Emission line models for the lowest-mass core collapse supernovae. I: Case study of a 9 $M_\odot$ one-dimensional neutrino-driven explosion

TL;DR

This study models nebular spectra of a 9 solar mass core-collapse supernova, comparing predictions with observations to identify the progenitors and explosion mechanisms of subluminous Type IIP supernovae.

Contribution

It provides the first detailed nebular spectral models for low-mass CCSNe and links these models to observed subluminous IIP supernovae, supporting their origin from low-mass Fe core progenitors.

Findings

Models match observed spectra of SN 1997D and SN 2008bk.

No evidence of high $^{58}$Ni/$^{56}$Ni ratios, ruling out ECSNe.

Sub-luminous IIP SNe likely originate from low-mass Fe core progenitors.

Abstract

A large fraction of core-collapse supernovae (CCSNe), 30-50%, are expected to originate from the low-mass end of progenitors with $M_{\rm ZAMS}~= 8-12~M_\odot$. However, degeneracy effects make stellar evolution modelling of such stars challenging, and few predictions for their supernova light curves and spectra have been presented. Here we calculate synthetic nebular spectra of a 9 $M_\odot$ Fe CCSN model exploded with the neutrino mechanism. The model predicts emission lines with FWHM$\sim$1000 km/s, including signatures from each deep layer in the metal core. We compare this model to observations of the three subluminous IIP SNe with published nebular spectra; SN 1997D, SN 2005cs, and SN 2008bk. The prediction of both line profiles and luminosities are in good agreement with SN 1997D and SN 2008bk. The close fit of a model with no tuning parameters provides strong evidence for an association of these objects with low-mass Fe CCSNe. For SN 2005cs, the interpretation is less clear, as the observational coverage ended before key diagnostic lines from the core had emerged. We perform a parameterised study of the amount of explosively made stable nickel, and find that none of these three SNe show the high $^{58}$Ni/$^{56}$Ni ratio predicted by current models of electron capture SNe (ECSNe) and ECSN-like explosions. Combined with clear detection of lines from O and He shell material, these SNe rather originate from Fe core progenitors. We argue that the outcome of self-consistent explosion simulations of low-mass stars, which gives fits to many key observables, strongly suggests that the class of subluminous Type IIP SNe is the observational counterpart of the lowest mass CCSNe.