A Post-perihelion Constraint On The CO2/H2O Ratio Of Interstellar Comet 3I/ATLAS From [O I] Forbidden Lines

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 I630.0/I636.4=2.91±0.21 matches the optically thin branching ratio (Storey Zeippen 2000), 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., Festou Feldman1981).

The derived ratio is significantly lower than the extremely CO₂-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 pre- and post-perihelion measurements, our result indicates that the CO₂/H₂O ratio decreased substantially across perihelion.

Yoshiharu Shinnaka, Ko Tsujimoto, Hideyo Kawakita, Hitomi Kobayashi, Jun-ichi Watanabe, Takafumi Ootsubo

Comments: 8 pages, 2 figures, 4 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2603.25002 [astro-ph.GA] (or arXiv:2603.25002v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2603.25002
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Submission history
From: Yoshiharu Shinnaka Dr.
[v1] Thu, 26 Mar 2026 04:02:55 UTC (111 KB)
https://arxiv.org/abs/2603.25002
Astrobiology, Astrochemistry,