Global Hierarchical Collapse In Molecular Clouds. Towards a
  Comprehensive Scenario
  We present a unified description of the scenario of Global Hierarchical
Collapse and fragmentation (GHC) in molecular clouds (MCs), owing to the
continuous decrease of the average Jeans mass in the contracting cloud. GHC
constitutes a regime of collapses within collapses, in which small-scale
collapses begin at later times, but occur on shorter timescales than
large-scale ones. The difference in timescales allows for most of the clouds'
mass to be dispersed by feedback from the first massive stars, maintaining the
global star formation rate low. All scales accrete from their parent
structures. The main features of GHC are: star-forming MCs are in an
essentially pressureless regime, which produces filaments that accrete onto
clumps and cores ("hubs"). The filaments constitute the collapse flow from
cloud to hub scales and may approach a quasi-stationary state; the molecular
and dense mass fractions of the clouds increase over time; the first (low-mass)
stars appear several Myr after global contraction began; more massive stars
appear after a few Myr in massive hubs resulting from the collapse of larger
scales; the minimum fragment mass may extend well into the brown-dwarf regime;
Bondi-Hoyle-Lyttleton accretion occurs at the protostellar and core scales,
accounting for a near-Salpeter IMF; the extreme anisotropy of the filamentary
network explains the difficulty in detecting large-scale infall signatures; the
balance between inertial and gravitationally-driven motions in clumps evolves
during the contraction; prestellar cores adopt Bonnor-Ebert-like profiles, but
are contracting ever since early times when they may appear to be unbound and
to require pressure confinement; stellar clusters develop radial age and mass
segregation gradients. Finally, we discuss the incompatibility between
supersonic turbulence and the observed scalings in the molecular hierarchy.
