J. Ballesteros-paredes et al., Clouds as turbulent density fluctuations: Implications for pressure confinement and spectral line data interpretation, ASTROPHYS J, 515(1), 1999, pp. 286-303
We examine the idea that diffuse H I and giant molecular clouds and their s
ubstructure form as density fluctuations induced by large-scale interstella
r turbulence. We do this by closely investigating the topology of the veloc
ity, density, and magnetic fields within and at the boundaries of the cloud
s emerging in high-resolution two-dimensional simulations of the interstell
ar medium (ISM) including self-gravity, magnetic fields, parameterized heat
ing and cooling, and a simple model for star formation. We find that the ve
locity field is continuous across cloud boundaries for a hierarchy of cloud
s of progressively smaller sizes. Cloud boundaries defined by a density-thr
eshold criterion are found to be quite arbitrary, with no correspondence to
any actual physical boundary, such as a density discontinuity. Abrupt velo
city jumps are coincident with the density maxima, which indicates that the
clouds are formed by colliding gas streams. This conclusion is also suppor
ted by the fact that the volume and surface kinetic terms in the Eulerian v
irial theorem for a cloud ensemble are comparable in general and by the top
ology of the magnetic field, which exhibits bends and reversals where the g
as streams collide. However, no unique trend of alignment between density a
nd magnetic features is observed. Both sub- and super-Alfvenic motions are
observed within the clouds. In light of these results, we argue that therma
l pressure equilibrium is irrelevant for cloud confinement in a turbulent m
edium, since inertial motions can still distort or disrupt a cloud, unless
it is strongly gravitationally bound. Turbulent pressure confinement appear
s self-defeating because turbulence contains large-scale motions that neces
sarily distort Lagrangian cloud boundaries or equivalently cause flux throu
gh Eulerian boundaries. We then discuss the compatibility of the present sc
enario with observational data. We find that density-weighted velocity hist
ograms are consistent with observational line profiles of comparable spatia
l and velocity resolution, exhibiting similar FWHMs and similar multicompon
ent structure. An analysis of the regions contributing to each velocity int
erval indicates that the histogram "features" do not come from isolated "cl
umps" but rather from extended regions throughout a cloud, which often have
very different total velocity vectors. Finally, we argue that the scenario
presented here may also be applicable to small scales with larger densitie
s (molecular clouds and their substructure, up to at least n similar to 10(
3)-10(5) cm(-3)) and conjecture that quasihydrostatic configurations cannot
be produced from turbulent fluctuations unless the thermodynamic behavior
of the flow becomes nearly adiabatic. We demonstrate, using appropriate coo
ling rates, that this will not occur except for very small compressions (le
ss than or similar to 10(-2) pc) or until protostellar densities are reache
d for collapse.