  We use a suite of 3D simulations of star-forming molecular clouds, with and
without stellar feedback, magnetic fields, and driven turbulence, to study the
compression and expansion rates of the gas as functions of density. We show
that, around the mean density, supersonic turbulence promotes rough equilibrium
between the amounts of compressing and expanding gas, consistent with
continuous gas cycling between high and low density states. We find that the
inclusion of protostellar jets produces rapidly expanding and compressing
low-density gas. We find that the gas mass flux peaks at the transition between
the lognormal and power-law forms of the density probability distribution
function (PDF). This is consistent with the transition density tracking the
post-shock density, which promotes an enhancement of mass at this density
(i.e., shock compression and filament formation). At high densities, the gas
dynamics are dominated by self-gravity: the compression rate in all of our runs
matches the rate of the run with only gravity, suggesting that processes other
than self-gravity have little effect at these densities. The net gas mass flux
becomes constant at a density below the sink formation threshold, where it
equals the star formation rate. The density at which the net gas mass flux
equals the star formation rate is one order of magnitude lower than our sink
threshold density, corresponds to the formation of the second power-law tail in
the density PDF, and sets the overall star formation rates of these
simulations.
