Pre- and post-perihelion observations of the carbon-depleted comet C/2023 A3 (Tsuchinshan-Atlas)

Introduction: Comets are among the most pristine remnants of the early Solar System [1], preserving volatile ices, refractory dust, and organic compounds that offer insights into the protoplanetary disk [2, 3, 4]. When approaching the Sun, sublimation processes release gas and dust, making comets key targets to study chemical and physical evolution. Molecular emissions in cometary comae reveal volatile inventories and photochemical processes. Comet C/2023 A3 (Tsuchinshan-ATLAS) is an Oort Cloud comet discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on January, 2023. This dynamically new comet reached its perihelion on September 27, 2024, at a distance of 0.39 au from the Sun. As the comet approached the inner Solar System, its activity increased significantly, with a substantial production of dust [5]. In this study, we analyze pre- and post-perihelion optical spectra of comet C/2023 A3, obtained with DOLORES at the Telescopio Nazionale Galileo (TNG) on May 1, 2024, and with the PEPSI spectrograph on the Large Binocular Telescope (LBT) on October 27, 2024. We derive the CN production rate and upper limits for other volatiles. Additionally, we present a post-perihelion high-resolution catalog of emission lines, and compare the emission features and elemental abundances of C/2023 A3 with those observed in other comets.

Methods: Due to the strong dust contamination and low gas emission observed pre-perihelion, only the CN emission band could be detected (see Fig. 1), while upper limits were derived for other molecular species (C₂, C₃, NH₂, see Fig. 2). Post-perihelion, thanks to high-resolution spectroscopy, we built a detailed catalog of emission lines for comet C/2023 A3.

Fig. 1. 𝑇𝑜𝑝 𝑝𝑎𝑛𝑒𝑙. Gaussian fit to the CN emission band centered at 3881.68 Å in the comet’s spectrum. The black line represents the observed spectrum, the red line is the fitted Gaussian profile, the blue dashed line indicates the central wavelength, and the shaded area corresponds to the total flux under the curve. 𝐵𝑜𝑡𝑡𝑜𝑚 𝑝𝑎𝑛𝑒𝑙. Zoomed-in view of the CN emission band and its Gaussian fit. The detailed view highlights the agreement between the fitted Gaussian profile (red line) and the observed data (black line), with the shaded area representing the integrated flux. The central wavelength is marked by the blue dashed line [10]. The CN production rate was determined by fitting a Gaussian profile to the CN (B–X) emission band at 3883 Å and applying a Haser model [8] to account for the spatial distribution in the coma. Fluxes were converted into production rates using standard fluorescence efficiencies and expansion velocities scaled to the heliocentric distance. For undetected species, upper limits were calculated based on the noise level around the expected emission wavelengths, assuming a 3σ threshold, and converted into 

production rates using the same formalism adopted for CN. The strong dust continuum was modeled and subtracted by fitting a low-order polynomial to emission-free regions and using a scaled solar analog spectrum. Following [9], the post-perihelion catalog includes both molecular bands (e.g., CN, C₂, NH₂) and atomic lines (e.g., Na I, K I), enabling a comprehensive analysis of the comet's volatile and atomic inventory. Emission line fluxes were measured by fitting Gaussian profiles after local subtraction of residual solar and telluric features, and were used to derive relative abundances and to compare C/2023 A3 with other comets.

Fig. 2. Top panel: Observed spectrum of comet C/2023 A3 in the spectral region of the C₂ bandhead at 5168.64 Å. The black line represents the observed spectrum, while the blue dashed line marks the expected central wavelength of the molecular feature. The shaded red region indicates the area where the emission would be expected, but no significant detection is observed. Middle panel: Same as the top panel but for the C₃ bandhead at 4003.65 Å. Bottom panel: Same as the previous panels but for the NH₂ bandhead at 5537.27 Å.

Results: Pre-perihelion observations revealed the presence of CN emission in comet C/2023 A3, with a derived production rate of (3.89 ± 0.21) × 10²⁵ molec/s. Due to the strong dust contamination, no other molecular emissions were detected, and we derived upper limits for C₂, C₃, and NH₂: Q(C₂) < 1.30 × 10²⁵ molec/s, Q(C₃) < 3.12 × 10²⁴ molec/s, and Q(NH₂) < 2.79 × 10²⁵ molec/s. The Afρ parameter, an indicator of dust production, was measured at 4329 ± 56 cm, confirming the comet's high dust activity. The production rate ratios log(Q(C₂)/Q(CN)) < −0.48 and log(Q(C₃)/Q(CN)) < −1.10 classify C/2023 A3 as a carbon-depleted comet [10]. Post-perihelion, high-resolution observations with PEPSI at the LBT allowed the construction of a detailed catalog of emission lines. We identified several molecular and atomic species, including CN, C₃, C₂, CH, CO⁺, Na, O I, NH₂, and K.

Conclusions: Comet C/2023 A3 (Tsuchinshan-ATLAS) exhibits characteristics typical of dynamically new, carbon-depleted comets. Pre-perihelion observation confirmed significant dust activity and indicates that C/2023 A3 is a carbon-depleted comet, consistent with previous classifications of dynamically new comets. Post-perihelion high-resolution spectroscopy enabled the detection of multiple molecular and atomic species, enriching our understanding of the comet's chemical inventory.
These results contribute to the broader characterization of volatile and elemental abundances in Oort Cloud comets, providing important constraints for models of Solar System formation and evolution.

Acknowledgements: This research used the facilities of the Italian Center for Astronomical Archive (IA2) operated by INAF at the Astronomical Observatory of Trieste.

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