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 HI 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.
|Number of pages||12|
|Journal||Astronomy & astrophysics|
|Publication status||Published - Nov-1994|
- INTERSTELLAR MEDIUM
- HIGH-VELOCITY CLOUDS
- MOLECULAR CLOUDS