Tianma Radio Telescope Successfully Captures Radio Signals from Comet 12P/Pons-Brooks

Recently, a joint research team led by the Shanghai Astronomical Observatory of the Chinese Academy of Sciences used the Tianma Radio Telescope to conduct multi-band radio observations of the returning comet 12P/Pons-Brooks (hereafter referred to as 12P). They systematically measured its water production rate during outburst activities and obtained the farthest detection of ammonia molecules in a Halley-type comet to date. The related research findings have been published in the international astronomy journal Astronomy & Astrophysics under the title "Pre-perihelion radio observations of comet 12P/Pons-Brooks with the Tianma Radio Telescope."

Comets contain a rich variety of icy components from the early formation of the solar system. As their orbital positions bring them closer to the Sun, these components sublimate due to solar heating, driving diverse cometary activity. The presence of these volatile ices indicates that comets have not undergone significant thermal evolution since their formation. Therefore, studying the composition of ices in comets helps us understand the thermal and chemical conditions of the primordial solar system 4.6 billion years ago.

12P is a Halley-type comet with an orbital period of approximately 71 years. Since its discovery in 1812, it has exhibited multiple outbursts during each return, a distinctive characteristic whose cause remains unsolved. During its 2024 return, 12P again experienced frequent outbursts, manifesting as short-term surges in overall brightness. As outbursts often involve the release of more gas from the nucleus, the outburst period of 12P provides an excellent opportunity to monitor its gas composition and changes, and to analyze the mechanisms and material sources of the outbursts.

During the observation window from late 2023 to early 2024, the research team conducted multiple observations of 12P in the L-band and K-band using the Shanghai 65-meter Tianma Radio Telescope. In the L-band observations, they successfully detected the 18-cm hydroxyl (OH) spectral line of 12P (Figure 1). Using a radiative transfer model, they calculated the water production rate and expansion velocity of 12P before and after several outbursts. Combining these results with previous research, they characterized the short-term and long-term activity changes of 12P influenced by outbursts (Figure 2a). At a heliocentric distance of 1 AU, 12P can release over 5 tons of water vapor per second, a level exceeding that of most short-period comets and some long-period comets, clearly demonstrating 12P's high activity. During outbursts, 12P's activity (using water as an example) can approximately double.

In the K-band observations, ammonia molecules (NH₃) were detected for the first time in a Halley-type comet at radio wavelengths with 3σ confidence, setting the record for the farthest detection of radio-wave ammonia in a comet. This observation measured the NH₃ production rate and relative abundance during one of 12P's outburst periods, finding that the relative abundance of NH₃ is at a high level among comets (Figure 2b). NH₃ has a relatively low sublimation temperature. For short-period comets like 12P, where more volatile substances (such as CO, CO₂) may have been depleted, the high abundance of NH₃ and its distribution within the nucleus might be one of the reasons for 12P's frequent outbursts.

Figure 1: Average 18 cm OH lines of 12P at different epochs (scaled to 1667 MHz).

Figure 2: (a) The variation of OH and water production rates for 12P with heliocentric distance; (b) The relationship between NH₃ abundance and heliocentric distance for different comets.

This research not only reveals the evolution of material release during 12P's outbursts but also provides new observational evidence for understanding the mechanisms of cometary activity and their internal composition.

The first author of the paper, Juncen Li, is a postdoctoral fellow at the Shanghai Astronomical Observatory. The main work of this study was completed during his PhD studies at the Purple Mountain Observatory. This work was supported by projects from the National Natural Science Foundation of China, Key Program of the National Natural Science Foundation of China, and key research and development project of the Ministry of Science and Technology.

Paper Link: https://doi.org/10.1051/0004-6361/202554867

Scientific Contacts: Xian Shi,  shi@shao.ac.cn,Yuehua Ma,  yhma@pmo.ac.cn


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