The nearby Type II supernova 2024ggi beyond 600 days: circumstellar interaction and dust formation
The Type II SN 2024ggi is one of the nearest core-collapse SNe to have exploded in recent decades. Its proximity has enabled extensive multiwavelength monitoring from early times through the nebular phase. Signatures of late-time interaction between the SN ejecta and hydrogen-rich circumstellar material (CSM) have recently been reported. Here we extend its study using late-time optical photometry and spectroscopy from 570 to 820 d after explosion. We investigate the emergence and evolution of late-time ejecta-CSM interaction and constrain the geometry of the interaction region and the properties of newly formed dust. Comparisons with other interacting SNe II and with synthetic spectra are presented. We model the H$α$ profile with the Monte Carlo radiative-transfer code Damocles, including both gas and dust components. Spectroscopic signatures of interaction precede the photometric flattening. The spectra evolve from ejecta-dominated to interaction-dominated as H$α$ develops a broad, boxy profile. The optical light curves subsequently flatten relative to the $^{56}$Co decay rate at approximately 650-670 d. The emission lines show a systematic blue-to-red asymmetry that requires a dust component newly formed within the ejecta. The inferred dust mass formed in the ejecta is $10^{-6}-10^{-5}$ M$_\odot$ if the grains are amorphous carbon or a carbon-silicate mixture, but can reach up to $10^{-3}$ M$_\odot$ for pure silicates.