We study the formation of giant dense cloud complexes and of stars within them using SPH numerical simulations of the collision of gas streams (''inflows'') in the WNM at moderately supersonic velocities. The collisions cause compression, cooling, and turbulence generation in the gas, forming a cloud that then becomes self-gravitating and begins to collapse globally. Simultaneously, the turbulent, nonlinear density fluctuations induce fast, local collapse events. The simulations show that (1) The clouds are not in a state of equilibrium. Instead, they undergo secular evolution. During its early stages, the cloud's mass and gravitational energy jE g j increase steadily, while the turbulent energy E k reaches a plateau. (2) When jE g j becomes comparable to E k , global collapse begins, causing a simultaneous increase in jE g j and E k that maintains a near-equipartition condition jE g j $ 2E k . (3) Longer inflow durations delay the onset of global and local collapse by maintaining a higher turbulent velocity dispersion in the cloud over longer times. (4) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. (5) The column densities of the local star-forming clumps closely resemble 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 ubiquity of H i halos around them. Also, within their assumptions, our simulations support the scenario of rapid star formation after MCs are formed, although long (k15 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.

We present a unified description of the scenario of global hierarchical collapse (GHC). GHC constitutes a flow regime of (non-homologous) collapses within collapses, in which all scales accrete from their parent structures, and small, dense regions begin to contract at later times, but on shorter time-scales than large, diffuse ones. The different time-scales allow for most of the clouds’ mass to be dispersed by the feedback from the first massive stars, maintaining the cloud-scale star formation rate low. Molecular clouds (MCs), clumps, and cores are not in equilibrium, but rather are either undergoing contraction or dispersal. The main features of GHC are as follows: (1) The gravitational contraction is initially very slow, and begins when the cloud still consists of mostly atomic gas. (2) Star-forming MCs are in an essentially pressureless regime, causing filamentary accretion flows from the cloud to the core scale to arise spontaneously. (3) Accreting objects have longer lifetimes than their own free-fall time, due to the continuous replenishment of material. (4) The clouds’ total mass and its molecular and dense mass fractions increase over time. (5) The clouds’ masses stop growing when feedback becomes important. (6) The first stars appear several megayears after global contraction began, and are of low mass; massive stars appear a few megayears later, in massive hubs. (7) The minimum fragment mass may well extend into the brown-dwarf regime. (8) Bondi–Hoyle–Lyttleton-like accretion occurs at both the protostellar and the core scales, accounting for an IMF with slope dN/dM ∝ M−2. (9) The extreme anisotropy of the filamentary network explains the difficulty in detecting large-scale infall signatures. (10) The balance between inertial and gravitationally driven motions in clumps evolves during the contraction, explaining the approach to apparent virial equilibrium, from supervirial states in low-column density clumps and from subvirial states in dense cores. (11) Prestellar cores adopt Bonnor–Ebert-like profiles, but are contracting ever since when they may appear to be unbound. (12) Stellar clusters develop radial age and mass segregation gradients. We also discuss the incompatibility between supersonic turbulence and the observed scalings in the molecular hierarchy. Since gravitationally formed filaments do not develop shocks at their axes, we suggest that a diagnostic for the GHC scenario should be the absence of strong shocks in them. Finally, we critically discuss some recent objections to the GHC mechanism.

We report on the filaments that develop self-consistently in a new numerical simulation of cloud formation by colliding flows. As in previous studies, the forming cloud begins to undergo gravitational collapse because it rapidly acquires a mass much larger than the average Jeans mass. Thus, the collapse soon becomes nearly pressureless, proceeding along its shortest dimension first. This naturally produces filaments in the cloud, and clumps within the filaments. The filaments are not in equilibrium at any time, but instead are long-lived flow features, through which the gas flows from the cloud to the clumps. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension. Moreover, the clumps also exhibit a hierarchical nature: the gas in a filament flows onto a main, central clump, but other, smaller-scale clumps form along the infalling gas. Correspondingly, the velocity along the filament exhibits a hierarchy of jumps at the locations of the clumps. Two prominent filaments in the simulation have lengths ∼ 15 pc, and masses ∼ 600M ⊙ above density n ∼ 10 3 cm −3 (∼ 2×10 3 M ⊙ at n > 50 cm −3 ). The density profile exhibits a central flattened core of size ∼ 0.3 pc and an envelope that decays as r −2.5 , in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ∼ 30M ⊙ Myr −1 pc −1 .

We investigate the collapse of non-spherical substructures, such as sheets and filaments, which are ubiquitous in molecular clouds. Such non-spherical substructures collapse homologously in their interiors but are influenced by an edge effect that causes their edges to be preferentially accelerated. We analytically compute the homologous collapse timescales of the interiors of uniform-density, self-gravitating filaments and find that the homologous collapse timescale scales linearly with the aspect ratio. The characteristic timescale for an edge-driven collapse mode in a filament, however, is shown to have a square-root dependence on the aspect ratio. For both filaments and circular sheets, we find that selective edge acceleration becomes more important with increasing aspect ratio. In general, we find that lower dimensional objects and objects with larger aspect ratios have longer collapse timescales. We show that estimates for star formation rates, based upon gas densities, can be overestimated by an order of magnitude if the geometry of a cloud is not taken into account.