CH3CN photochemistry at hot core margins

Abstract

CH3CN is a species observed to be much more abundant in ultracompact hot molecular cores (UCHCs) than can be accounted for by dark cloud gas phase chemistry. The close proximity of UCHCs to newly formed OB stars suggests that an impinging far ultraviolet (FUV) radiation field might contribute to the chemical processing at the core margins. In this paper, the photochemistry of gas phase CH3CN formation is explored assuming an enhanced FUV radiation field interacts with dense neutral material evaporated from grain mantle ices. We are able to show that high CH3CN/CH3OH abundance ratios can be generated from CH3OH and NH3 precursors in such a photon‐dominated margin as a realistic alternative to a grain surface origin. We show that the observed CH3CN/CH3OH column density ratio towards the UCHC in G34.3+0.15 can be interpreted as arising from photochemical generation of CH3CN in the gas phase.

Pressure dependent low temperature kinetics for CN + CH3CN: competition between chemical reaction and van der Waals complex formation

“…In ionospheric models, such as those developed by Krasnopolsky, 51 some reactions with cations (HCNH + , C 2 H 5 + ) are also sink processes for CH 3 CN. In the interstellar medium, gas phase formation of CH 3 CN is anticipated to come from three processes: CN À + CH 3 -CH 3 CN + e À , CH 3 CNH + + e À -CH 3 CN + H and a radiative association mechanism CN + CH 3 -CH 3 CN + hn. [52][53][54] However, gas-grain chemistry is expected to also provide a significant route for acetonitrile formation either from direct addition of CH 3 and CN or from successive hydrogenations starting with C 2 N. 55 It is worth noting here that laboratory experiments on ice analogs demonstrated quite recently that acetonitrile can be formed via VUV irradiation of a pure ethylamine (C 2 H 5 NH 2 ) ice maintained at 20 K, 17 a route not presently considered in astrochemical networks to our knowledge. Further evaporation of the grain surface can then release CH 3 CN in the gas phase environment provided that the grain surface is heated up to about 90 K. On the other hand, potential processes for destruction of acetonitrile include dissociation and ionization by cosmic rays or photons and a large number of ion-CH 3 CN reactions (see the KIDA and the UMIST rate12 databases at respectively http://kida.obs.u-bordeaux1.fr and http://udfa.ajmarkwick.net/ for details).…”