2017MNRAS.469.2219L

Methyl isocyanate (CH₃NCO) belongs to a select group of interstellar molecules considered to be relevant precursors in the formation of larger organic compounds, including those with peptide bonds. The molecule has only been detected in a couple of high-mass protostars and potentially on comets. A formation route on icy grains has been postulated for this molecule but experimental evidence is lacking. Here we extend the range of environments where methyl isocyanate is found and unambiguously identify CH₃NCO through the detection of 43 unblended transitions in the ALMA Protostellar Interferometric Line Survey (PILS) of the low-mass solar-type protostellar binary IRAS 16293-2422. The molecule is detected towards both components of the binary with a ratio HNCO/CH₃NCO ∼ 4-12. The isomers CH₃CNO and CH₃OCN are not identified, resulting in upper abundance ratios of CH₃NCO/CH₃CNO > 100 and CH₃NCO/CH₃OCN > 10. The resulting abundance ratios compare well with those found for related N-containing species towards high-mass protostars. To constrain its formation, a set of cryogenic UHV experiments is performed. VUV irradiation of CH₄:HNCO mixtures at 20 K strongly indicate that methyl isocyanate can be formed in the solid state through CH₃ and (H)NCO recombinations. Combined with gas-grain models that include this reaction, the solid-state route is found to be a plausible scenario to explain the methyl isocyanate abundances found in IRAS 16293-2422.