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\n3-phase modelling of gas and grain chemistry of cold cores. In this model, both\nthe mantle and the surface are considered as chemically active. We also take\ninto account the competition among reaction, diffusion and evaporation. The\nmodel predictions are confronted to ice observations in the envelope of\nlow-mass and massive young stellar objects as well as toward background stars.\nModelled gas-phase abundances are compared to species observed toward TMC-1\n(CP) and L134N dark clouds. We find that our model successfully reproduces the\nobserved ice species. It is found that the reaction-diffusion competition\nstrongly enhances reactions with barriers and more specifically reactions with\nH2, which is abundant on grains. This finding highlights the importance to have\na good approach to determine the abundance of H2 on grains. Consequently, it is\nfound that the major N-bearing species on grains go from NH3 to N2 and HCN when\nthe reaction-diffusion competition is accounted. In the gas-phase and before\nfew 10^5 yrs, we find that the 3-phase model does not have a strong impact on\nthe observed species compared to the 2-phase model. After this time, the\ncomputed abundances dramatically decrease due to the strong accretion on dust,\nwhich is not counterbalanced by the desorption less efficient than in the\n2-phase model. This strongly constrains the chemical-age of cold cores to be of\nthe order of few 10^5 yrs.\n