Filaments in Simulations of Molecular Cloud Formation
  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 ~600 Msun above density n ~ 10^3 cm-3 (~2x10^3 Msun 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 ~30 Msun
Myr^-1 pc^-1.
