Molecular cloud evolution. I. molecular cloud and thin cold neutral medium sheet formation. Astrophys

We discuss molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). Initially, a shocked layer forms, and within it, a thin cold layer. An analytical model and high-resolution one-dimensional simulations predict the thermodynamic conditions in the cold layer. After 1 Myr of evolution, the layer has column density 2:5; 1019 cm2, thickness 0.03 pc, temperature 25 K, and pressure 6650 K cm3. These conditions are strongly reminiscent of those recently reported by Heiles and coworkers for cold neutral medium sheets. In the one-dimensional simulations, the inflows into the sheets produce line profileswith a central line ofwidth0.5 km s1 and broad wings of width1 km s1. Three-dimensional numerical simulations show that the cold layer develops turbulentmotions and increases its thickness until it becomes a fully three-dimensional turbulent cloud. Fully developed turbulence arises on times ranging from 7.5 Myr for inflow Mach numberM1; r 2:4 to>80 Myr for M1; r 1:03. These numbers should be considered upper limits. The highest density turbulent gas (HDG, n> 100 cm3) is always overpressured with respect to the meanWNMpressure by factors of 1.5–4, even thoughwe do not include self-gravity. The intermediate-density gas (IDG, 10 < n/cm3 < 100) has a significant pressure scatter that increases withM1,r, so that at M1; r 2:4 a significant fraction of the IDG is at a higher pressure than the HDG. Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compressive process that forms the clouds themselves and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic. Subject headings: instabilities — ISM: clouds — shock waves — turbulence Online material: color figure, mpeg animations 1