2020A&A...642A..33D

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 one-dimensional steady-state non-local thermodynamic equilibrium radiative transfer calculations of typeII SNe at 300 d after explosion. Guided by the results obtained from a large set of stellar evolution simulations, we craft ejecta models for typeII SNe from the explosion of a 12, 15, 20, and 25M_☉ 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 Lyα and Lyβ. Our spectra show a strong sensitivity to ⁵⁶Ni mixing since it determines where decay power is absorbed. Even at 300 d 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 [OI] λλ 6300, 6364 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 would weaken the OI doublet flux in the resulting nebular SNII spectrum. This process may occur in nature, with a greater occurrence in higher mass progenitors, and this may explain in part the preponderance of progenitor masses below 17M_☉ that are inferred from nebular spectra.