A Super-Alfv\'enic Model of Dark Clouds
P. Padoan, A. Nordlund ;
ApJ, 1999, 526, 279
ABSTRACT:Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfvén waves, but the possibility that these
motions are super-Alfvénic has not been ruled out.
In this work we
report the results of numerical experiments in two opposite regimes:
ℳA~1 and ℳA>>1, where ℳA is the
initial Alfvénic Mach number-the ratio of the rms velocity to the
Alfvén speed.
Our results show that models with
ℳA>>1 are consistent with the observed properties of molecular clouds that
we have tested (statistics of extinction measurements, distribution of
integrated antenna temperature, Zeeman-splitting measurements of magnetic
field strength, line width versus integrated antenna temperature of
molecular emission-line spectra, statistical B-n relation, and scatter in that
relation), while models withℳA~1 have properties that are in
conflict with the observations.
We find that both the density and the magnetic
field in molecular clouds may be very intermittent.
The statistical
distributions of the magnetic field and gas density are related by a power law, with an
index that decreases with time in experiments with decaying
turbulence.
After about one dynamical time it stabilizes at
B~n0.4.
Magnetically dominated cores form early in the evolution, while later on the
intermittency in the density field wins out, and also cores with a weak field can be
generated by mass accretion along magnetic field lines.
KEYWORDS: ism: clouds, ism: kinematics and dynamics, ism: magnetic fields, magnetohydrodynamics: mhd, shock waves, turbulence
PERSOKEY:turbulence, ,
CODE: padoan99a