Infrared diagnostics of late-time core-collapse supernova spectra

TL;DR

This work develops NLTE infrared radiative-transfer models for late-time core-collapse supernova ejecta, focusing on atoms and ions while neglecting molecules and dust to isolate atomic cooling. Using updated isotopic inputs, macroscopic mixing via shuffled-shells, and a CMFGEN-based framework, the authors simulate both H-rich Type II and H-deficient Type Ib/c explosions from 200 to 500 days after explosion, predicting 1–30 μm spectra and key diagnostic lines. They find that [Ne II] 12.810 μm is a robust indicator of progenitor mass in certain regimes and that Ni- and Fe-group infrared lines encode explosive nucleosynthesis and ejecta structure, though their evolution is highly model-dependent and not a universal Ni-decay power proxy. The study anticipates JWST-driven advances in constraining massive-star deaths, provides diagnostic guidelines, and outlines future work incorporating molecular cooling and dust formation.

Abstract

We present nonlocal thermodynamic equilibrium radiative transfer calculations of red supergiant and He-star explosions, extending previous work to focus on the infrared emission from atoms and ions in the ejecta during the nebular-phase (i.e., ~200 to ~500d) -- molecules and dust are ignored. We cover non-rotating solar-metallicity progenitors spanning an initial mass between 10 and about 40Msun and exploding as Type II or Ibc supernovae (SNe). Both photometrically and spectroscopically, the SN II models evolve distinctly from the SN Ibc models primarily because of the greater ejecta kinetic-energy-to-mass ratio in the latter, which leads to a greater gamma-ray escape together with a lower density and a higher ionization in our H-deficient ejecta. Type II SN models remain optically luminous at all times, whereas Type Ibc models progressively brighten in the infrared (which holds 80% of their luminosity at 500d), causing strong infrared lines such as [NeII]12.810mic and [NiII]6.634mic to evolve essentially at constant luminosity. The strength of [NeII]12.810mic exhibits a complicated dependence with either He-core or preSN mass because of the additional impact of ejecta ionization -- this line radiates alone up to 20% of the SN luminosity after ~300d in our Type Ibc SN models. The numerous infrared Ni lines are found to be good tracers of the material that underwent explosive nucleosynthesis and can thus be used directly to constrain the level of 56Ni mixing in core-collapse SNe. The evolution of the integrated flux in infrared Fe and Co lines shows a great amount of diversity, which compromises their use as a diagnostic of the 56Ni-decay power source in our models. Future spectroscopic observations of core-collapse SNe by JWST will provide unprecedented information on the emission from atoms and ions in their ejecta, delivering critical constraints on massive star explosions.

Figures (17)

• Figure 1: Bolometric light curves for our model sample. The dashed line corresponds to the decay-power emitted in model s21p5.
• Figure 2: Infrared spectrum of Type II SN model s15p2 at 350 d for an assumed distance of 10 Mpc. A logarithmic scale is used for the $x$-axis. Labels indicate the main contributor to each emission line (i.e., primarily H, Ne, Ar, and iron-group elements). The color coding indicates the associated species. (See Section \ref{['sect_s15p2']} for discussion.)
• Figure 3: Evolution of some spectral properties for model s15p2 over the time span from 200 to 500 d after explosion. We show the evolution of the cumulative flux integrated from 0.1 to 30.0 $\mu$m (left) and the evolution of the percentage, fractional luminosity relative to the bolometric luminosity of a few strong lines in the infrared (right). (See Section \ref{['sect_s15p2']} for discussion.)
• Figure 4: Infrared Ni lines in the models s15p2 at 350 d. We show the quantity $\lambda F_\lambda$ using the total flux (solid) and that due to Ni only (dashed) centered at the rest wavelength of two strong lines of [Ni i] and [Ni ii] (see label). A distance of 10 Mpc is assumed. The inset shows the Ni mass fraction versus velocity at 350 d, which thus accounts for all stable Ni isotopes (those that after explosion were neither $^{56}\rm{N}$i nor $^{57}\rm{N}$i -- these arise here at an 80 % level from $^{58}\rm{N}$i). (See Section \ref{['sect_s15p2']} for discussion.)
• Figure 5: Same as Fig. \ref{['fig_spec_s15p2_350d']} bur for the Type Ib SN model he6p0 at 200 d and a distance of 10 Mpc.