Discovery of allenyl acetylene, H2CCCHCCH, in TMC-1

Abstract

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 1 K and a column density of 1.2 0.2 × 1013 cm-2. The other well known isomer of this molecule, methyl diacetylene (CH3C4H), has also been observed and we derived a similar rotational temperature, Tr  =  7.0 0.3 K, and a column density for its two states (A and E) of 6.5 0.3 × 1012 cm-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 3 upper limit of 2.5 × 1012 cm-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.