Velocity Field Statistics in Star-forming Regions. I. Centroid Velocity Observations
M. S. Miesch, J. Scalo, J. Bally;
ApJ, 1999, 524, 895
ABSTRACT:The probability density functions (pdfs) of molecular line centroid velocity fluctuations, and of line centroid velocity fluctuation differences
at different spatial lags, are estimated for several nearby molecular
clouds with active internal star formation.
The data consist of over 75,000
^13CO line profiles divided among 12 spatially and/or kinematically
distinct regions.
These regions range in size from less than 1 to more than 40 pc
and are all substantially supersonic, with centroid fluctuation Mach
numbers ranging from about 1.5 to 7.
The centroid pdfs are constructed using
three different types of estimators.
Although three regions (all in Mon R2)
exhibit nearly Gaussian centroid pdfs, the other regions show strong evidence
for non-Gaussian pdfs, often nearly exponential, with possible evidence
for power-law contributions in the far tails.
Evidence for nearly
exponential centroid pdfs in the neutral H I component of the interstellar medium is
also presented, based on older published data for optical absorption lines
and H I emission and absorption lines.
These strongly non-Gaussian pdfs
disagree with the nearly Gaussian behavior found for incompressible
turbulence (except possibly shear flow turbulence) and simulations of decaying
mildly supersonic turbulence.
Spatial images of the largest magnitude
centroid velocity differences for the star-forming regions appear less
filamentary than predicted by decay simulations dominated by vortical
interactions.
No evidence for the scaling of difference pdf kurtosis with Reynolds
number, as found in incompressible turbulence experiments and simulations,
is found.
We conclude that turbulence in both star-forming molecular
clouds and diffuse H I regions involves physical processes that are not
adequately captured by incompressible turbulence or by mildly supersonic decay
simulations.
The variation with lag of the variance and kurtosis of the difference pdfs
is presented as a constraint on future simulations, and we evaluate and
discuss the implications of the large scale and Taylor scale Reynolds numbers
for the regions studied here.
KEYWORDS: ism: clouds, ism: kinematics and dynamics, ism: molecules, stars: formation, turbulence
PERSOKEY:turbulence, ,
CODE: miesch99