Astrochemistry

From Emission To Absorption: The FAST Observation Of The OH 18-cm Lines From The Comet C2025/A6

By Keith Cowing
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astro-ph.EP
December 29, 2025
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From Emission To Absorption: The FAST Observation Of The OH 18-cm Lines From The Comet C2025/A6
Optical image of comet C/2025 A6 obtained on 21st October 2025 (18:35–18:50, UTC+8) with the 70-cm UCASST telescope. The image combines 5-min exposures in each of the R, V, and B filters and shows a coma with an apparent diameter of ∼8–10 arcmin. The telescope is equipped with a 2048×2048 detector with a pixel size of 13.5 µm, providing a field of view of about 20.8’×20.8′, at the time of the observation, 1′ corresponds to a projected scale of ∼25,700 km at the comet. — astro-ph.EP

We observed comet C/2025 A6 with FAST telescope equipped with the ultra-wideband receiver from 23rd October to 8th November 2025 and detected the OH 18-cm lines for the first time.

The OH lines underwent a reversal from emission to absorption from 23rd October to 5th November, which is mainly caused by variations in the heliocentric velocity. Through trapezoidal fitting of the OH line profiles, we derive expansion velocities of the water that rise as the heliocentric distance decreases.

Based on these results, we estimated the OH production rates of C/2025 A6 for 23rd October, 26th October, 4th November, and 5th November and it presents a significant upward trend.

Dongyue Jiang, Lei Qian, Minglei Guo, Qiaoli Hao, Menglin Huang, Peng Jiang, Hongfei Liu, Chun Sun, Xingyi Wang, Qingliang Yang, Naiping Yu, Lei Zhao, Yutao Zhao, Liyun Zhang, Yichi Zhang, Tongjie Zhang, Zhichen Pan

Comments: 7 pages, 2 figures, 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2512.21969 [astro-ph.EP](or arXiv:2512.21969v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.21969
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Submission history
From: Dongyue Jiang
[v1] Fri, 26 Dec 2025 10:33:46 UTC (24,156 KB)
https://arxiv.org/abs/2512.21969
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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