Sethulakshmi Vazhayil

arXiv2026

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$α$) 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.

astro-ph.HE↗

arXiv2026

Optical and near-infrared nebular-phase spectroscopy of SN 2024ggi: constraints on the structure of the inner ejecta, progenitor mass, and dust

We present optical and near-infrared (NIR) spectroscopic observations of the nearby Type II supernova SN\,2024ggi from 250 and 581 days after the explosion. Comparing the evolution of the [\ion{O}{1}] at 6300, 6363 \textÅ doublet normalized to the continuum with spectral models from the literature, we estimate a progenitor star zero-age main-sequence mass ($M_{\mathrm{ZAMS}}$) of $\approx 14$ M$_\odot$. This value is consistent with $M_{\mathrm{ZAMS}}$ reported in the literature from independent methodologies. The nebular spectra are used to study the structure of the inner ejecta. The broad H$α$ line has a full-width at half maximum (FWHM) of $\simeq 3900$ km s$^{-1}$, with small deviations from a symmetric Gaussian profile centred at zero velocity, and the [\ion{O}{1}] doublet is blue-shifted by $\approx -940$ km s$^{-1}$. In the NIR, the nebular spectra reveal double-peaked emission features of \ion{Mg}{1} and [\ion{Fe}{2}] lines between +250 and +319 days, suggesting a bipolar distribution of intermediate mass and iron peak elements in the line-of-sight. Such a double-peaked feature in these NIR lines has not been previously reported. No corresponding asymmetries are observed in the hydrogen lines, suggesting that the asymmetry is mostly confined to intermediate mass and iron peak elements in the innermost core of the supernova ejecta. Additionally, we detect first-overtone carbon monoxide (CO) emission at 2.3,$μ$m between 250 and 319 days, and a blueshift in the emission lines of H$α$, [\ion{O}{1}], \ion{Mg}{1}], and [\ion{Fe}{2}] in the +581 day optical spectrum, consistent with dust formation in the ejecta.

astro-ph.SR↗