2021A&A...647L...3C

We present the discovery in TMC-1 of allenyl acetylene, H₂CCCHCCH, through the observation of nineteen lines with a signal-to-noise ratio ∼4-15. For this species, we derived a rotational temperature of 7±1K and a column density of 1.2±0.2x10¹³cm^–2. The other well known isomer of this molecule, methyl diacetylene (CH₃C₄H), has also been observed and we derived a similar rotational temperature, T_r=7.0±0.3K, and a column density for its two states (A and E) of 6.5±0.3x10¹²cm^–2. Hence, allenyl acetylene and methyl diacetylene have a similar abundance. Remarkably, their abundances are close to that of vinyl acetylene (CH₂CHCCH). We also searched for the other isomer of C₅H₄, HCCCH₂CCH (1.4-Pentadiyne), but only a 3σ upper limit of 2.5x10¹²cm^–2 to the column density can be established. These results have been compared to state-of-the-art chemical models for TMC-1, indicating the important role of these hydrocarbons in its chemistry. The rotational parameters of allenyl acetylene have been improved by fitting the existing laboratory data together with the frequencies of the transitions observed in TMC-1.