Emission line models for the lowest mass core-collapse supernovae -- II. Full 3D NLTE radiative transfer modelling of a $9.0\,M_\odot$ neutrino-driven explosion

TL;DR

This paper presents a 3D NLTE nebular-phase spectral study of a $9.0\,M_\\odot$ neutrino-driven CCSN using the ExTraSS code, now including photoionization and photoexcitation. The authors compare synthetic spectra across viewing angles to observations of SN 1997D and SN 2016bkv, finding good matches for several key lines (e.g., Mg I], [O I], [Ca II], [Fe II]) while H$\alpha$ and Fe I/Ca I features require adjustments. They show that 3D ejecta morphologies lead to distributed $\gamma$-ray deposition, reduced sharp-shell effects, and substantial viewing-angle variability in line luminosities and centroids, with variations up to ~20–50% depending on the line. The results support a low-mass Fe-CCSN (and potential ECSN) interpretation for these SNe, while highlighting remaining physics gaps (e.g., electron scattering, dust, larger nuclear networks) and the need for more 3D models to robustly diagnose explosion energetics from nebular spectra.

Abstract

The nebular phase of a supernova (SN) occurs several months to years after the explosion, when the ejecta become mostly optically thin yet there still is sufficient radioactive material to keep the supernova bright. The asymmetries created by the explosion are encoded into the line profiles of the emission lines which appear in the nebular phase. In order to make accurate predictions for these line profiles, Non-Local Thermodynamic Equilibrium (NLTE) radiative transfer calculations need to be carried out. In this work, we use \texttt{ExTraSS} (EXplosive TRAnsient Spectral Simulator) -- which was recently upgraded into a full 3D NLTE radiative transfer code (including photoionization and line-by-line transfer effects) -- to carry out such calculations. \texttt{ExTraSS} is applied to a 3D explosion model of a $9.0\,M_\odot$ H-rich progenitor which is evolved into the homologous phase. Synthetic spectra are computed and the lines from different elements are studied for varying viewing angles. The model spectra are also compared against observations of SN 1997D and SN 2016bkv. The model is capable of creating good line profile matches for both SNe, and reasonable luminosity matches for He, C, O, and Mg lines for SN 1997D -- however H$α$ and Fe I lines are too strong.

Figures (14)

• Figure 1: Mass fractions of the angle-averaged ejecta for s9.0. The 13 $\alpha-$nuclei from $^{1}$H to $^{52}$Fe are shown (the $^{54}$Fe is added to $^{52}$Fe), together with $^{56}$Ni (which contains the $^{56}$Ni, $^{56}$Co and $^{56}$Fe and all 'element X'). The neutron star mass ($1.355\,M_\odot$, janka2024interplay) is excluded in this plot. The top axis indicates the velocity of the ejecta. The total ejecta masses for each element are also listed in Table \ref{['tab:StockMass']}. A 3D rendering, up to 2.8 d, of the $^{56}$Ni is also shown in stockinger2020three.
• Figure 2: The cumulative $\gamma$-ray energy deposition (coloured lines) and cumulative mass (black). Solid lines show the cumulative fraction, while dashed lines show the fraction per unit velocity. The 400 day deposition curve in the 1D model of \ref{['cite.jerkstrand2018emission']}18 is shown in dotted brown: the dense "wall" present in 1D gives a sharp rise at $\sim400\,\text{km}\,\text{s}^{-1}$ which does not happen in 3D. Instead, there is a distributed deposition in the $100-400\,\text{km}\,\text{s}^{-1}$ range (receiving around $40\,\%$ of the total power).
• Figure 3: Electron fraction ($x_e$, top) and temperature (bottom) in the s9.0 model at 400 days. Angle-averaged values are plotted as dark blue lines, and $1\sigma$ angle variation as light blue shaded region. Also shown are cumulative distributions of H, O, NiCoFeX, and total ejecta.
• Figure 4: The spectrum of s9.0 at $400\,$days, colour coded by emission origin, in the wavelength range $4000 - 10000\,\hbox{\normalfont\AA}$, for the observer that is most directly approached by the neutron star. The spectrum has a resolution of R=2665.
• Figure 5: The same as Figure \ref{['fig:optical_spectrum']}, but with boosted $\sigma_\text{PI}$ for neutral Ca and Fe (see text).