Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model

Abstract

We present an extended version of the 2-phase gas-grain code NAUTILUS to the 3phase modelling of gas and grain chemistry of cold cores. In this model, both the mantle and the surface are considered as chemically active. We also take into account the competition among reaction, diffusion and evaporation. The model predictions are confronted to ice observations in the envelope of low-mass and massive young stellar objects as well as toward background stars. Modelled gas-phase abundances are compared to species observed toward TMC-1 (CP) and L134N dark clouds. We find that our model successfully reproduces the observed ice species. It is found that the reaction-diffusion competition strongly enhances reactions with barriers and more specifically reactions with H 2 , which is abundant on grains. This finding highlights the importance to have a good approach to determine the abundance of H 2 on grains. Consequently, it is found that the major N-bearing species on grains go from NH 3 to N 2 and HCN when the reaction-diffusion competition is accounted. In the gasphase and before few 10 5 yrs, we find that the 3-phase model does not have a strong impact on the observed species compared to the 2-phase model. After this time, the computed abundances dramatically decrease due to the strong accretion on dust, which is not counterbalanced by the desorption less efficient than in the 2-phase model. This strongly constrains the chemical-age of cold cores to be of the order of few 10 5 yrs.

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

“…All of the work presented in this section was done using a modified version of the three-phase gas-grain model presented in detail by Ruaud et al 6,21 The third phase was turned off so mantle chemistry was not considered. The adopted gas phase network is kida.uva.2014.…”

“…In the previous section we presented results from a modified version of the three-phase gas-grain model presented in detail by Ruaud et al 6,21 The third phase was turned off so mantle chemistry was not considered. The modifications made were necessary to explore high temperature gas-grain chemistry and explore how coupling this with gas phase processes will affect molecular abundances of molecules in astrophysical environments.…”

“…A species can be adsorbed in a shallow chemisorption well or conversely in a deep physisorption well. In 6 this sense whether something is chemisorbed or physisorbed can be a matter of degree and this is an important distinction to make in the present work. 17 Physisorption processes have already been treated at low temperatures, where atoms and molecules don't have sufficient energy to cross into a chemisorption well, so we'll be looking at high temperature grain reactions where chemisorption is believed to dominate.…”

“…44,46 The main difference between how photo-desorption is treated in these high temperature regimes is the scaling factor, we restrict desorption to the first monolayer of a bare grain rather than the first two monolayers for an ice mantle considered in previous models. 6 This is a very inefficient process; the rate constant is 10-20 orders of magnitude smaller than thermal desorption or reactive desorption. Photo-desorption is an indirect process which is driven by photo-absorption in sub-surface molecular layers and an energy transfer to the surface.…”

“…In this section we compare the new high temperature gas-grain code with the Nautilus-three Phase Model presented in detail by Ruaud et al and look at the major effects of the new chemisorption processes. 6 In section four we summarize the results and try to identify any shortcomings in this approach.…”