2022A&A...663L...2C

Cyclic C₅H (c-C₅H), the radical formed by substituting an ethynyl group CCH for the hydrogen atom in the c-C₃H radical, has been detected for the first time in the space. The c-C₅H species is an isomer of the well-known linear radical l-C₅H and is ∼6 kcal mol^–1 less stable. A total of 17 rotational transitions were detected for the c-C₅H species in TMC-1 within the 31.0-50.3 GHz range using the Yebes 40m radio telescope. We derive a column density of (9.0 ± 0.9) x 10¹⁰ cm^–2 for c-C₅H. Additionally, we observed 12 lines for l-C₅H and derive a column density for it of (1.3 ± 0.3) x 10¹² cm^–2, which results in an abundance ratio c-C₅H/l-C₅H of 0.069. This is in sharp contrast with the value found for the analogue system c-C₃H/l-C₃H, whose ratio is 5.5 in TMC-1. We discuss the formation of c-C₅H and conclude that this radical is probably formed in the reaction of atomic carbon with diacetylene.