Methyl isocyanate in Orion

Methyl isocyanate (CH_3_NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified.

We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH_3_NCO in Orion is only a factor of ten below those of HNCO and CH_3_CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface.