The CO/H2O Abundance Ratio in the Dynamically New Comet C/2023 A3 (Tsuchinshan-ATLAS) using Forbidden Oxygen Emission Lines

Comets are considered among the most pristine objects in the solar system. The main components of cometary ices are H₂O, CO₂, and CO (Mumma & Charnley 2011). These chemical compositions reflect the physico-chemical evolution of the early solar system, particularly the formation temperatures of cometary ices. For comets that have frequently approached the Sun within a few au (e.g., a Jupiter-family comet), volatile species such as CO₂ (sublimation temperature T_sub ~70 K) and CO (T_sub ~25 K) are thought to have been selectively sublimated from the surface of the nucleus due to the solar heating. However, it remains unclear whether the observed abundances of CO₂ and CO relative to H₂O in such comets reflect their primordial compositions or are the result of evolutionary effects. To address this issue, we focus on a comet making its first approach to the inner solar system from the Oort Cloud, a so-called Dynamically New Comet (DNC). DNCs are thought to be among the most pristine icy bodies, as they have undergone little thermal alteration by solar heating since their formation in the solar nebula 4.6 billion years ago.

The abundance ratios of H₂O:CO₂:CO in comets have been determined by two methods: (1) direct measurements using space-based infrared facilities (e.g., Ootsubo et al. 2012), and (2) indirect estimates based on the intensity ratios of three forbidden emission lines of atomic oxygen (photodissociation products of primary volatile species) obtained through high-resolution optical spectroscopy (e.g., Shinnaka et al. 2020). In the second method, the CO₂/H₂O abundance ratio is derived from the intensity ratio between the "green line" at 557.7 nm and the "red lines" at 630.0 and 636.4 nm of the atomic oxygen forbidden emissions (Decock et al. 2013, McKay et al. 2016).

              To investigate the abundance ratios of CO₂ relative to H₂O in the DNC C/2023 A3 (Tsuchinshan-ATLAS), we conducted high-resolution spectroscopic observations with the High Dispersion Spectrograph (HDS) on the Subaru Telescope atop Maunakea in April 2024 (at a heliocentric distance r_h = 2.8 au during the inbound orbit), October 2024 (at r_h = 0.9 au, where the nucleus surface becomes warm enough for all volatiles to fully sublimate), and June 2025 (at r_h = 4.1 au during the outbound orbit). We discuss the heliocentric dependence of chemical compositions based on our new data and comparison with previous studies.

References

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