Near-Infrared and Optical Observations of Type Ic SN 2021krf: Luminous Late-time Emission and Dust Formation

TL;DR

This study presents detailed optical and near-infrared observations of SN 2021krf, revealing dust formation in the ejecta, and models its progenitor and energy sources, including radioactive decay and a central pulsar engine.

Contribution

It provides the first evidence of dust formation in a Type Ic supernova ejecta and models the progenitor with two possible masses using radiation hydrodynamics.

Findings

Detection of dust formation indicated by NIR and optical spectra.

Progenitor models with C-O star masses of 3.93 and 5.74 M$_{ ext{odot}}$.

Late-time light curves suggest additional energy from a central pulsar.

Abstract

We present near-infrared (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2021krf obtained between days 13 and 259 at several ground-based telescopes. The NIR spectrum at day 68 exhibits a rising $K$-band continuum flux density longward of $\sim$ 2.0 $\mu$m, and a late-time optical spectrum at day 259 shows strong [O I] 6300 and 6364 \r{A} emission-line asymmetry, both indicating the presence of dust, likely formed in the SN ejecta. We estimate a carbon-grain dust mass of $\sim$ 2 $\times$ 10$^{-5}$ M$_{\odot}$ and a dust temperature of $\sim$ 900 - 1200 K associated with this rising continuum and suggest the dust has formed in SN ejecta. Utilizing the one-dimensional multigroup radiation hydrodynamics code STELLA, we present two degenerate progenitor solutions for SN 2021krf, characterized by C-O star masses of 3.93 and 5.74 M$_{\odot}$, but with the same best-fit $^{56}$Ni mass of 0.11 M$_{\odot}$ for early times (0-70 days). At late times (70-300 days), optical light curves of SN 2021krf decline substantially more slowly than that expected from $^{56}$Co radioactive decay. Lack of H and He lines in the late-time SN spectrum suggests the absence of significant interaction of the ejecta with the circumstellar medium. We reproduce the entire bolometric light curve with a combination of radioactive decay and an additional powering source in the form of a central engine of a millisecond pulsar with a magnetic field smaller than that of a typical magnetar.