Pre-perihelion radio observations of comet 12P/Pons-Brooks with the Tianma Radio Telescope

¹ Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, 200030 Shanghai, PR China
⁴ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago, Chile
⁵ Planetary Environmental and Astrobiological Research Laboratory (PEARL), School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China
⁶ Key Laboratory of Radio Astronomy, Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China
⁷ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

^★ Corresponding authors: yhma@pmo.ac.cn; shi@shao.ac.cn

Received: 30 March 2025
Accepted: 5 August 2025

Abstract

Context. The multiple outburst events of comet 12P/Pons-Brooks during its 2024 apparition offer a unique window into highly active volatile releasing processes not observable during quiescent periods. Radio observations are capable of measuring specific cometary gas species, providing valuable insights into the chemical composition and activity mechanism of comets.

Aims. We performed radio observations of comet 12P/Pons-Brooks with the Tianma 65 m Radio Telescope, targeting the OH and NH₃ inversion lines at 18 and 1.3 cm, respectively. By monitoring 12P at different heliocentric distances on its inbound journey, we aim to provide insights into the comet's volatile composition and outburst behavior.

Methods. Four observations were carried out between December 2023 and March 2024, when the comet was approaching the Sun from 2.22 to 1.18 AU. We conducted 18 cm OH lines observations on four separate days using the cryogenically cooled receiver system of the telescope to derive the H₂O production rate. During 12P's outburst on December 14, we also conducted observations targeting the NH₃ emission.

Results. OH 18 cm lines were clearly detected with a signal-to-noise ratio of ∼4 σ (peak intensity), yielding estimates on corresponding water production rates of approximately 10²⁹ molec. s⁻¹. A tentative detection of NH₃ was made at the ∼3 σ level during the outburst phase, but the detection needs to be verified.

Conclusions. Our observations provide information on the outgassing behavior of 12P/Pons-Brooks during its 2024 apparition. The water production rate of 12P, derived from the 18 cm OH lines, is consistent with measurements obtained in other works. The possible detection of NH₃ during an outburst suggests possible connections between the subsurface volatile reservoir and the outburst mechanism. These results could further our understanding of the composition and activity of Halley-type comets.