Low-luminosity Type II supernovae - III. SN 2018hwm, a faint event with an unusually long plateau

In this work, we present photometric and spectroscopic data of the low-luminosity (LL) Type IIP supernova (SN) 2018hwm. The object shows a faint (Mr = -15 mag) and very long (∼130 d) plateau, followed by a 2.7 mag drop in the r band to the radioactive tail. The first spectrum shows a blue continuum with narrow Balmer lines, while during the plateau the spectra show numerous metal lines, all with strong and narrow P-Cygni profiles. The expansion velocities are low, in the 1000-1400 km s-1 range. The nebular spectrum, dominated by H α in emission, reveals weak emission from [O i] and [Ca ii] doublets. The absolute light curve and spectra at different phases are similar to those of LL SNe IIP. We estimate that 0.002 M of 56Ni mass were ejected, through hydrodynamical simulations. The best fit of the model to the observed data is found for an extremely low explosion energy of 0.055 foe, a progenitor radius of 215 R, and a final progenitor mass of 9-10 M. Finally, we performed a modelling of the nebular spectrum, to establish the amount of oxygen and calcium ejected. We found a low M(16O)$approx 0.02, mathrm{ M}_{odot }$, but a high M(40Ca) of 0.3 M. The inferred low explosion energy, the low ejected 56Ni mass, and the progenitor parameters, along with peculiar features observed in the nebular spectrum, are consistent with both an electron-capture SN explosion of a superasymptotic giant branch star and with a low-energy, Ni-poor iron core-collapse SN from a 10-12 M red supergiant.