SN 2021dbg: A Luminous Type IIP-IIL Supernova Exploding from a Massive Star with a Layered Shell

TL;DR

SN 2021dbg is a luminous Type IIP-IIL supernova from a massive red supergiant with a layered shell, showing signs of circumstellar interaction and significant nickel production, bridging features of both supernova subclasses.

Contribution

This study provides detailed observations and analysis of SN 2021dbg, revealing its layered shell structure and transitional nature between SNe IIP and IIL, with implications for progenitor mass-loss history.

Findings

High luminosity sustained for 385 days due to CSM interaction and nickel decay.

Progenitor was a ~20 M_sun red supergiant with a thick hydrogen shell.

Estimated CSM mass near progenitor was (1.0--2.0) x 10^-3 M_sun.

Abstract

We present extensive observations and analysis of supernova (SN) 2021dbg, utilizing optical photometry and spectroscopy. For approximately 385 days following the explosion, SN 2021dbg exhibited remarkable luminosity, surpassing most SNe II. This initial high luminosity is potentially attributed to the interaction between the ejected material and the surrounding circumstellar material (CSM), as evidenced by the pronounced interaction signatures observed in its spectra. The subsequent high luminosity is primarily due to the significant $^{56}$Ni ($0.17 \pm 0.05$ M$_{\odot}$) produced in the explosion. Based on the flux of flash emission lines detected in the initial spectra, we estimate that the CSM mass near the progenitor amounted to $\sim$(1.0--2.0) $\times 10^{-3}$ M$_{\odot}$, likely resulting from intense stellar wind activity 2--3 yr preceding the explosion. Considering the bolometric light curve, nebular spectrum modeling, and mass-loss rate, we suggest that the progenitor of SN 2021dbg was a red supergiant (RSG) with a mass of $\sim 20$ M$_{\odot}$ and a radius of 1200 R$_{\odot}$. This RSG featured a thick hydrogen shell, which may have contained a region with a sharp decrease in material density, electron density, and temperature, contributing to its layered structure. This object demonstrates mixed features of SNe IIP and SNe IIL, making it as a transitional event linking the above two subclasses of SNe II.