Discovery of linear propadienone: Study of the chemistry of linear and cyclic H${}_2$C${}_3$O and H${}_2$C${}_3$S in TMC-1

Abstract

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 isotopologue c-H${}_2$${}^{13}$CCCO, and c-HDCCCO of c-H${}_2$C${}_3$O. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the frequency range 31.0-50.3 GHz, complemented with data between 71.6-116.0 GHz, and carried out with Yebes-40m and IRAM-30m telescopes, respectively. We obtain a total column density of 3.7$\times$10${}^{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-by-line 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.