Rapid Formation of Molecular Clouds and Stars in the Solar Neighborhood

Observations of both star-forming regions and young, gas-free stellarassociations indicate that most nearby molecular clouds form stars only over ashort time span before dispersal; large-scale flows in the diffuse interstellarmedium have the potential for forming clouds sufficiently rapidly, and forproducing stellar populations with ages much less than the lateral crossingtimes of their host molecular clouds. We identify four important factors forunderstanding rapid star formation and short cloud lifetimes. First, much ofthe accumulation and dispersal of clouds near the solar circle might occur inthe atomic phase; only the high-density portion of a cloud's lifecycle is spentin the molecular phase, thus helping to limit molecular cloud ``lifetimes''.Second, once a cloud achieves a high enough column density to form $\h2$ andCO, gravitational forces become larger than typical interstellar pressureforces; thus star formation can follow rapidly upon molecular gas formation andturbulent dissipation in limited areas of each cloud complex. Third, typicalmagnetic fields are not strong enough to prevent rapid cloud formation andgravitational collapse. Fourth, rapid dispersal of gas by newly-formed stars,and reduction of shielding by a small expansion of the cloud after the firstevents of star formation, might limit the length of the star formation epochand the lifetime of a cloud in its molecular state. This picture emphasizes theimportance of large-scale boundary conditions for understanding molecular cloudformation, and implies that star formation is a highly dynamic, rather thanquasi-static, process.Comment: 41 pages, 10 figures, accepted by Ap