Circumstellar Interaction Signatures in the Low-luminosity Type II SN 2021gmj

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 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 an Fe ii velocity of similar to 1800 km 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 (MNi)-Ni-56 = 0.014 +/- 0.001 M-circle dot. The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity H alpha 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-circle dot in our hydrodynamic-modeling light-curve analysis is also consistent with our spectroscopy. We observe a broad feature near 4600 angstrom, 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.