SN 2021gno: a Calcium-rich transient with double-peaked light curves

TL;DR

SN 2021gno is a Calcium-rich transient with double-peaked light curves, rapid evolution, and spectral features dominated by calcium, modeled as a highly-stripped massive star explosion with circumstellar material.

Contribution

This study provides detailed UV and optical observations of SN 2021gno, revealing its unique double-peaked light curve and modeling its explosion as a highly-stripped star with circumstellar material.

Findings

SN 2021gno exhibits double-peaked light curves among Calcium-rich transients.

The explosion is consistent with a highly-stripped star of 0.8 solar masses ejecta.

Presence of extended circumstellar material explains early cooling phase.

Abstract

We present extensive ultraviolet (UV) and optical photometric and optical spectroscopic follow-up of supernova (SN)~2021gno by the "Precision Observations of Infant Supernova Explosions" (POISE) project, starting less than two days after the explosion. Given its intermediate luminosity, fast photometric evolution, and quick transition to the nebular phase with spectra dominated by [Ca~II] lines, SN~2021gno belongs to the small family of Calcium-rich transients. Moreover, it shows double-peaked light curves, a phenomenon shared with only four other Calcium-rich events. The projected distance from the center of the host galaxy is not as large as other objects in this family. The initial optical light-curve peaks coincide with a very quick decline of the UV flux, indicating a fast initial cooling phase. Through hydrodynamical modelling of the bolometric light curve and line velocity evolution, we found that the observations are compatible with the explosion of a highly-stripped massive star with an ejecta mass of $0.8\,M_\odot$ and a $^{56}$Ni mass of $0.024~M_{\odot}$. The initial cooling phase (first light curve peak) is explained by the presence of an extended circumstellar material comprising $\sim$$10^{-2}\,M_{\odot}$ with an extension of $1100\,R_{\odot}$. We discuss if hydrogen features are present in both maximum-light and nebular spectra, and its implications in terms of the proposed progenitor scenarios for Calcium-rich transients.