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Astron. Astrophys. 340, 241-256 (1998)

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Non-equilibrium chemistry in the dissipative structures of interstellar turbulence

K. Joulain 1, E. Falgarone 1, G. Pineau des Forêts 2 and D. Flower 3

1 Radioastronomie, CNRS, URA 336, Ecole Normale Supérieure, 24 rue Lhomond, F-75005 Paris, France
2 D.A.E.C, CNRS, URA 173, Observatoire de Paris, F-92195 Meudon Principal Cedex, France
3 Physics Department, The University, Durham DH1 3LE, UK

Received 5 May 1998 / Accepted 16 September 1998

Abstract

We study the chemical evolution of low density gas trapped in a vortex, representative of the dissipative structures of turbulence in the diffuse medium. A magnetic field is present and is close to being aligned locally with the vorticity. The chemical evolution is concentrated within tiny regions ([FORMULA] 10 AU) and short timescales ([FORMULA] a few 100 yr). It is controlled by the sharp temperature rise following the passage through layers where viscous dissipation is intense, and by the ion-neutral drift in layers where the tangential velocity of the neutrals is large. The facts that these two processes are closely associated in space and time, and that the amount of energy available in the dissipative structures is large, could explain, without fine-tuning the parameters of the model, the salient features of the observations of molecular species in diffuse gas: the large column densities of CH+, OH and HCO+, the remarkable proportionality of the OH and HCO+ column densities, the similarity of the OH and HCO+ (resp. CH and CH+) line centroids, and the fact that the OH-rich gas seen in absorption is not always detected in emission. A large number of such vortices must be intercepted at any time on any line of sight to reproduce the observed column densities but we show that less than one percent of the gas column density need to be in those chemically active regions. The turbulent energy dissipated in all these structures is, on average, smaller than that available in the turbulent cascade of the diffuse medium. Last, the dependence of our results on the gas density confirms that this 'hot' chemistry must arise in low density gas in order to meet the requirements of the observations.

Key words: ISM: evolution – ISM: kinematics and dynamics – ISM: molecules – turbulence

Send offprint requests to: E. Falgarone

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998
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