A Postperihelion Constraint on the CO ₂ /H ₂ O Ratio of Interstellar Comet 3I/ATLAS from [O <scp>I</scp> ] Forbidden Lines

Abstract We present high-resolution optical spectroscopy of interstellar comet 3I/ATLAS (C/2025 N1) obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope on UT 2026 January 7, when the comet was on its outbound trajectory at a heliocentric distance of r h = 2.87 au. The spectra cover the forbidden atomic oxygen lines, [O I ], at 557.7, 630.0, and 636.4 nm. The [O I ] red-doublet intensity ratio I 630.0 / I 636.4 = 2.91 ± 0.21 matches the optically thin branching ratio (∼3), indicating that optical-depth effects are small and that our relative flux calibration is reliable. We measure a green-to-red [O I ] intensity ratio of G/R = I 557.7 /( I 630.0 + I 636.4 ) = 0.339 ± 0.027. This value is higher than those of most solar system comets at similar heliocentric distances, but comparable to that of the interstellar comet 2I/Borisov. From the measured G/R ratio in 3I/ATLAS, we estimate the CO 2 /H 2 O abundance ratio under the assumption that H 2 O and CO 2 are the dominant parents of O( 1 S) and O( 1 D) with other oxygen-bearing species expected to have a smaller influence under typical conditions. The derived ratio is significantly lower than the extremely CO 2 -rich composition reported from infrared observations on the inbound trajectory at r h ∼3.3 au, yet higher than typical values measured for comets in the solar system. Together with published preperihelion and postperihelion measurements, our result indicates that the CO 2 /H 2 O ratio decreased substantially across perihelion.