Discovery of linear propadienone

We report the first detection in space of propadienone, the linear isomer (l-H 2 C 3 O) of cyclopropenone (c-H 2 C 3 O). We also report the first detection of the isotopologues c-H 2 13 CCCO and c-HDCCCO of c-H 2 C 3 O. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0–50.3 GHz, complemented with data between 71.6–116.0 GHz, and carried out with Yebes-40 m and IRAM-30 m telescopes, respectively. We obtain a total column density of 3.7 × 10 10 cm −2 for l-H 2 C 3 O at an excitation temperature of 4.8 K. We find that the isomer is about eight times less abundant than the cyclic one. We also report a detailed line-byline study of cyclopropenethione (c-H 2 C 3 S) to compare the abundance of the O and S isomers. We find that cyclic O-isomers are more abundant than cyclic S-isomers; however, the opposite trend is found for the most stable linear isomers, with l-H 2 C 3 S being more than one order of magnitude more abundant than l-H 2 C 3 O. A comprehensive theoretical chemical analysis shows that the abundances of the H 2 C 3 O and H 2 C 3 S isomers are controlled by different formation pathways. In particular, while l-H 2 C 3 O is potentially produced by dissociative electron recombination reactions, ion-neutral chemistry is more effective at producing l-H 2 C 3 S and c-H 2 C 3 S.