Optical and near-infrared nebular-phase spectroscopy of SN 2024ggi: constraints on the structure of the inner ejecta, progenitor mass, and dust
We present optical and near-infrared (NIR) spectroscopic observations of the nearby Type II supernova SN\,2024ggi from 250 and 581 days after the explosion. Comparing the evolution of the [\ion{O}{1}] at 6300, 6363 \textÅ doublet normalized to the continuum with spectral models from the literature, we estimate a progenitor star zero-age main-sequence mass ($M_{\mathrm{ZAMS}}$) of $\approx 14$ M$_\odot$. This value is consistent with $M_{\mathrm{ZAMS}}$ reported in the literature from independent methodologies. The nebular spectra are used to study the structure of the inner ejecta. The broad H$α$ line has a full-width at half maximum (FWHM) of $\simeq 3900$ km s$^{-1}$, with small deviations from a symmetric Gaussian profile centred at zero velocity, and the [\ion{O}{1}] doublet is blue-shifted by $\approx -940$ km s$^{-1}$. In the NIR, the nebular spectra reveal double-peaked emission features of \ion{Mg}{1} and [\ion{Fe}{2}] lines between +250 and +319 days, suggesting a bipolar distribution of intermediate mass and iron peak elements in the line-of-sight. Such a double-peaked feature in these NIR lines has not been previously reported. No corresponding asymmetries are observed in the hydrogen lines, suggesting that the asymmetry is mostly confined to intermediate mass and iron peak elements in the innermost core of the supernova ejecta. Additionally, we detect first-overtone carbon monoxide (CO) emission at 2.3,$μ$m between 250 and 319 days, and a blueshift in the emission lines of H$α$, [\ion{O}{1}], \ion{Mg}{1}], and [\ion{Fe}{2}] in the +581 day optical spectrum, consistent with dust formation in the ejecta.