A recent study has revealed the activity enhancement process of the long-period comet C/2023 A3 (Tsuchinshan–ATLAS) at large heliocentric distances for the first time. The findings characterized the evolutionary features of its distant activity and provided observational evidence for understanding the activity mechanisms and evolutionary patterns of long-period comets at large heliocentric distances.

The study, led by a research team from the Shanghai Astronomical Observatory (SHAO), in collaboration with the Purple Mountain Observatory (PMO), was published in The Astrophysical Journal on May 25.

At heliocentric distances beyond 5 au, where sublimation driven by water ice is traditionally considered weak, cometary activity mechanisms remain poorly understood and observational samples are scarce. To address this, the team conducted continuous monitoring of C/2023 A3 from March to July 2023 using the high-precision telescope at the PMO Yao'an Observation Station. A total of 21 nights of high-quality data were obtained, covering the comet's journey from 7.2 au to 5.9 au.

The analysis revealed that both the dust production rate and the effective scattering cross-section of the comet increase continuously as heliocentric distance decreases. The activity index exhibits two distinct evolutionary phases, with distant-stage activity intensity significantly exceeding that of typical comets. The coma colors show a unique evolutionary pattern, suggesting the combined effects of gas emission and dust particle size variation.

This study filled the observational gap for comet C/2023 A3 (Tsuchinshan–ATLAS) in the distant range of 5 au to 7 au. It also provided a highly valuable and typical observational sample, as well as empirical support, for establishing a robust classification framework for the distant activity of long-period comets.

The first author of the paper is WU Yingqi, a master's student at SHAO, and the corresponding author is SHI Jianchun. This research was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences Telescope Operation Maintenance and Upgrading Fund, and the China Manned Space Engineering Program.