A post-perihelion constraint on the CO${}_2$/H${}_2$O ratio of interstellar comet 3I/ATLAS from [O I] 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 $r_{\mathrm{h}}=2.87$ 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 $I_{630.0}/I_{636.4}=2.91\pm 0.21$ matches the optically thin branching ratio ($\sim$3; \citealt{StoreyZeippen2000}), 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 $G/R=I_{557.7}/(I_{630.0}+I_{636.4})=0.339\pm 0.027$. 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 (e.g., \citealt{FestouFeldman1981}). The derived ratio is significantly lower than the extremely CO${}_2$-rich composition reported from infrared observations on the inbound trajectory at $r_{\mathrm{h}}\sim 3.3$ au, yet higher than typical values measured for comets in the Solar System. Together with published pre- and post-perihelion measurements, our result indicates that the CO${}_2$/H${}_2$O ratio decreased substantially across perihelion.

8 pages, 2 figures, 4 tables