Molecular Cloud Evolution. II. From Cloud Formation to the Early Stages of Star Formation in Decaying Conditions

We present a numerical study of the formation of dense cloud complexes and of stars within them via the collision of two opposite streams of self-gravitating, thermally bistable diffuse interstellar gas. We find that: a) The clouds are NOT in a state of equilibrium. Instead, they are continually evolving, increasing their mass and gravitational energy Eg, until the latter becomes comparable to the turbulent energy Ek, at which time global, and later local, collapse set in. b) After this time, the cloud begins to contract gravitationally as a whole, producing a simultaneous increase in |Eg| and Ek, satisfying a near-equipartition condition |Eg|~2Ek, a result that explains the apparent ``virialized'' state of MCs. c) Longer inflow durations delay the onset of both global and local collapse, by maintaining a constant turbulent velocity dispersion in the cloud. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) At the onset of star formation, the column densities of the local star- forming clumps are typically 0. 5-2 X 10^21 pc, very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. At that time, the bulk of the cloud is still expected to remain atomic. Within their framework and assumptions, our simulations thus support the scenario of rapid star formation AFTER MCs are formed, although long (> 15 Myr) accumulation periods are probably spent in the atomic phase, during which the clouds build up their gravitational energy.