A&A 495, 847-867 (2009)
DOI: 10.1051/0004-6361:200810803

Models of turbulent dissipation regions in the diffuse interstellar medium

B. Godard¹, E. Falgarone¹, and G. Pineau des Forêts<sup>2, 1

1</sup>  LRA/LERMA, CNRS UMR 8112, École Normale Supérieure & Observatoire de Paris, Paris, France
    e-mail: godard@lra.ens.fr
²  Institut d'Astrophysique Spatiale, CNRS UMR 8617, Université Paris-Sud, Orsay, France

Received 14 August 2008 / Accepted 11 December 2008

Abstract
Aims. Supersonic turbulence is a large reservoir of suprathermal energy in the interstellar medium. Its dissipation, because it is intermittent in space and time, can deeply modify the chemistry of the gas. This is clearly seen in the framework of shock chemistry. Intense turbulent dissipation also occurs in regions of large velocity shears, sharing with shocks the property of intermittency. Whether these bursts of dissipation, short-lived and localized, have a measurable impact on molecular abundances in the diffuse medium, and how the chemical enrichment they drive compares to observations, are the questions we address here.
Methods. We further explore a hybrid method to compute the chemical and thermal evolution of a magnetized dissipative structure, under the energetic constraints provided by the observed properties of turbulence in the cold neutral medium. For the first time, we model a random line of sight by taking into account the relative duration of the bursts with respect to the thermal and chemical relaxation timescales of the gas. The key parameter is the turbulent rate of strain a due to the ambient turbulence. With the gas density, it controls the size of the dissipative structures, therefore the strength of the burst. It also sets the relative importance of viscous dissipation and ion-neutral friction in the gas heating and chemical enrichment.
Results. For a large range of rates of strain and densities, the models of turbulent dissipation regions (TDR) reproduce the CH⁺ column densities observed in the diffuse medium and their correlation with highly excited H₂. They do so without producing an excess of CH. As a natural consequence, they reproduce the abundance ratios of HCO⁺/OH and HCO⁺/H₂O, and their dynamic range of about one order of magnitude observed in diffuse gas. Large C₂H and CO abundances, also related to those of HCO⁺, are another outcome of the TDR models that compare well with observed values. Neutral carbon exceeds the abundance expected at ionization equilibrium, in agreement with fine-structure line observations. The abundances and column densities computed for CN, HCN and HNC are one order of magnitude above PDR model predictions, although still significantly smaller than observed values. The dependence of our results on the rate of strain and density reveals that the chemical enhancements are in better agreement with observations if the dissipation is dominated by ion-neutral friction, involving shear structures of thickness ~100 AU.

Key words: astrochemistry -- turbulence -- ISM: molecules -- ISM: kinematics and dynamics -- ISM: structure -- ISM: clouds

© ESO 2009

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