The evolution of C₄H and c-C₃H₂ in molecular cores

¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
² Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, PR China
³ Research Center for Computational Earth and Space Science, Zhejiang Laboratory, Hangzhou 311100, PR China
⁴ Department of Physics, Xi’an Jiaotong-Liverpool University, 111 Ren’ai Road, Dushu Lake Science and Education Innovation District, Suzhou 215123, Jiangsu Province, PR China
⁵ State Key Laboratory of Radio Astronomy and Technology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁶ Department of Astronomy, School of Physics and Astronomy, and Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai 200240, PR China
⁷ State Key Laboratory of Dark Matter Physics, Shanghai Jiao Tong University, Shanghai 200240, PR China
⁸ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁹ Korea Astronomy and Space Science Institute, No. 776, Daedeok-daero, Yuseong-gu, Daejeon, Republic of Korea
¹⁰ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹¹ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, PR China

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

Received: 7 February 2026
Accepted: 2 March 2026

Abstract

Context. Linear C₄H and cyclic c-C₃H₂, as small unsaturated hydrocarbons, are the key precursors to complex organic molecules and are critical components of the interstellar medium. However, observational constraints on the evolution of these molecules in late-stage massive star-forming regions remain scarce.

Aims. We present on-the-fly mapping observations of C₄H 9–8 lines, c-C₃H₂ 2–1, H¹³CO⁺ 1–0, and H42α toward a sample of 22 massive star-forming regions using the IRAM 30 m telescope. Our aim is to further explore the evolution of these carbon-chain molecules by combining observational results obtained in cold cores.

Methods. We employed H¹³CO⁺ 1–0 and H42α as tracers to probe the positions of molecular cloud cores and ionised hydrogen regions (H II regions), respectively. One chemical model in particular, which includes gas, dust grain surface, and icy mantle phases for C₄H and c-C₃H₂ molecules, was used to make comparisons with observed abundances.

Results. From mapping observations targeting 31 regions across 22 sources, C₄H 9–8 (J = 19/2–17/2) and C₄H 9–8 (J = 17/2–15/2) were detected in only 17 regions, while H¹³CO⁺ 1–0 and c-C₃H₂ 2–1 were successfully detected in all 31 regions. We find that the emission of C₄H 9–8 and c-C₃H₂ 2–1 is concentrated at the edges of H42α emission regions. The C₄H/H¹³CO⁺ and c-C₃H₂/H¹³CO⁺ relative abundance ratios range from 0.17 to 1.77 (median ~0.57) and 1.42 to 6.69 (median ~4.19), respectively, with a median C₄H/c-C₃H₂ ratio of 0.13. By combining the observational results of cold cores, we find that C₄H/H¹³CO⁺ and c-C₃H₂/H¹³CO⁺ ratios show a strong decreasing trend as molecular cores evolve.

Conclusions. The decreasing trends in C₄H/H¹³CO⁺ and c-C₃H₂/H¹³CO⁺ ratios imply that small unsaturated hydrocarbons can be consumed and converted into other organic molecules during the evolution of molecular cores. The spatial concentration of C₄H and c-C₃H₂ emission at the edges of H42α regions further supports their role as precursors in the chemical pathways that lead to complex organic molecules in the interstellar medium.