2021A&A...650L...9C

We report the first detection in space of the cumulene carbon chain l-H₂C₅. A total of eleven rotational transitions, with J_up=7-10 and K_a=0 and 1, were detected in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40 m radio telescope. We derived a column density of (1.8±0.5)x10¹⁰cm^–2. In addition, we report observations of other cumulene carbenes detected previously in TMC-1 in order to compare their abundances with the newly detected cumulene carbene chain. We find that l-H₂C₅ is ∼4.0 times less abundant than the larger cumulene carbene l-H₂C₆, while it is ∼300 and ∼500 times less abundant than the shorter chains l-H₂C₃ and l-H₂C₄. We discuss the most likely gas-phase chemical routes to these cumulenes in TMC-1 and stress that chemical kinetics studies able to distinguish between different isomers are needed to shed light on the chemistry of C_nH₂ isomers with n > 3.