Sensitivity of gas-grain chemical models to surface reaction barriers

Effect from a key carbon-insertion reaction, C + H₂ → CH₂

¹ Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, 1000 Ljubljana, Slovenia
e-mail: matjaz.simoncic@fkkt.uni-lj.si
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: semenov@mpia.de
³ Department of Chemistry, Ludwig Maximilian University, Butenandtstr. 5-13, 81377 Munich, Germany
⁴ Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute for Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany

Received: 3 February 2020
Accepted: 20 March 2020

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 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 (n_H ∈ 10³, 10¹² 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 = 2500 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 + H₂ → CH₂ 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 (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 N₂ as 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.