Sensitivity of gas-grain chemical models to surface reaction barriers

M Simončič,Th Henning,C Jäger,S Krasnokutski,D Semenov

doi:10.1051/0004-6361/202037648

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 Ea 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 (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) and Ea = 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 CHn 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.

Highlights

To date, more than 200 interstellar molecular species have been discovered1 (McGuire 2018)
A variety of complex organic molecules such as CH3OH, CH3CN, HCOOCH3, CH3CHO, and CH3OCH3 have been found in the interstellar medium (ISM)
We recall that the same molecule can fall into both categories of the species with the highest increased or decreased abundances because it depends on physical conditions

Summary

Introduction

More than 200 interstellar molecular species have been discovered (McGuire 2018). These molecules include terrestrial-like stable molecules, more exotic unstable radicals, ions, complex species, macromolecules, and various ices. Among these complex species, a variety of complex organic molecules such as CH3OH, CH3CN, HCOOCH3, CH3CHO, and CH3OCH3 have been found in the interstellar medium (ISM). CO, CS, carbon chains, and cyanopolyynes are mainly formed through ion-molecule and dissociative recombination reactions in the gas phase. Some simple key molecules such as H2, NH3, H2O, CH4, and most of the complex organics are mainly synthesized through

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Discussion

Conclusion