  Nebular phase spectra of core-collapse supernovae (SNe) provide critical and
unique information on the progenitor massive star and its explosion. We present
a set of 1-D steady-state non-local thermodynamic equilibrium radiative
transfer calculations of type II SNe at 300d after explosion. Guided by results
for a large set of stellar evolution simulations, we craft ejecta models for
type II SNe from the explosion of a 12, 15, 20, and 25Msun star. The ejecta
density structure and kinetic energy, the 56Ni mass, and the level of chemical
mixing are parametrized. Our model spectra are sensitive to the adopted line
Doppler width, a phenomenon we associate with the overlap of FeII and OI lines
with Lyalpha and Lybeta. Our spectra show a strong sensitivity to 56Ni mixing
since it determines where decay power is absorbed. Even at 300d after
explosion, the H-rich layers reprocess the radiation from the inner metal rich
layers. In a given progenitor model, variations in 56Ni mass and distribution
impact the ejecta ionization, which can modulate the strength of all lines.
Such ionization shifts can quench CaII line emission. In our set of models, the
OI6300 doublet strength is the most robust signature of progenitor mass.
However, we emphasize that convective shell merging in the progenitor massive
star interior can pollute the O-rich shell with Ca, which will weaken the
OI6300 doublet flux in the resulting nebular SN II spectrum. This process may
occur in Nature, with a greater occurrence in higher mass progenitors, and may
explain in part the preponderance of progenitor masses below 17Msun inferred
from nebular spectra.
