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 MV=−15.45 and Fe~II velocity of ∼1800kms−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±0.001 M⊙. The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity Hα 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⊙ in our hydrodynamic-modeling light curve analysis is also consistent with our spectroscopy. We observe a broad feature near 4600 \AA, 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.
@article{arxiv.2401.04027,
title = {Circumstellar interaction signatures in the low luminosity type II SN 2021gmj},
author = {Nicolas Meza-Retamal and Yize Dong and K. Azalee Bostroem and Stefano Valenti and Lluis Galbany and Jeniveve Pearson and Griffin Hosseinzadeh and Jennifer E. Andrews and David J. Sand and Jacob E. Jencson and Daryl Janzen and Michael J. Lundquist and Emily T. Hoang and Samuel Wyatt and Peter J. Brown and D. Andrew Howell and Megan Newsome and Estefania Padilla Gonzalez and Craig Pellegrino and Giacomo Terreran and Vladimir Kouprianov and Daichi Hiramatsu and Saurabh W. Jha and Nathan Smith and Joshua Haislip and Daniel E. Reichart and Manisha Shrestha and F. Fabián Rosales-Ortega},
journal= {arXiv preprint arXiv:2401.04027},
year = {2024}
}