Abstract:
We present a set of nonlocal thermodynamic equilibrium steady-state calculations of radiative transfer for one-year old type II supernovae (SNe) starting from state-of-the-art explosion models computed with detailed nucleosynthesis. This grid covers single-star progenitors with initial masses between 9 and 29Msun, all evolved with KEPLER at solar metallicity and ignoring rotation. The [OI]6300,6364A line flux generally grows with progenitor mass, and Halpha exhibits an equally strong and opposite trend. The [CaII]7291,7323A strength increases at low 56Ni mass, low explosion energy, or with clumping. This CaII doublet, which forms primarily in the explosively-produced Si/S zones, depends little on the progenitor mass, but may strengthen if Ca^+ dominates in the H-rich emitting zones or if Ca is abundant in the O-rich zones. Indeed, Si-O shell merging prior to core collapse may boost the CaII doublet at the expense of the OI doublet, and may thus mimic the metal line strengths of a lower mass progenitor. We find that the 56Ni bubble effect has a weak impact, probably because it is too weak to induce much of an ionization shift in the various emitting zones. Our simulations compare favorably to observed SNe II, including SN2008bk (e.g., 9Msun model), SN2012aw (12Msun model), SN1987A (15Msun model), or SN2015bs (25Msun model with no Si-O shell merging). SNe II with narrow lines and a low 56Ni mass are well matched by the weak explosion of 9-11Msun progenitors. The nebular-phase spectra of standard SNe II can be explained with progenitors in the mass range 12-15Msun, with one notable exception for SN2015bs. In the intermediate mass range, these mass estimates may increase by a few Msun with allowance for clumping of the O-rich material or CO molecular cooling.
The full paper is available here

To access the spectra for some of these models, click on the following links (each zipped tar file contains the multi-epoch spectra and a list relating model index to the time since explosion):

sn2_neb.tgz