Discovery of the acetyl cation, CHCO, in space and in the laboratory.

Using the Yebes 40m and IRAM 30m radiotelescopes, we detected two series of harmonically related lines in space that can be fitted to a symmetric rotor. The lines have been seen towards the cold dense cores TMC-1, L483, L1527, and L1544. High level of theory calculations indicate that the best possible candidate is the acetyl cation, CHCO, which is the most stable product resulting from the protonation of ketene. We have produced this species in the laboratory and observed its rotational transitions = 10 up to = 27. Hence, we report the discovery of CHCO in space based on our observations, theoretical calculations, and laboratory experiments. The derived rotational and distortion constants allow us to predict the spectrum of CHCO with high accuracy up to 500 GHz. We derive an abundance ratio (HCCO)/(CHCO)~44. The high abundance of the protonated form of HCCO is due to the high proton affinity of the neutral species. The other isomer, HCCOH, is found to be 178.9 kJ mol above CHCO. The observed intensity ratio between the =0 and =1 lines, ~2.2, strongly suggests that the and symmetry states have suffered interconversion processes due to collisions with H and/or H, or during their formation through the reaction of with HCCO.