Effects of turbulent diffusion on the chemistry of diffuse clouds

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⁺, H, 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 H.