An optical to infrared study of type II SN2024ggi at nebular times

TL;DR

Pan-chromatic nebular spectroscopy of SN 2024ggi (Keck optical/NIR and JWST MIR) at ~275 d and ~400 d after explosion tests a standard explosion model for a red supergiant progenitor. The authors compare the data to the s15p2 radiative-transfer model for a $15 Msun$ progenitor, finding broad agreement from $0.3-21 μm$, with CO fundamental emission contributing about $5%$ of the total luminosity and little microscopic $^{56}$Ni mixing inferred from the coherent line widths. The optical spectrum shows a dense Fe forest while the IR reveals numerous Ni, Co, and Ar lines; all lines share similar widths ≤ $2000$ km s^{-1}$, implying efficient macroscopic mixing of the inner ejecta. The results support a $15 Msun$ progenitor, highlight the role of CO cooling in shaping certain lines (e.g., [O I] 0.632 μm), and demonstrate the value of panchromatic nebular spectroscopy in constraining ejecta composition, mixing, and molecular processes in core-collapse SNe.

Abstract

We present 0.3-21mic observations at ~275d and ~400d for Type II supernova (SN) 2024ggi, combining ground-based optical and near-infrared data from the Keck I/II telescopes and space-based infrared data from the James Webb Space Telescope. Although the optical regions dominate the observed flux, SN2024ggi is bright at infrared wavelengths (65%/35% falls each side of 1mic). SN2024ggi exhibits a plethora of emission lines from H, He, intermediate-mass elements (O, Na, Mg, S, Ar, Ca), and iron-group elements (IGEs; Fe, Co, and Ni) -- all lines have essentially the same width, suggesting efficient macroscopic chemical mixing of the inner ejecta at <~2000km/s and little mixing of 56Ni at larger velocities. Molecular emission in the infrared range is dominated by the CO fundamental, which radiates about 5% of the total SN luminosity. A molecule-free radiative-transfer model based on a standard red-supergiant star explosion (i.e., ~1e51erg, 0.06Msun of 56Ni from a 15.2Msun progenitor) yields a satisfactory match throughout the optical and infrared at both epochs. The SN2024ggi CO luminosity is comparable to the fractional decay-power absorbed in the model C/O-rich shell -- accounting for CO cooling would likely resolve the model overestimate of the [OI]0.632mic flux. The relative weakness of the molecular emission in SN2024ggi and the good overall match obtained with our molecule-free model suggests negligible microscopic mixing -- about 95% of the SN luminosity is radiated by atoms and ions. Lines from IGEs, which form from explosion ashes at such late times, are ideal diagnostics of the magnitude of 56Ni mixing in core-collapse SN ejecta. Stable Ni, clearly identified in SN2024ggi (e.g., [NiII]6.634mic), is probably a common product of massive-star explosions.

Figures (9)

• Figure 1: Comparison between the optical to infrared spectroscopic observations of SN 2024ggi at 266--291 d (red; see Section \ref{['sect_obs']}) with model s15p2 interpolated to the same epoch and scaled by a factor of 0.75 (black), each panel covering from top to bottom the optical, near-infrared and mid-infrared ranges. The data were corrected for redshift and reddening and the model was scaled to the SN distance. The flux shift corresponding to the same model but at $\pm$ 30 d is shown as light/dark grey. Labels indicate the main emission features. For easier comparison, the radiative-transfer model flux was augmented by a composite blackbody spectrum, which may arise from molecular emission (dashed; the blackbody temperatures and radii are 1230/400 K and $0.8/5.0\times 10^{15}$ cm). The region between 8 and 10 $\mu$m, which appears featureless with only weak SiO emission in SN 2024ggi, is shown in Fig. \ref{['fig_obs_only']}.
• Figure 2: Same as Fig. \ref{['fig_mod_vs_obs_jan25']} but now for the observations of SN 2024ggi at around 400 d and the contemporaneous model s15p2.
• Figure 3: Fractional luminosity integrated from 0.3 up to 21 $\mu$m for the first epoch of observations of SN 2024ggi and model s15p2 at 275 d. The data were corrected for redshift and reddening. Linear interpolation is used to infer the flux in regions without data.
• Figure 4: Illustration of the mass of the O-rich shell (black) and the C/O-rich shell (red) in the explosion models of sukhbold_ccsn_16 having a zero-age main sequence mass ($M_{\rm ZAMS}$) in the range 9.0 to 26.5 $M_{\odot}$. The inset zooms in on the lower mass progenitors and their very low metal yields.
• Figure 5: Evolution of the percentage fraction of the bolometric flux that is radiated in [Ar ii] 6.983 $\mu$m (left), [Ne ii] 12.810 $\mu$m (middle), and [O i] 0.632 $\mu$m (right) for a sample of core-collapse SN models from dessart_ir_25.