IIn event SN 2023usc was sampled over five epochs from +12 to +155 days post-explosion. The nearly featureless (except H α ) late-time (+62 days onward) spectra of SN 2023usc suggests a novel explosion route for this type IIn event. Assuming a circumstellar medium (CSM) model created by multiepoch ejection of material from the pre-explosion progenitor, we present here a comparative study of both events with several other type IIn / interacting supernovae in progenitors with persistent signatures of a CSM. We find that true narrow lines ( v ≪ 1000 km s −1 ) emerge in the early (∼+10 days) spectra only in a few events (SN 2017hcc, SN 2023usc and SN 2010jl) initially classified as type IIn in our sample. In most cases, the line velocity hovers at ∼1000 km s −1 even in the very early epochs. CSM line velocity being indicative of its extent and opacity, this suggests that progenitors with highly extended CSMs, which are also optically transparent in their outer regions, may be relatively rare.