Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models
E. C. Ostriker, J. M. Stone, C. F. Gammie;
ApJ, 2001, 546, 980
ABSTRACT:We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with
parameters chosen to represent conditions in giant molecular clouds (GMCs).
We
present results of three model cloud simulations in which the mean magnetic
field strength is varied (B0=1.4-14 μG for GMC parameters),
but an identical initial turbulent velocity field is introduced.
We
describe the energy evolution, showing that (1) turbulence decays rapidly,
with the turbulent energy reduced by a factor 2 after 0.4-0.8 flow crossing
times (~2-4 Myr for GMC parameters), and (2) the magnetically supercritical
cloud models gravitationally collapse after time ~6 Myr, while the
magnetically subcritical cloud does not collapse.
We compare density, velocity,
and magnetic field structure in three sets of model ``snapshots'' with
matched values of the Mach number ℳ~9,7,5.
We show that the
distributions of volume density and column density are both approximately
log-normal, with mean mass-weighted volume density a factor 3-6 times the
unperturbed value, but mean mass-weighted column density only a factor 1.1-1.4
times the unperturbed value.
We introduce a spatial binning algorithm to
investigate the dependence of kinetic quantities on spatial scale for regions of
column density contrast (ROCs) on the plane of the sky.
We show that the
average velocity dispersion for the distribution of ROCs is only weakly
correlated with scale, similar to mean size-line width distributions for clumps
within GMCs.
We find that ROCs are often superpositions of spatially
unconnected regions that cannot easily be separated using velocity information;
we argue that the same difficulty may affect observed GMC clumps.
We
suggest that it may be possible to deduce the mean 3D size-line width relation
using the lower envelope of the 2D size-line width distribution.
We analyze
magnetic field structure and show that in the high-density regime
nH2>~103 cm-3, total magnetic field strengths increase with
density with logarithmic slope ~1/3-2/3.
We find that mean line-of-sight
magnetic field strengths may vary widely across a projected cloud and are not
positively correlated with column density.
We compute simulated interstellar
polarization maps at varying observer orientations and determine that the
Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the
plane-of sky magnetic field strength, provided the dispersion in polarization
angles is <~25°.
KEYWORDS: ism: clouds, ism: molecules, magnetohydrodynamics: mhd, methods: numerical, stars: formation
PERSOKEY:turbulence, ,
CODE: ostriker2001