Maté B, Molpeceres G, Tanarro I, Peláez RJ, Guillemin JC, Cernicharo J, Herrero VJ. Stability of CH
3NCO in astronomical ices under energetic processing. A laboratory study.
THE ASTROPHYSICAL JOURNAL 2018;
861:61. [PMID:
30185993 PMCID:
PMC6120682 DOI:
10.3847/1538-4357/aac826]
[Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
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 νa-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 μm of water-ice, the molecule could survive for ~ 109 yr.
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