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A post-perihelion constraint on the CO$_{2}$/H$_{2}$O ratio of interstellar comet 3I/ATLAS from [O I] forbidden lines

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

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.

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 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 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 . 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 ratio in 3I/ATLAS, we estimate the CO/HO abundance ratio under the assumption that HO and CO are the dominant parents of O(S) and O(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-rich composition reported from infrared observations on the inbound trajectory at 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/HO ratio decreased substantially across perihelion.

Paper Structure

This paper contains 6 sections, 1 equation, 2 figures.

Table of Contents

  1. Introduction
  2. Observations And Data Reduction
  3. Results and Discussion
  4. [O I] Line Measurements and Intensity Ratios
  5. CO$_{2}$/H$_{2}$O ratio inferred from the [O I] green-to-red ratio
  6. Summary and Conclusions

Figures (2)

  • Figure 1: Spectra of the three [O I] emission lines around 557.73 nm (top), 630.03 nm (middle), and 636.37 nm (bottom) in 3I/ATLAS observed on UT 2026 January 7. Wavelengths are given in the comet rest frame. Our best-fit single-Gaussian profiles for the three [O I] lines are overplotted as red curves. Vertical ticks indicate the cometary [O I] lines, and vertical dotted lines denote the positions of the telluric [O I] lines. The telluric oxygen emission lines are clearly separated from the cometary lines. We neglect possible contamination of the [O I] lines by other species (C$_2$ and NH$_2$), because their expected intensities are below the measurement uncertainty.
  • Figure 2: The green-to-red intensity ratio of [O I] (G/R) in 3I (filled circle), compared with measurements for a sample of Solar System comets: Jupiter-family comets (JFCs; open triangles), Oort Cloud comets (OCCs; open squares), and Halley-type comets (HTCs; crosses) from Decock2013McKay2015ShinnakaKawakita2016Shinnaka2020, as well as C/2016 R2 from Opitom2019 and 2I/Borisov from Opitom2021.