Spatial intensity profiles of forbidden atomic oxygen emission lines in C/2023 A3 (Tsuchinshan-ATLAS)

  Comets, icy small bodies formed approximately 4.6 billion years ago during the formation of the solar system and which have remained in distant regions for most of their lifetimes, are thought to preserve pristine information about the early solar system. Therefore, studying comets is essential for understanding the processes and materials (ice and dust) involved in the formation of the solar system.

  The primary constituents of cometary ices are H₂O, CO₂, and CO. Once these molecules sublimate from the nucleus surface, they are photo-dissociated into fragments by the solar UV radiation, producing atomic oxygen through several reaction channels. Not only are oxygen atoms in the electronic ground state ³P produced, but also excited states ¹D and ¹S. These excited states are meta-stable, with lifetimes of ～110 seconds for the ¹D state and ～1 seconds for the ¹S state. Oxygen atoms in the ¹D state emit forbidden lines at 6300 Å and 6364 Å (red lines), while those in the ¹S state emit the forbidden line at 5577 Å (green line). Measuring the intensity ratio between the green and red lines I₅₅₇₇ / (I₆₃₀₀ + I₆₃₆₄), known as the green-to-red (G/R) ratio, allows for estimating the relative abundance of CO₂ with respect to H₂O (CO₂/H₂O) in the coma (Cochran & Cochran 2001; Furusho et al. 2006; Huffman et al. 2024 and references therein).

  However, this method has is based on the assumption that all excited oxygen atoms emit the photons as forbidden emission. In the inner coma, where the gas density is sufficiently high, the oxygen atoms in the ¹D and ¹S state can collide with water molecules and are de-excited to the ground state without emitting photons. This collisional quenching is especially significant for oxygen O(¹D) due to its longer radiative lifetime. As a result, quenched oxygen atoms do not contribute to the observed emission spectrum, potentially leading to an overestimation of the CO₂ abundance derived from the G/R ratio (Decock et al. 2015). Therefore, it is necessary to correct for this effect in the G/R ratio in order to accurately determine the CO₂/H₂O ratio.

  To address this issue, it is necessary to investigate the spatial distribution of the forbidden emission of atomic oxygen (i.e., the G/R ratio as a function of the distance from the nucleus). Decock et al. (2015) demonstrated that the G/R ratio varies significantly with nucleocentric distance, increasing markedly within 1000 km due to the quenching effects.

  In this study, we aim to more accurately estimate the CO₂/H₂O ratio by analyzing the spatial distribution of forbidden atomic oxygen lines. We conducted high-dispersion optical spectroscopic observations of comet C/2023 A3 (Tsuchinshan-ATLAS) on October 31 and November 1, 2024 (at heliocentric distance of 0.9 au), using the High Dispersion Spectrograph (HDS) mounted on the Subaru Telescope at the summit of Maunakea, Hawaii. Based on the obtained data, we discuss the spatial distribution of forbidden emission lines and compare the observed profiles with model results.