Free access article

A&A 433, 1-13 (2005)
DOI: 10.1051/0004-6361:20041474

Thermal condensation in a turbulent atomic hydrogen flow

E. Audit¹ and P. Hennebelle<sup>2

1</sup>  Service d'Astrophysique, CEA/DSM/DAPNIA/SAp, C. E. Saclay, 91191 Gif-sur-Yvette Cedex, France
    e-mail: edouard.audit@cea.fr
²  Laboratoire de radioastronomie millimétrique, UMR 8112 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
    e-mail: patrick.hennebelle@ens.fr

(Received 15 June 2004 / Accepted 30 October 2004 )

Abstract
We present a numerical and analytical study of the thermal fragmentation of a turbulent flow of interstellar hydrogen. We first present the different dynamical processes and the large range of spatial (and temporal) scales that need to be adequately represented in numerical simulations. Next, we present bidimensional simulations of turbulent converging flows which induce the dynamical condensation of the warm neutral phase into the cold phase. We then analyse the cold structures and the fraction of unstable gas in each simulation, paying particular attention to the influence of the degree of turbulence. When the flow is very turbulent a large fraction of the gas remains in the thermally unstable domain. This unstable gas forms a filamentary network. We show that the fraction of thermally unstable gas is strongly correlated with the level of turbulence of the flow. We then develop a semi-analytical model to explain the origin of this unstable gas. This simple model is able to quantitatively reproduce the fraction of unstable gas observed in the simulations and its correlation with turbulence. Finally, we stress the fact that even when the flow is very turbulent and in spite of the fact that a large fraction of the gas is maintained dynamically in the thermally unstable domain, the classical picture of a 2-phase medium with stiff thermal fronts and local pressure equilibrium turns out to be still relevant in the vicinity of the cold structures.

Key words: hydrodynamics -- instabilities -- ISM: kinematics and dynamics -- ISM: structure -- ISM: clouds

© ESO 2005

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.