Measuring the fractal structure of interstellar clouds
M. G. R. Vogelaar, B. P. Wakker;
AaA, 1994, 291, 557
ABSTRACT:To study the structure of interstellar matter we have applied the concept of fractal curves to the brightness contours of maps of interstellar
clouds and from these estimated the fractal dimension for some of them.
We
used the so-called perimeter-area relation as the basis for these
estimates.
We concentrate on the influence of measurement limitations on the
accuracy of the determination of the fractal dimension from
observations.
Problems and pitfalls are discussed in detail.
We show that to obtain reliable
results the signal-to-noise ratio (if defined properly for extended objects)
must be higher than about 20.
This is not a strict rule, however.
Further,
not only is a large-enough dynamic range of enclosed contour areas needed
(a criterion which is related to the signal-to-noise ratio), but it is
necessary (but not necessarily sufficient) that the smallest area is at least 25
pixels.
Even when all these criteria are fulfilled, the accuracy one can expect for
the fractal dimension is only of the order of 0.05.
Estimates of the
fractal dimension are presented for two different cloud types, high-velocity
H I clouds (HVCs) and infrared cirrus.
These estimates generally range
between 1.35 and 1.5, somewhat higher than found in previous, similar
studies.
However, for the one object that this and other studies have in common, we find the
same value of 1.23.
KEYWORDS: astronomical models, fractals, hydrogen clouds, infrared cirrus (astronomy), interstellar matter, brightness, contours, signal to noise ratios
PERSOKEY:turbulence, ,
CODE: vogelaar94