High resolution spectroscopy of comet C/2023 A3 (Tsuchinshan–ATLAS) from LBT/PEPSI observations

Studying the composition of comets is pivotal to investigate the early stages of the Solar System formation and evolution. In this context, the first and only passage of the dynamically new and remarkably bright comet C/2023 A3 (Tsuchinshan–ATLAS) in the inner Solar System offered a great opportunity to investigate the composition of a pristine Oort cloud comet. Unfortunately, previous observations of this object detected only the CN emission. As a consequence, the chemical composition of C/2023 A3 coma is still largely uncharacterised. We present a high resolution (R=130000) spectrum of C/2023 A3 at 0.83 AU from the Sun obtained with the PEPSI instrument at the Large Binocular Telescope (LBT). While pre-perihelion showed only the emission of CN, we also detect the usual cometary emissions, including Na I, [OI], CN, C 2 , C 3 and NH 2 . Their identification allows to investigate the chemical composition of the coma, still largely uncharacterised for this comet. The calculated production rates and their ratios confirm that C/2023 A3 (a) is depleted in carbon-chain molecules, in agreement with pre-perihelion observations, (b) shows a water-dominated outgassing and (c) has a enhanced ammonia abundance, consistent with the sublimation of ammonium salts. • We present a high-resolution spectrum of dynamically new comet C/2023 A3 (Tsuchinshan–ATLAS). • Unlike previous observations, we detect CN, C 2 , C 3 , NH 2 O[I] and Na. • We confirm a carbon-chain depletion on C/2023 A3 coma. • We identify water-dominated activity for the O[I] emission. • we estimate a typical NH3/H2O, suggesting formation in cold part of the protoplanetary disk.