Stability of CH₃NCO in astronomical ices under energetic processing. A laboratory study

Methyl isocyanate (CH₃NCO) was recently found in hot cores and suggested on comet 67P/CG. The incorporation of this molecule into astrochemical networks requires data on its formation and destruction. In this work, ices of pure CH₃NCO and of CH₃NCO(4-5%)/H₂O mixtures deposited at 20 K were irradiated with a UV D₂ lamp (120-400 nm) and bombarded by 5 keV electrons to mimic the secondary electrons produced by cosmic rays (CRs). The destruction of CH₃NCO was studied using IR spectroscopy. After processing, the ν_a-NCO band of CH₃NCO disappeared and IR bands corresponding to CO, CO₂, OCN^- and HCN/CN^- appeared instead. The products of photon and electron processing were very similar. Destruction cross sections and half-life doses were derived from the measurements. Water ice provides a good shield against UV irradiation (half-life dose of ~ 64 eV molecule^-1 for CH₃NCO in water-ice), but not so good against high-energy electrons (half-life dose ~ 18 eV molecule^-1). It was also found that CH₃NCO does not react with H₂O over the 20-200 K temperature range. These results indicate that hypothetical CH₃NCO in the ices of dense clouds should be stable against UV photons and relatively stable against CRs over the lifetime of a cloud (~ 10⁷ yr), and could sublime in the hot core phase. On the surface of a Kuiper belt object (the original location of comet 67P/CG) the molecule would be swiftly destroyed, both by photons and CRs, but embedded below just 10 microm of water-ice, the molecule could survive for ~ 10⁹ yr.