Discovery of CH3CHCO in TMC-1 with the QUIJOTE line survey

We report the detection of methyl ketene towards TMC-1 with the QUIJOTE line survey. Nineteen rotational transitions with rotational quantum numbers ranging from J = 3 up to J = 5 and Ka ≤ 2 were identified in the frequency range 32.0–50.4 GHz, 11 of which arise above the 3σ level. We derived a column density for CH3CHCO of N = 1.5 × 1011 cm−2 and a rotational temperature of 9 K. Hence, the abundance ratio between ketene and methyl ketene, CH2CO/CH3CHCO, is 93. This species is the second C3H4O isomer detected. The other, trans-propenal (CH2CHCHO), corresponds to the most stable isomer and has a column density of N = (2.2 ± 0.3)×1011 cm−2, which results in an abundance ratio CH2CHCHO/CH3CHCO of 1.5. The next non-detected isomer with the lowest energy is cis-propenal, which is therefore a good candidate for future discovery. We have carried out an in-depth study of the possible gas-phase chemical reactions involving methyl ketene to explain the abundance detected, achieving good agreement between chemical models and observations.