Turbulent velocity structure in molecular clouds
V. Ossenkopf, M. M. Mac Low;
AaA, 2002, 390, 307
ABSTRACT:We compare velocity structure observed in the Polaris Flare molecular cloud at scales ranging from 0.015 pc to 20 pc to the velocity structure of a
suite of simulations of supersonic hydrodynamic and MHD turbulence
computed with the ZEUS MHD code.
We examine different methods of
characterising the structure, including a scanning-beam method that provides an
objective measurement of Larson's size-linewidth relation, structure
functions, velocity and velocity difference probability distribution
functions (PDFs), and the Delta -variance wavelet transform, and use them to
compare models and observations.
The Delta -variance is most sensitive to
characteristic scales and scaling laws, but is limited in its application by a lack of
intensity weighting so that its results are easily dominated by observational
noise in maps with large empty areas.
The scanning-beam size-linewidth
relation is more robust with respect to noisy data.
Obtaining the global
velocity scaling behaviour requires that large-scale trends in the maps not be
removed but treated as part of the turbulent cascade.
We compare the true
velocity PDF in our models to simulated observations of velocity centroids and
average line profiles in optically thin lines, and find that the line profiles
reflect the true PDF better unless the map size is comparable to the total
line-of-sight thickness of the cloud.
Comparison of line profiles to velocity
centroid PDFs can thus be used to measure the line-of-sight depth of a cloud.
The
observed density and velocity structure is consistent with supersonic
turbulence with a driving scale at or above the size of the molecular cloud and
dissipative processes below 0.05 pc.
Ambipolar diffusion could explain the
dissipation.
Over most of the observed range of scales the velocity structure is that of a
shock-dominated medium driven from large scale.
The velocity PDFs exclude
small-scale driving such as that from stellar outflows as a dominant process in the
observed region.
In the models, large-scale driving is the only process that
produces deviations from a Gaussian PDF shape consistent with observations,
almost independent of the strength of driving or magnetic field.
Strong
magnetic fields impose a clear anisotropy on the velocity field, reducing the
velocity variance in directions perpendicular to the field.
KEYWORDS: ism: clouds, ism: magnetic fields, turbulence, ism: kinematics and dynamics, mhd
PERSOKEY:statistical analysis, turbulence, simulation, co, ,
CODE: ossenkopf2002