Detection and modelling of CH3NC in TMC-1

Abstract

ABSTRACT Two closely related isomeric pairs of cyanides, CH3[CN/NC] and H2C[CN/NC], are studied in cold, dark interstellar cloud conditions. In contrast to the diverse detections of methyl cyanide (CH3CN) in space, methyl isocyanide (CH3NC) has previously only been observed in warm and hot star-forming regions. We report the detection of CH3NC in the cold pre-stellar core Taurus Molecular Cloud (TMC-1) using the Green Bank Telescope with a detection significance of 13.4σ. Hyperfine transitions in H2CCN and quadrupole interactions in CH3CN and CH3NC were matched to a spectral line survey from the Green Bank Telescope Observations of TMC-1: Hunting for Aromatic Molecules large project on the Green Bank Telescope, resulting in abundances with respect to hydrogen of $1.92^{+0.13}_{-0.07} \times 10^{-9}$ for the cyanomethyl radical (H2CCN), $5.02^{+3.08}_{-2.06} \times 10^{-10}$ for CH3CN, and $2.97^{+2.10}_{-1.37} \times 10^{-11}$ for CH3NC. Efforts to model these molecules with the three-phase gas-grain code nautilus in TMC-1 conditions overproduce both CH3CN and CH3NC, though the ratio of ∼5.9 per cent is consistent across observations and models of these species. This may point to missing destruction routes in the model. The models capture the larger abundance of H2CCN well. Dissociative recombination is found to be the primary production route for these molecules, and reactions with abundant ions are found to be the primary destruction routes. H + CH3NC is investigated with transition state theory as a potential destruction route, but found to be too slow in cold cloud conditions to account for the discrepancy in modelled and observed abundances of CH3NC.