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

Abstract

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 Ðlaments of 10È20]2È5 pc, most have been formed by H I Ñows in Myr, explaining the [3 absence of postÈT Tauri stars in the region with ages Myr. Observations in the 21 cm line of the H I Z3 ““ halos ÏÏ around the Taurus molecular gas show many features (broad asymmetric proÐles, 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 Ðlamentary structures. This rapid evolution is possible because the H I Ñows producing and disrupting the cloud have much higher velocities (5È10 km s~1) than are present in the molecular gas resulting from the colliding Ñows. The simulations suggest that such Ñows can occur from the global ISM turbulence without requiring a single triggering event such as a supernova explosion. Subject