Sensitivity of gas-grain chemical models to surface reaction barriers: Effect from a key carbon-insertion reaction, C + H$_2$ $\rightarrow$ CH$_2$

Abstract

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 barrier E a of the important surface reaction between some of the most abundant species: C and H 2 (surface C + surface H 2 → surface CH 2 ). Methods. We used the gas-grain code ALCHEMIC to model the time-dependent chemical evolution over a 2D grid of densities (n H ∈ 10 3 , 10 12 cm −3 ) and temperatures (T ∈ 10, 300 K), assuming UV-dark (A V = 20 mag) and partly UV-irradiated (A V = 3 mag) conditions that are typical of the dense interstellar medium. We considered two values for the energy barrier of the surface reaction, E a = 2 500 K (as originally implemented in the networks) and E a = 0 K (as measured in the laboratory and computed by quantum chemistry simulations). Results. We find that if the C + H 2 → CH 2 surface reaction is barrierless, a more rapid conversion of the surface carbon atoms into methane ice occurs. Overproduction of the CH n hydrocarbon 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 (C 1 ) and di-carbonic (C 2 ) hydrocarbons, CO 2 , CN, HCN, HNC, HNCO, CS, H 2 CO, H 2 CS, CH 2 CO, and CH 3 OH (in either gas and/or ice). The abundances of key species, CO, H 2 O, and N 2 as well as O, HCO + , N 2 H + , NH 3 , 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.