Molecular cloud evolution. I. Molecular cloud and thin CNM sheet formation

摘要: 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, thin cold layer. An analytical model high-resolution 1D simulations predict thermodynamic conditions After $\sim 1$ Myr of evolution, has column density 2.5 \times 10^{19} \psc$, thickness 0.03$ pc, temperature 25$ K pressure 6650$ $\pcc$. These are strongly reminiscent those recently reported Heiles coworkers for sheets. In simulations, inflows into sheets produce line profiles with central width 0.5 \kms$ broad wings 1 \kms$. 3D numerical show that develops turbulent motions increases its thickness, until it becomes fully three-dimensional cloud. Fully developed turbulence arises on times ranging from 7.5$ inflow Mach number $\Mr = 2.4$ to $> 80$ 1.03$. numbers should be considered upper limits. The highest-density gas (HDG, $n > 100 \pcc$) is always overpressured respect mean WNM factors 1.5--4, even though we do not include self-gravity. intermediate-density (IDG, $10 < n [{\rm cm}^ {-3}] 100$) significant scatter $\Mr$, so at 2.4$, fraction IDG higher than HDG. Our results suggest least part excess clouds can generated compressive process forms themselves, CNM may formed transiently this mechanism, when only weakly supersonic.