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We present an extended version of the 2-phase gas-grain code NAUTILUS to the3-phase modelling of gas and grain chemistry of cold cores. In this model, boththe mantle and the surface are considered as chemically active. We also takeinto account the competition among reaction, diffusion and evaporation. Themodel predictions are confronted to ice observations in the envelope oflow-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 theobserved ice species. It is found that the reaction-diffusion competitionstrongly enhances reactions with barriers and more specifically reactions withH2, which is abundant on grains. This finding highlights the importance to havea good approach to determine the abundance of H2 on grains. Consequently, it isfound that the major N-bearing species on grains go from NH3 to N2 and HCN whenthe reaction-diffusion competition is accounted. In the gas-phase and beforefew 10^5 yrs, we find that the 3-phase model does not have a strong impact onthe observed species compared to the 2-phase model. After this time, thecomputed abundances dramatically decrease due to the strong accretion on dust,which is not counterbalanced by the desorption less efficient than in the2-phase model. This strongly constrains the chemical-age of cold cores to be ofthe order of few 10^5 yrs.

Gas and grain chemical composition in cold cores as predicted by the Nautilus 3-phase model