A 3 mm molecular line survey toward IRC+10216

¹ School of Physics and Astronomy, Sun Yat-sen University, 2 Daxue Road, Tangjia, Zhuhai 519082, Guangdong Province, China
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi 830011, China
³ CSST Science Center for the Guangdong-Hongkong-Macau Greater Bay Area, Sun Yat-Sen University, Guangdong Province, China
⁴ School of Mathematics and Physics, Jinggangshan University, 28 Xueyuan Road, Qingyuan District, Ji’an 343009, Jiangxi Province, China
⁵ Xinjiang Key Laboratory of Radio Astrophysics, 150 Science 1-Street, Urumqi, Xinjiang 830011, China
⁶ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, China

^★ Corresponding author: zhangyong5@email.sysu.edu.cn

Received: 24 March 2025
Accepted: 13 May 2025

Abstract

Context. IRC+10216 is the brightest infrared source in the northern sky, known for its rich chemical composition. It is often used as a standard reference for studying the circumstellar envelope (CSE) of carbon-rich stars. While pioneering 3 mm spectral surveys have laid foundational datasets, their system temperature limitations rendered spectral line detection thresholds inadequate for probing the source’s complex organic molecule inventory at this band, which made superseding observations necessary.

Aims. We aim to gain an unbiased view regarding circumstellar chemistry and investigate whether IRC+10216 is typical or anomalous in terms of its chemical composition.

Methods. We carried out an in-depth spectral line survey of the circumstellar envelope of IRC+10216 utilizing the Arizona Radio Observatory 12m telescope. We achieved complete spectral sampling across the 90-116 GHz atmospheric window (λ = 2.6-3.3 mm). Results. A total of 214 emission lines belonging to 43 molecular species are identified in the CSE of IRC+10216, among which 28 lines are newly detected in this object and four emission lines remain unidentified. The excitation temperatures and column densities of 16 molecules are determined through rotation diagrams. We estimate the isotopic ratios of carbon, oxygen, and silicon elements. For the majority of the molecular species, the line intensity ratios between IRC+10216 and CIT 6 are inversely proportional to the square of their distance, which suggests that the chemical processes occurring within them are similar. Nevertheless, there is evidence suggesting that the emission of C₄H and C₃N in IRC+10216 is unusually strong.

Conclusions. These observations stand as the most sensitive and unbiased line survey of IRC+10216 within the λ = 3 window carried out by a single-dish telescope. They offer a valuable reference for the astronomical community. They can facilitate comparative studies of the circumstellar chemistry of carbon-rich evolved stars and can act as a guiding framework for sensitive interferometric molecular mappings.