Molecular Cloud Evolution II. From cloud formation to the early stages of star formation in decaying conditions

Abstract

We study the formation of giant dense cloud complexes and of stars within them by means of SPH numerical simulations of the mildly supersonic collision of gas streams (``inflows'') in the warm neutral medium (WNM). The resulting compressions cause cooling and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and undergoes global collapse. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulations show that: a) The clouds are not in a state of equilibrium. Instead, they undergo secular evolution. Initially, their mass and gravitational energy |Eg| increase steadily, while the turbulent energy Ek reaches a plateau. b) When |Eg| becomes comparable to Ek, global collapse begins, causing a simultaneous increase in both that maintains a near-equipartition condition |Eg| ~ 2 Ek. c) Longer inflow durations delay the onset of global and local collapse, by maintaining a higher turbulent velocity dispersion in the cloud over longer times. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) The column densities of the local star-forming clumps are very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. The MC formation mechanism discussed here naturally explains the apparent ``virialized'' state of MCs and the ubiquitous presence of HI halos around them. Within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (>~ 15 Myr) accumulation periods do occur during which the clouds build up their gravitational energy, and which are expected to be spent in the atomic phase.