2023A&A...671L...6F

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 K_a ≤ 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.