Optical and Near-infrared Nebular-phase Spectroscopy of SN 2024ggi: Constraints on the Structure of the Inner Ejecta, Progenitor Mass, and Dust*

Abstract We present optical and near-infrared (NIR) spectroscopic observations of the nearby Type II supernova SN 2024ggi ranging from 250 to 581 days after the explosion. Comparing the evolution of the [O i ] at 6300, 6363 Å doublet normalized to the continuum with spectral models from the literature, we estimate a progenitor star zero-age main-sequence mass ( M ZAMS ) of ≈14 M ⊙ . This value is consistent with M 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 of ≃3900 km s −1 , with small deviations from a symmetric Gaussian profile centred at zero velocity that persist until the penultimate spectrum obtained 459 days after explosion, and the [O i ] doublet is blueshifted by ≈−940 km s −1 . In the NIR, the nebular spectra reveal double-peaked emission features of [Mg i] and [Fe ii ] 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, indicating that the asymmetry is confined to intermediate-mass and iron-peak elements in the innermost ejecta. Additionally, we detect first-overtone carbon monoxide emission at 2.3 μ m between 250 and 319 days and a blueshift in the emission lines of H α , [O i ], [Mg i ], and [Fe ii ] first observed in the +581 days optical spectrum, consistent with dust formation in the ejecta.