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 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 ∼1800 km s −1 at 50 days past explosion. Using the expanding photosphere method, we derive a distance of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>17.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Mpc. From the tail of the light curve we obtain a radioactive nickel mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:msup> <mml:mrow/> <mml:mn>56</mml:mn> </mml:msup> <mml:mi>Ni</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 0.014 ± 0.001 M ⊙ . The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity H α 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 Å, 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.