Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges

Abstract

We survey the current situation regarding chemical modelling of the synthesis of molecules in the interstellar medium. The present state of knowledge concerning the rate coefficients and their uncertainties for the major gas-phase processes-ion-neutral reactions, neutral-neutral reactions, radiative association, and dissociative recombination-is reviewed. Emphasis is placed on those key reactions that have been identified, by sensitivity analyses, as 'crucial' in determining the predicted abundances of the species observed in the interstellar medium. These sensitivity analyses have been carried out for gas-phase models of three representative, molecule-rich, astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the expanding circumstellar envelope IRC +10216. Our review has led to the proposal of new values and uncertainties for the rate coefficients of many of the key reactions. The impact of these new data on the predicted abundances in TMC-1 and L134N is reported. Interstellar dust particles also influence the observed abundances of molecules in the interstellar medium. Their role is included in gas-grain, as distinct from gas-phase only, models. We review the methods for incorporating both accretion onto, and reactions on, the surfaces of grains in such models, as well as describing some recent experimental efforts to simulate and examine relevant processes in the laboratory. These efforts include experiments on the surface-catalyzed recombination of hydrogen atoms, on chemical processing on and in the ices that are known to exist on the surface of interstellar grains, and on desorption processes, which may enable species formed on grains to return to the gas-phase.

Surface formation routes of interstellar molecules: hydrogenation reactions in simple ices

“…The general conclusion that the three channels (O/O 2 / O 3 ? H) are strongly linked, is of importance for astrochemical models focusing on water formation under interstellar conditions (Wakelam et al 2010). …”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…Model results are then compared with observational results to attempt to verify whether results are valid. 24 Modelling the chemistry in astrophysical environments is normally done using one of the following techniques: 1) the rate equation approach, 2) the master equation approach, 3) kinetic Monte Carlo method. [20][21][25][26] Each has its own advantages but in this research a rate equation approach will be utilized.…”

“…In this method, each reaction in the network is assigned a rate coefficient which is usually temperature dependent. For a gas phase reaction between two neutral species or an ion and a neutral, the rate coefficient will follow a modified Arrhenius rate law as follows: 24,27 (1.1) k(T) = α * ( T 300 )…”

“…These parameters are generally determined experimentally and sometimes it's necessary to extrapolate values outside the set temperature range, which leads to some uncertainty in determining the rate constant. 24,28 We'll be using the chemical model Nautilus which has over 11,000 reactions and over 700 species. We've added 316 reactions and 18 species (see tables in appendix).…”

“…In this case the potential is anisotropic and depends on both the distance between reactants and the orientation of the permanent dipole, the long-range force is no longer central in nature. 24 For the wide temperature range used here a method adopted from the trajectory scaling model of Su & Chesnavich is used. 31 This approach is also described by Herbst & Woon and is based on the following unitless parameter x: 32…”