SN 2017hcc and SN 2023usc a comparative spectroscopic study of type IIn supernovae

We report on a spectroscopic study of the bright, nearby type-IIn supernova SN 2017hcc, and SN 2023usc using data obtained from the Himalayan Chandra Telescope (HCT). SN 2017hcc is well-studied event, and our sampling covers 7 epochs, starting from +14,d post explosion, and continuing into the nebular stage, at +411,d. The type-IIn event SN 2023usc was sampled over 5 epochs from +12,d to +155,d post explosion. The nearly featureless (except H$\alpha$) late time (+62,d onward) spectra of SN 2023usc, suggests a novel explosion route for this type-IIn event. Assuming a CSM model created by multi-epoch 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 ($v\ll 1000$,km,s${}^{-1}$) emerge in the early ($\sim$ +10,d) spectra only in 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 $\sim 1000$,km,s${}^{-1}$ even in the very early epochs. CSM line velocity being indicative of its extent and opacity, this suggests that progenitors with a highly extended CSMs, which are also optically transparent in their outer regions may be relatively rare.