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Lee Hartmann (Harvard-Smithsonian CfA), J. Ballesteros-Paredes (Inst. de Astronom\'ia, U. Nacional Aut\'onoma de M\'exico \&\ Harvard-Smithsonian CfA), E. Vazquez-Semadeni (Inst. de Astronom\'ia, U. Nacional Aut\'onoma de M\'exico)
We suggest that molecular clouds can be formed and disrupted on short timescales by compressions in large scale streams in the ISM. In particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc \times 2-5 pc, has been formed by HI flows in < 3~Myr, and this rapid formation explains the absence of post-T Tauri stars in the region with ages \gtrsim 3~Myr. 21 cm line observations of the HI ``halos'' around the Taurus molecular gas show many features (broad asymmetric profiles, velocity shifts of HI relative to 12CO, etc.) predicted by our MHD numerical simulations, in which where large-scale HI streams collide to produce dense filamentary structures. Our simulations indicate that the lifetimes of the clouds formed in this way depend strongly on the size scales, but structures with the characteristics of Taurus form and disperse in only a few Myr. This rapid evolution is possible because the HI flows producing and disrupting the cloud have much higher velocities (5 - 10 km/sec) than present in the molecular gas resulting from the colliding flows. The simulations suggest that such flows can occur from ISM turbulence without requiring a single triggering event such as a SN explosion.
These results are contrary to the widely accepted result that molecular clouds last for 10 Myr or more, but are consistent with a picture of the turbulent interstellar medium in which clouds in (magneto) hydrostatic equilibrium are rare.
The author(s) of this abstract have provided an email address for comments about the abstract: lhartmann@cfa.harvard.edu