Discovery of the cyclic C5H radical in TMC-1

Authors: Cabezas, C.; Agundez, M.; Fuentetaja, R.; Endo, Y.; Marcelino, N.; Tercero, B.; Pardo, J. R.; de Vicente, P.; Cernicharo, J.

Journal: ASTRONOMY & ASTROPHYSICS

Publication date: 2022/07/01

DOI: 10.1051/0004-6361/202244063

Abstract: 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 similar to 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 10(10) 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 10(12) 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.