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

We suggest that molecular clouds can be formed on short timescales by compressions from large scale streams in the interstellar medium (ISM). In particular, we argue that in the Taurus-Auriga complex, with filaments of 10-20 × 2-5 pc, most have been formed by H I flows in ≲3 Myr, explaining the absence of post-T Tauri stars in the region with ages ≳3 Myr. Observations in the 21 cm line of the H I "halos" around the Taurus molecular gas show many features (broad asymmetric profiles, velocity shifts of H I relative to ¹²CO) predicted by our MHD numerical simulations, in which large-scale H I streams collide to produce dense filamentary structures. This rapid evolution is possible because the H I flows producing and disrupting the cloud have much higher velocities (5-10 km s^-1) than are present in the molecular gas resulting from the colliding flows. The simulations suggest that such flows can occur from the global ISM turbulence without requiring a single triggering event such as a supernova explosion.