Optical and near-infrared nebular-phase spectroscopy of SN 2024ggi: constraints on the structure of the inner ejecta, progenitor mass, and dust

TL;DR

This study uses optical and near-infrared nebular-phase spectroscopy of SN 2024ggi to constrain the inner ejecta structure and the progenitor mass. By identifying key nebular lines, measuring line profiles, and detecting CO overtone emission, the authors infer a bipolar inner-core geometry and evidence for dust formation at late times. Comparing the [O I] 6300,6363 Å flux ratio to nebular models yields a progenitor ZAMS mass of about $14$ $M_\odot$, consistent with independent estimates. The results demonstrate the power of nebular-phase spectroscopy in constraining core-collapse SN progenitors and inner ejecta geometry in nearby Type II SNe, with implications for explosion asymmetries and dust formation processes.

Abstract

We present optical and near-infrared (NIR) spectroscopic observations of the nearby Type II supernova SN\,2024ggi from 250 and 420 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, 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 from 250 to 319 days in the NIR.

Figures (7)

• Figure 1: Nebular-phase spectral time series of SN 2024ggi shown in black. Upper panel: Optical spectra from 269 to 420 days post-explosion, with SN 1999em Leonard02, SN 2015bs anderson18, and SN 2023ixf Michel25 shown for comparison. The 400d spectrum of SN 2015bs is smoothed with a Savitzky–Golay filter. Lower panel: Near-IR spectra from 250 to 319 days. The 319d spectrum of SN 2024ggi is smoothed. NIR spectra of SN 1987A Bouchet91 and SN 2017eaw rho18 are included for comparison. Vertical lines mark rest-frame wavelengths of key identified ions.
• Figure 2: FWHM velocities from single Gaussian fits to the H$\alpha$, [O1] 6300, 6363 Å doublet, Mg1 1.504$~\mu$m, and [Fe2] 1.644$~\mu$m line profiles.
• Figure 3: Near-IR and optical [Fe2] and Mg1 line profiles comparison. Upper left: Comparison between [Fe2] 1.257 $\mu$m and [Fe2] 1.664 $\mu$m emission at 250 days. Upper right: Comparison between Mg1 1.504 $\mu$m and 1.711$\mu$m emission at 250d. In both panels, a double-peaked profile is observed. Middle left: [Fe2] 1.644 $\mu$m from SN 2024ggi at 250d (black) compared with SN 1987A Bouchet91, and SN 2017eaw rho18. Middle right: Mg1 1.504 $\mu$m from SN 2024ggi, compared with SN 1987A Bouchet91, and SN 2017eaw rho18. Bottom left: [Fe2] 7155 Å emission profile. The emission displays a broad and flat profile. Bottom right: Mg1] 4571 Å emission profile. At 385 days post first light, a blueshift of $\sim$ 1.300 km s $^{-1}$ is observed.
• Figure 4: Line profile of the emission lines affected for dust formation. The grey dotted line denotes zero velocity at redshift $z$ = 0.002435. The black dashed line marks the peak of the emission of each ion.
• Figure 5: Comparison between nebular spectra of SN 2024ggi, SN 2023ixf Jacobsongalan25, and SN 2004et Fabbri11. Broad features in the red and blue sections of the H$\alpha$ emission profile are observed