Turbulent Flow–driven Molecular Cloud Formation: A Solution to the Post–T Tauri Problem?

We suggest that molecular clouds can be formed on short time scales bycompressions from large scale streams in the interstellar medium (ISM). Inparticular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc$\times$ 2-5 pc, most have been formed by H I flows in $\lesssim 3$Myr,explaining the absence of post-T Tauri stars in the region with ages $\gtrsim3$ Myr. Observations in the 21 cm line of the H I `halos' around the Taurusmolecular gas show many features (broad asymmetric profiles, velocity shifts ofH I relative to $^{12}$CO) predicted by our MHD numerical simulations, in whichlarge-scale H I streams collide to produce dense filamentary structures. Thisrapid evolution is possible because the H I flows producing and disrupting thecloud have much higher velocities (5-10 kms) than present in the molecular gasresulting from the colliding flows. The simulations suggest that such flows canoccur from the global ISM turbulence without requiring a single triggeringevent such as a SN explosion.Comment: 26 pages, 12 ps figures. Apj accepte