Postperihelion Coma Composition of the Interstellar Comet 3I/ATLAS from Optical Spectroscopy

Abstract We present multiepoch optical spectroscopy of the interstellar comet 3I/ATLAS obtained between 2025 December and 2026 January (heliocentric distances of 1.8–3.3 au), yielding postperihelion production rates and mixing ratios for CN, C 3 , C 2 , CH, and gaseous metals (Fe i and Ni i ). Our results show that the coma is less depleted in C 2 after perihelion than before, indicative of subsurface activation or compositional heterogeneity. The outgassing profiles reveal a pronounced perihelion asymmetry: the CN and metal production rates decline more gradually outbound than inbound, consistent with the reported behavior of H 2 O and implying a change in the comet’s activity pattern across perihelion. Despite being metal rich relative to its H 2 O content, 3I/ATLAS follows the metal–CO correlation observed in comets of diverse origins, suggesting that gaseous metal release is more closely linked to a CO-bearing volatile reservoir than to H 2 O, potentially in the form of metal carbonyls. In addition, the [O i ] λ 6300 emission shows a significant residual after subtracting the expected contributions from H 2 O, CO 2 , and CO, which may reflect systematic uncertainties in the photodissociation yields of those molecules or a contribution from additional oxygen-bearing parents.