2026-04-15 京都産業大学
図１：恒星間天体アトラス彗星（3I/ATLAS; C/2025 N1）。（クレジット：国立天文台）
＜関連情報＞
酸素禁制線による恒星間彗星3I/ATLASの近日点通過後におけるCO2/H2O 比の制約
A post-perihelion constraint on the CO2/H2O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines
Yoshiharu Shinnaka, Ko Tsujimoto, Hideyo Kawakita, Hitomi Kobayashi, Jun-ichi Watanabe, Takafumi Ootsubo
The Astronomical Journal [Submitted on 26 Mar 2026]
https://doi.org/10.48550/arXiv.2603.25002
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 Γh=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 Ｉ630.0+I636.4=2.91±0.21 matches the optically thin branching ratio (～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＝Ｉ557.7／（Ｉ630.0+I636.4)=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 CO2/H2O abundance ratio under the assumption that H2O and CO2 are the dominant parents of O(1S) and O(1D), 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 CO2-rich composition reported from infrared observations on the inbound trajectory at Γh ~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 CO2/H2O ratio decreased substantially across perihelion
