The Evolution Of C4H And c-C3H2 In Molecular Cores

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. We present on-the-fly mapping observations of C₄H 9-8 lines, c-C₃H₂ 2-1, H¹³CO⁺ 1-0, and H42 toward a sample of 22 massive star-forming regions using the IRAM 30m telescope.

Our aim is to further explore the evolution of these carbon-chain molecules by combining observational results obtained in cold cores. We employed H¹³CO⁺ 1-0 and H42a as tracers to probe the positions of molecular cloud cores and ionised hydrogen regions (HII 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. 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 H42a emission regions. The C₄H/H¹³CO⁺ and c-C₃H₂/H¹³CO⁺ relative abundance ratios range from 0.17 to 1.77 and 1.42 to 6.69, 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.

The decreasing trends in C₄H/H¹³CO⁺ and c-C₃3H₂/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 H42a regions further supports their role as precursors in the chemical pathways that lead to complex organic molecules in the interstellar medium.

Yijia Liu, Junzhi Wang, Ningyu Tang, Yajiang Lu, Donghui Quan, Juan Li, Kai Yang, Shu Liu, Yuqiang Li, Siqi Zheng, Chao Ou

Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2605.08790 [astro-ph.GA] (or arXiv:2605.08790v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2605.08790
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Journal reference: A&A, 708, A254 (2026)
Related DOI:
https://doi.org/10.1051/0004-6361/202659367
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Submission history
From: Yijia Liu
[v1] Sat, 9 May 2026 08:21:00 UTC (2,292 KB)
https://arxiv.org/abs/2605.08790

Astrobiology, astrochemistry, astrophysics,