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[119.05] Thin CNM Sheet and Molecular Cloud Formation

E. Vazquez-Semadeni (CRyA-UNAM), D. Ryu (Chungnam National University, Korea), T. Passot (Obs. de la cote d'Azur, France), R. Gonzalez, A. Gazol (CRyA-UNAM)

We analyze the scenario of molecular cloud formation by large-scale supersonic compressions in the diffuse warm neutral medium (WNM). An analytical model and high-resolution 1D simulations predict that when the inflow Mach number M_r ~1, a thin cold layer forms within the shocked gas. After ~1 Myr of evolution, the layer has column density ~2.5 \times 10¹⁹ cm^-2, thickness ~0.03 pc, temperature ~25 K and pressure ~6650 K cm^-3. In the simulations, the sheets have line profiles with a central line of width ~ 0.5 km s^-1 and broad wings of width ~1 km s^-1, which correspond to the inflowing speed of the gas, and do not imply excessively short lifetimes for the sheets. These sheets are reminiscent of those recently observed by Heiles and coworkers.

3D numerical simulations show that at later times the cold layer becomes dynamically unstable, through a nonlinear thin shell-like instability occurring at the boundary of the thin shell. Fully developed turbulence arises on times ranging from ~5 Myr for M_r=2.4 to ~100 Myr for M_r=1.03. In the turbulent regime, the highest-density gas (HDG, n > 100 cm^-3) is always overpressured with respect to the mean WNM pressure by factors 2--5, even though we do not include self-gravity. The intermediate-density gas (IDG, 10-3] < 100) has a significant pressure scatter at a given value of the density, and increases with M_r. The ratio of internal to kinetic energy density changes from the inflow to the IDG and the HDG, and increases with density in the most turbulent runs.

Our results suggest that the turbulence and at least part of the excess pressure in molecular clouds can be generated by the compression that forms the clouds themselves, and that thin CNM sheets may be formed transiently by this mechanism, when the compressions are only weakly supersonic.

This work has been funded by CONACYT grant 36571-E to E.V.-S., Korea Research Foundation grant KRF-2004-015-C00213 to D.R., and the French national program PCMI to T.P.

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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.