Abstract
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 type II SNe at 300 d after explosion. Guided by the results obtained from a large set of stellar evolution simulations, we craft ejecta models for type II SNe from the explosion of a 12, 15, 20, and 25 M⊙ 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 Fe II and O I lines with Ly α and Ly β. 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 Ca II line emission. In our set of models, the [O I] 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 O I doublet flux in the resulting nebular SN II 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 17 M⊙ that are inferred from nebular spectra.
Highlights
Nebular-phase spectroscopy provides critical information on the properties of massive star explosions and type II supernovae (SNe)
In about 50% of the models produced with these parameters, the Si-rich and the O-rich shells merged during Si burning, producing a single Si-rich and O-rich shell with a nearly uniform composition
The N to He mass ratio in the He-rich shell is around 0.005, with a scatter of about 50% from low to high mass progenitors. These are representative composition properties for our set of MESA simulations evolved with the network approx21.net, which is routinely used for massive star explosions
Summary
Nebular-phase spectroscopy provides critical information on the properties of massive star explosions and type II supernovae (SNe). In about 50% of the models produced with these parameters, the Si-rich and the O-rich shells merged during Si burning, producing a single Si-rich and O-rich shell with a nearly uniform composition (all elements are microscopically mixed) This has a considerable impact on the appearance of a core-collapse SN at nebular times (Fransson & Chevalier 1989). In the O-rich shell, O is about 104 times more abundant than Ca, whose mass fraction in that shell is equal to the original metallicity in our MESA simulations The N to He mass ratio in the He-rich shell is around 0.005, with a scatter of about 50% from low to high mass progenitors These are representative composition properties for our set of MESA simulations evolved with the network approx21.net, which is routinely used for massive star explosions. Using state-of-the-art explosion models with a fully consistent composition computed with a huge network is straightforward but is delayed to our study
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