Low luminosity Type II supernovae -- IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

Abstract

Photometric and spectroscopic data for two Low Luminosity Type IIP Supernovae (LL SNe IIP) are presented. SN 2020cxd reaches a peak absolute magnitude $M_r$ = -13.90 $\pm$ 0.05 mag two days after explosion, subsequently settling on a plateau for $\sim$120 days. Through the luminosity of the late light curve tail, we infer a synthesized ${}^{56}$Ni mass of (1.8$\pm$0.5) $\times$ 10${}^{-3}$ M${}_{\odot }$. During the early evolutionary phases, optical spectra show a blue continuum ($T$ $>$ 8000 K) with broad Balmer lines displaying a P Cygni profile, while at later phases Ca II, Fe II, Sc II and Ba II lines dominate the spectra. Hydrodynamical modelling of the observables yields $R$ $\simeq$ 575 $R_{\odot }$ for the progenitor star, with $M_{ej}$ = 7.5 M${}_{\odot }$ and $E$ $\simeq$ 0.097 foe emitted during the explosion. This low-energy event originating from a low-mass progenitor star is compatible with both the explosion of a red supergiant (RSG) star and with an Electron Capture Supernova arising from a super asymptotic giant branch star. SN 2021aai reaches a maximum luminosity of $M_r$ = -16.4 mag (correcting for $A_V$=1.9 mag), and displays a remarkably long plateau ($\sim$140 days). The estimated ${}^{56}$Ni mass is (1.4$\pm$0.5) $\times$ 10${}^{-2}$ M${}_{\odot }$. The expansion velocities are compatible with those of other LL SNe IIP (few 10${}^3$ km s${}^{-1}$). The physical parameters obtained through hydrodynamical modelling are $R$ $\simeq$ 575 R${}_{\odot }$, $M_{ej}$ = 15.5 M${}_{\odot }$ and $E$ = 0.4 foe. SN 2021aai is therefore interpreted as the explosion of a RSG, with properties that bridge the class of LL SNe IIP with standard SN IIP events.

22 pages, 17 figures, submitted to MNRAS