摘要：Context.The feasibility of contemporary gas-grain astrochemical models depends on the availability of accurate kinetics data, in particular, for surface processes.Aims.We study the sensitivity of gas-grain chemical models to the energy barrierEaof the important surface reaction between some of the most abundant species: C and H2(surface C + surface H2→ surface CH2).Methods.We used the gas-grain code ALCHEMIC to model the time-dependent chemical evolution over a 2D grid of densities (nH∈ 103, 1012 cm−3) and temperatures (T∈ 10, 300 K), assuming UV-dark (AV= 20 mag) and partly UV-irradiated (AV= 3 mag) conditions that are typical of the dense interstellar medium. We considered two values for the energy barrier of the surface reaction,Ea= 2500 K (as originally implemented in the networks) andEa= 0 K (as measured in the laboratory and computed by quantum chemistry simulations).Results.We find that if the C + H2→ CH2surface reaction is barrierless, a more rapid conversion of the surface carbon atoms into methane ice occurs. Overproduction of the CHnhydrocarbon ices affects the surface formation of more complex hydrocarbons, cyanides and nitriles, and CS-bearing species at low temperatures ≲10−15 K. The surface hydrogenation of CO and hence the synthesis of complex (organic) molecules become affected as well. As a result, important species whose abundances may change by more than a factor of two at 1 Myr include atomic carbon, small mono-carbonic (C1) and di-carbonic (C2) hydrocarbons, CO2, CN, HCN, HNC, HNCO, CS, H2CO, H2CS, CH2CO, and CH3OH (in either gas and/or ice). The abundances of key species, CO, H2O, and N2as well as O, HCO+, N2H+, NH3, NO, and most of the S-bearing molecules, remain almost unaffected.Conclusions.Further accurate laboratory measurements and quantum chemical calculations of the surface reaction barriers will be crucial to improve the accuracy of astrochemical models.
