Detection of methyl isocyanate (CH₃NCO) in a solar-type protostar

Abstract

We report the detection of the pre-biotic molecule CH₃NCO in a solar-type protostar, IRAS16293-2422 B. A significant abundance of this species on the surface of the comet 67P/Churyumov-Gerasimenko has been proposed, and it has recently been detected in hot cores around high-mass protostars. We observed IRAS16293-2422 B with the Atacama Large Millimeter Array in the 90 to 265 GHz range, and detected eight unblended transitions of CH₃NCO. From our Local Thermodynamic Equilibrium analysis, we derived an excitation temperature of 110 ± 19 K and a column density of (4.0 ± 0.3) × 10¹⁵ cm^-2, which results in an abundance of ≤(1.4 ± 0.1) × 10^-10 with respect to molecular hydrogen. This implies a CH₃NCO/HNCO and CH₃NCO/NH₂CHO column density ratios of ∼0.08. Our modelling of the chemistry of CH₃NCO suggests that both ice surface and gas phase formation reactions of this molecule are needed to explain the observations.