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Issue A&A
Volume 469, Number 3, July III 2007
Page(s) 949 - 961
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20066807



A&A 469, 949-961 (2007)
DOI: 10.1051/0004-6361:20066807

Effects of turbulent diffusion on the chemistry of diffuse clouds

P. Lesaffre, M. Gerin, and P. Hennebelle

LERMA, CNRS UMR8112, Observatoire de Paris and ENS/LRA, 24 rue Lhomond, 75231 Paris Cedex 05, France
    e-mail: pierre.lesaffre@lra.ens.fr

(Received 24 November 2006 / Accepted 19 April 2007)

Abstract
Aims.We probe the effect of turbulent diffusion on the chemistry at the interface between a cold neutral medium (CNM) cloudlet and the warm neutral medium (WNM).
Methods.We perform moving grid, multifluid, 1D, hydrodynamical simulations with chemistry including thermal and chemical diffusion. The diffusion coefficients are enhanced to account for turbulent diffusion. We post-process the steady-states of our simulations with a crude model of radiative transfer to compute line profiles.
Results.Turbulent diffusion spreads out the transition region between the CNM and the WNM. We find that the CNM slightly expands and heats up: its CH and H2 content decreases due to the lower density. The change of physical conditions and diffusive transport increase the H+ content in the CNM which results in increased OH and H2O. Diffusion transports some CO out of the CNM. It also brings H2 into contact with the warm gas with enhanced production of CH+, H3+, OH and H2O at the interface. O lines are sensitive to the spread of the thermal profile in the intermediate region between the CNM and the WNM. Enhanced molecular content at the interface of the cloud broadens the molecular line profiles and helps exciting transitions of intermediate energy. The relative molecular yields are found higher for bigger clouds.
Conclusions.Turbulent diffusion can be the source of additional molecular production and should be included in chemical models of the interstellar medium (ISM). It also is a good candidate for the interpretation of observational problems such as warm H2, CH+ formation and presence of H3+.


Key words: astrochemistry -- turbulence -- diffusion -- ISM: clouds -- ISM: molecules -- ISM: lines and bands



© ESO 2007


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