SN 2018hfm : A Low-Energy Type II Supernova with Prominent Signatures of Circumstellar Interaction and Dust Formation

TL;DR

SN 2018hfm is a low-energy Type II supernova exhibiting strong signs of circumstellar interaction and ongoing dust formation, with unique light curve features and spectral signatures indicating a small ejecta mass and low explosion energy.

Contribution

This study provides detailed photometric and spectroscopic analysis of SN 2018hfm, highlighting its low energy, small ejecta mass, and evidence of dust formation, which are novel insights for low-energy Type II supernovae.

Findings

Low ejecta mass (~1.3 M) and explosion energy (~10^50 erg)

Significant circumstellar interaction evidenced by box-like emission profiles

Continuous dust formation observed from +66.7 to +389.4 days after explosion

Abstract

We present multiband optical photometric and spectroscopic observations of an unusual Type II supernova, SN 2018hfm, which exploded in the nearby (d = 34.67 Mpc) dwarf galaxy PGC 1297331 with a very low star-formation rate (0.0270 M yr-1) and a subsolar metallicity environment(~ 0.5 Z). The V-band light curve of SN 2018hfm reaches a peak with value of -18.69+/-0.64 mag, followed by a fast decline(4.42+/-0.13 mag (100d)-1). After about 50 days, it is found to experience a large flux drop (~3.0 mag in V), and then enters into an unusually faint tail, which indicates a relatively small amount of 56Ni synthesized during the explosion. From the bolometric light curve, SN 2018hfm is estimated to have low ejecta mass (~1.3M) and low explosion energy(~10 50 erg) compared with typical SNe II. The photospheric spectra of SN 2018hfm are similar to those of other SNe II, with P Cygni profiles of the Balmer series and metal lines, while at late phases the spectra are characterised by box-like profiles of HU emission, suggesting significant interaction between the SN ejecta and circumstellar matter. These box-like emission features are found to show increasing asymmetry with time, with the red-side component becoming gradually weaker, indicating that dust is continuously formed in the ejecta. Based on the dust-estimation tool damocles, we find that the dust increases from ~10 -6M to 10 -4 - 10 -3 M between +66.7 d and +389.4 d after explosion.