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

TL;DR

This study compares the spectroscopic evolution of two nearby type-IIn supernovae, SN 2017hcc and SN 2023usc, revealing insights into their circumstellar environments and explosion mechanisms.

Contribution

It provides the first detailed spectroscopic comparison of SN 2017hcc and SN 2023usc, highlighting differences in early narrow line emergence and suggesting rare progenitor CSM configurations.

Findings

Early narrow lines appear only in a few type-IIn SNe.

SN 2023usc shows featureless late-time spectra, indicating a novel explosion route.

Progenitors with highly extended, transparent CSMs are rare.

Abstract

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.