  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 ab initio calculations indicate that the best
possible candidate is the acetyl cation, CH3CO+, 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 Ju = 10 up to Ju =
27. Hence, we report the discovery of CH3CO+ in space based on our
observations, theoretical calculations, and laboratory experiments. The derived
rotational and distortion constants allow us to predict the spectrum of CH3CO+
with high accuracy up to 500 GHz. We derive an abundance ratio
N(H2CCO)/N(CH3CO+) = 44. The high abundance of the protonated form of H2CCO is
due to the high proton affinity of the neutral species. The other isomer,
H2CCOH+, is found to be 178.9 kJ/mol above CH3CO+. The observed intensity ratio
between the K=0 and K=1 lines, 2.2, strongly suggests that the A and E symmetry
states have suffered interconversion processes due to collisions with H and/or
H2, or during their formation through the reaction of H3+ with H2CCO.
