A spectral-line survey of CIT 6 between 30 and 50 GHz with the eQ receiver at NRO 45 m

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
² Simons Astrophysics Group at ICISE, Institute for Interdisciplinary Research in Science and Education (IFIRSE), ICISE, 07 Science Avenue, Quy Nhon Nam, Gia Lai, Vietnam
³ Institute for Astronomy and Space Quantum Communications (IAS), Vietnam
⁴ National Astronomical Observatory of Japan (NAOJ), National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Graduate Institute for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ Department of Astronomy, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
⁷ Institute of Physics, Vietnam Academy of Science and Technology, 10 Dao Tan, Giang Vo, Hanoi, Vietnam
⁸ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, National Taiwan University, No. 1, Sec., Roosevelt Road, Taipei 10617, Taiwan
⁹ Faculty of Social Information Studies, Otsuma Women’s University, Chiyoda-ku, Tokyo 102-8357, Japan
¹⁰ Department of Astronomy and Earth Sciences, Tokyo Gakugei University, 4-1-1 Nukuikitamachi, Koganei, Tokyo 184-8501, Japan
¹¹ Center for Astronomy, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
¹² Nobeyama Radio Observatory, National Astronomical Observatory of Japan (NAOJ), National Institutes of Natural Sciences (NINS), 462-2 Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305, Japan
¹³ Department of Physics, Graduate School of Science, Osaka Metropolitan University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
¹⁴ Institute of Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
¹⁵ The American University of Paris, 2bis, Passage Landrieu, 75007 Paris, France
¹⁶ Center for Space Technology Research and Development, Vietnam National Space Center, Hanoi, Vietnam

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 November 2025
Accepted: 12 December 2025

Abstract

Context. At the end of the long H-burning phase (main sequence), low-mass to intermediate-mass stars evolve into asymptotic giant branch (AGB) stars that can have carbon-rich envelopes depending on the initial C/O ratio. In their circumstellar envelopes (CSEs), dust particles and molecules are formed and shed back to the interstellar space. Therefore, these stars significantly contribute to the galactic astro-chemical evolution.

Aims. To shed light on the chemical properties of carbon-rich CSEs, especially carbon- and silicon-bearing molecules, we performed parallel spectral-line surveys of IRC+10216 and CIT 6, the brightest and second-brightest carbon-rich star envelopes on the sky.

Methods. We conducted 30−50 GHz observations towards both sources using a high-sensitivity-wide-band extended Q-band receiver (eQ) of the Nobeyama 45−m telescope. We then analysed data of CIT 6 and used data of IRC+10216 for comparison purposes. We applied the rotational-diagram method to derive their rotational temperatures and column densities for HC₅N and HC₇N. For other molecules, we assumed an excitation temperature to derive their column densities.

Results. Molecular column densities in CIT 6 are systematically lower than those of IRC+10216, typically by one order of magnitude. Silicon- and sulfur-bearing species such as SiS and CS show the strongest depletion, whereas carbon-chain molecules (HC_nN, C₆H) remain relatively prominent, indicating that carbon-chain formation is still efficient in CIT 6. Rotational temperatures are higher in CIT 6, which is consistent with the fact that emission arises from warmer and more compact regions of its envelope. Both sources show low ¹²C/¹³C ratios and mildly sub-solar ²⁸Si/²⁹Si values, which are non-solar isotopic ratios.

Conclusions. Both envelopes display canonical, carbon-rich AGB chemistry and comparable isotopic compositions. CIT 6, however, shows slightly higher excitation temperatures, stronger carbon-chain growth, and deeper depletion of Si- and S-bearing species. These signatures point to a more evolved circumstellar environment, where dust condensation and shock processing further modulated the molecular composition.