2026/03/26 by Yoshiharu Shinnaka, Ko Tsujimoto, Hideyo Kawakita +3 · 2 voices
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.3847/1538-3881/ae578d
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 rh = 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; \citealtStoreyZeippen2000), 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., \citealtFestouFeldman1981). The derived ratio is significantly lower than the extremely CO2-rich composition reported from infrared observations on the inbound trajectory at rh ∼ 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.