On the Origin of Sodium Atoms in the Comae and Trails of Comets

Background: The observed abundance ratios of alkali species in ground-based spectra of comets deviate from solar composition, suggesting alkali ejection from phenoxides reacting with carbon dioxide at the nucleus surface (alkali-phenoxide carbonylation). Methods: Here, we search for the alkali emissions in spectra of the coma and of the trail of Comet C/2023 A3 (Tsuchinshan-ATLAS) exploiting the double-fiber entrance of the high-resolution PEPSI spectrograph at the 8.4 m Large Binocular Telescope. Results: Spectra sampling the nucleus yield Na/K ratios 3.6 times higher than the chondritic value, and even higher ratios sampling the trail. This fact excludes photodesorption as the main sodium source, leaving sodium-phenoxide carbonylation at the surface of the main nucleus and the trail mininuclei as the primary sodium source. Conclusions: The nucleus temperature and the faint KI line exclude potassium-phenoxide carbonylation. For the first time, KI is detected in the trail of an Oort cloud comet, suggesting potassium photodesorbed from the trail mininuclei. Sodium-phenoxide carbonylation is at least six times more efficient than sodium photodesorption if the Na/K ratio in the C/2023 A3 nuclei is chondritic. Trails composed of sub-km-sized mininuclei may be common features of Oort cloud comets.