Circumstellar interaction signatures in the low luminosity type II SN 2021gmj

Abstract

We present comprehensive optical observations of SN~2021gmj, a Type II supernova (SN~II) discovered within a day of explosion by the Distance Less Than 40~Mpc (DLT40) survey. Follow-up observations show that SN~2021gmj is a low-luminosity SN~II (LL~SN~II), with a peak magnitude $M_V=-15.45$ and Fe~II velocity of $\sim 1800\mathrm{km}{\mathrm{s}}^{-1}$ at 50 days past explosion. Using the expanding photosphere method, we derive a distance of $17.8_{-0.4}^{+0.6}$~Mpc. From the tail of the light curve we obtain a radioactive nickel mass of $0.014\pm 0.001$ M${}_{\odot }$. The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity H$\hat{I}\pm$ in absorption. Analytical shock-cooling models of the light curve cannot reproduce the fast rise, supporting the idea that the early-time emission is partially powered by the interaction of the SN ejecta and CSM. The inferred low CSM mass of 0.025 M${}_{\odot }$ in our hydrodynamic-modeling light curve analysis is also consistent with our spectroscopy. We observe a broad feature near 4600 Ã , which may be high-ionization lines of C, N, or/and He~II. This feature is reproduced by radiation-hydrodynamic simulations of red supergiants with extended atmospheres. Several LL~SNe~II show similar spectral features, implying that high-density material around the progenitor may be common among them.

Accepted version at ApJ