A&A 399, 553-565 (2003)
Turbulent diffusion and mixing in interstellar dark clouds with adsorption onto grainsC. J. Yate and T. J. Millar
Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK
(Received 17 April 2002 / Accepted 6 December 2002 )
This paper reports the results of models of dark cloud chemistry incorporating a depth dependent density distribution with diffusive mixing and adsorption onto grains. The model is based on the approach taken by Xie et al. (1995), with the addition of grain accretion effects. Without diffusion, the central regions of the cloud freeze out in less than 107 years. Freeze-out time is dependent on density, so the diffuse outer region of the cloud remains abundant in gas for about an order of magnitude longer. We find that fairly small amounts of diffusive mixing can delay freeze-out at the centre of the model cloud for a time up to an order of magnitude greater than without diffusion, due to material diffusing inward from the edges of the cloud. The gas-phase lifetime of the cloud core can thus be increased by up to an order of magnitude or more by this process. We have run three different grain models with various diffusion coefficients to investigate the effects of changing the sticking parameters.
Key words: astrochemistry -- turbulence -- diffusion -- ISM: abundances -- ISM: dust, extinction
Offprint request: T. J. Millar, Tom.Millar@umist.ac.uk
© ESO 2003