Discovery of the cyclic C5H radical in TMC-1

TL;DR

This paper reports the first detection of the cyclic C5H radical in space, measuring its abundance and comparing it to its linear isomer, providing insights into its formation in the interstellar medium.

Contribution

First detection of cyclic C5H in space, with detailed spectral data and abundance analysis, expanding understanding of interstellar carbon chain radicals.

Findings

Detected 17 rotational transitions of c-C5H in TMC-1

Derived a column density of (9.0 +/- 0.9)e10 cm-2 for c-C5H

Abundance ratio c-C5H/l-C5H is 0.069

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 6 kcal/mol 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)e10 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)e12 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.