Mapping 3-D Explosive Nucleosynthesis in Type II Supernova 2024ggi with Infrared Emission Lines

Abstract

We present analysis and modeling of optical and infrared (IR) spectroscopy of the Type II supernova (SN II) 2024ggi obtained with ground-based instruments and the James Webb Space Telescope (JWST) at phases of ~265 - 400 days. The near- and mid-IR spectra reveal diverse iron-group emission-line morphologies, including double-peaked profiles in [Ni I] 3.119 and 11.998 $μ$m, [Fe II] 1.644 and 17.931 $μ$m, and [Co I] 12.255 $μ$m, alongside Gaussian profiles in [Ni II] 1.939 $μ$m, [Co II] 10.520 $μ$m, and [Ni I] 7.505 and 11.304 $μ$m. These differences imply both chemical inhomogeneity and aspherical ionization of inner ejecta, consistent with expectations from the $^{56}$Ni bubble effect. Modeling of double-peaked profiles supports an ejecta distribution with polar enhancements as large as ~7 for Ni/Co/Fe-rich material and ~2 for intermediate-mass elements. LTE estimates imply a stable Ni mass of $M_{\rm Ni}\approx1.3\times10^{-3}$ M$_{\odot}$, but electron densities near critical values indicate departures from LTE. Comparisons to non-LTE radiative transfer models favor a progenitor mass of ~12 - 15.2 M$_{\odot}$. We show that a simple mapping between elemental mass distribution and projected velocity reproduces line profiles produced in a CMFGEN radiative transfer calculation. We apply this property to 3-D neutrino-driven explosion simulations and predict Ni emission profiles for varying viewing angles. We find that only energetic 3-D explosion models of high-mass progenitors reproduce the observed extent of Ni mixing in SN 2024ggi, conflicting with progenitor masses inferred from radiative transfer models. These results demonstrate the utility of resolved nebular IR lines as direct probes of the 3-D distribution of explosively synthesized material in core-collapse SNe.