Letter to the Editor

First detection of radio recombination lines of ions heavier than helium

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China
e-mail: liuxunchuan@shao.ac.cn; liutie@shao.ac.cn; zshen@shao.ac.cn
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712-1205, USA
⁴ Department of Astronomy, Yunnan University, Kunming 650091, PR China
⁵ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, PR China
⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁷ Institute of Astronomy and Astrophysics, Academia Sinica, Roosevelt Road, Taipei 10617, Taiwan, R.O.C.
⁸ Center for Intelligent Computing Platforms, Zhejiang Laboratory, Hangzhou 311100, PR China
⁹ Nobeyama Radio Observatory, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 462-2 Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305, Japan
¹⁰ Institute of Astronomy and Astrophysics, School of Mathematics and Physics, Anqing Normal University, Anqing, PR China
¹¹ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China
¹² Physical Research Laboratory, Navrangpura, Ahmedabad, Gujarat, 380009, India

Received: 13 January 2023
Accepted: 7 February 2023

Abstract

We report the first detection of radio recombination lines (RRLs) of ions heavier than helium. In a highly sensitive multi-band (12–50 GHz) line survey toward Orion KL with the TianMa 65-m Radio Telescope (TMRT), we successfully detected more than fifteen unblended α lines of RRLs of singly ionized species (XII) recombined from XIII. The Ka-band (26–35 GHz) spectrum also shows tentative signals of β lines of ions. The detected lines can be successfully crossmatched with the rest frequencies of RRLs of C II and/or O II. This finding greatly expands the scope of our understanding of ion RRLs, since prior to this work, only two blended lines (105α and 121α) of He II had been reported. Our detected lines can be fitted simultaneously under assumption of local thermodynamic equilibrium (LTE). An abundance of C III and O III of 8.8×10⁻⁴ was obtained, avoiding the complexities of optical and infrared observations and the blending of RRLs of atoms. It is consistent with but approaches the upper bound of the value (10⁻⁴–10⁻³) estimated from optical and infrared observations. The effects of dielectronic recombination may contribute to enhancing the level populations even at large n. We expect future observations using radio interferometers could break the degeneracy between C and O, while also helping to reveal the ionization structure and dynamical evolution of various ionized regions.