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We demonstrate that the observationally inferred rapid onset of starformation after parental molecular clouds have assembled can be achieved byflow-driven cloud formation of atomic gas, using our previous three-dimensionalnumerical simulations. We post-process these simulations to approximate COformation, which allows us to investigate the times at which CO becomesabundant relative to the onset of cloud collapse. We find that global gravityin a finite cloud has two crucial effects on cloud evolution. (a) Lateralcollapse (perpendicular to the flows sweeping up the cloud) leads to rapidlyincreasing column densities above the accumulation from the one-dimensionalflow. This in turn allows fast formation of CO, allowing the molecular cloud to``appear'' rapidly. (b) Global gravity is required to drive the dense gas tothe high pressures necessary to form solar-mass cores, in support of recentanalytical models of cloud fragmentation. While the clouds still appear``supersonically turbulent'', this turbulence is relegated to playing asecondary role, in that it is to some extent a consequence of gravitationalforces.

Rapid Molecular Cloud and Star Formation: Mechanisms and Movies