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

Abstract

In this work, we present photometric and spectroscopic data of the low-luminosity Type IIP supernova (SN) 2018hwm. The object shows a faint ($M_r=-15$ mag) and very long ($\sim$130 days) 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$\alpha$ 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 low-luminosity SNe IIP. We estimate that 0.0085 $M_{\odot }$ of ${}^{56}$Ni 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 0.075 foe, a progenitor radius of 845 $R_{\odot }$ and a final progenitor mass of 9-10 $M_{\odot }$. Finally, we performed a modeling of the nebular spectrum, to establish the amount of oxygen and calcium ejected. We found a low M(${}^{16}$O)$\approx 0.02$ $M_{\odot }$, but a high M(${}^{40}$Ca) of 0.3 $M_{\odot }$. The inferred low explosion energy, the low ejected ${}^{56}$Ni 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 super-asymptotic giant branch star and with a low-energy, Ni-poor iron core-collapse SN from a 10-12 $M_{\odot }$ red supergiant.

15 pages, 8 figures, 6 tables, updated the progenitor parameters after the submission of an Erratum

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