Determining structure in molecular clouds
J. P. Williams, E. J. De Geus, L. Blitz;
ApJ, 1994, 428, 693
ABSTRACT:We descibe an automatic, objective routine for analyzing the clumpy structure in a spectral line position-position-velocity data cube.
The
algorithm works by first contouring the data at a multiple of the rms noise of the
observations, then searches for peaks of emission which locate the clumps, and then
follows them down to lower intensities.
No a proiri clump profile is
assumed.
By creating simulated data, we test the performance of the algorithm and
show that a contour map most accurately depicts internal structure at a
contouring interval equal to twice the rms noise of the map.
Blending of clump
emission leads to small errors in mass and size determinations and in severe
cases can result in a number of clumps being misidentified as a single unit,
flattening the measured clump mass spectrum.
The algorithm is applied to two real
data sets as an example of its use.
The Rosette molecular cloud is a
'typical' star-forming cloud, but in the Maddalena molecular cloud high-mass
star formation is completely absent.
Comparison of the two clump lists
generated by the algorithm show that on a one-to-one basis the clumps in the
star-forming cloud have higher peak temperatures, higher average densities, and
are more gravitationally bound than in the non-star-forming
cloud.
Collective properties of the clumps, such as temperature-size-line-width-mass
relations appear very similar, however.
Contrary to the initial results
reported in a previous paper (Williams & Blitz 1993), we find that the
current, more thoroughly tes ted analysis finds no significant difference in
the clump mass spectrum of the two clouds.
KEYWORDS: evolution (development), mass spectra, molecular clouds, size (dimensions), spectral line width, star formation, stellar structure, temperature, algorithms, least squares method, line spectra, mapping, numerical analysis, simulation
CODE: williams94