A method for detection of structure

Context. In order to understand the evolution of molecular clouds it is important to identify the departures from self-similarity associated with the scales of self-gravity and the driving of turbulence. Aims. A method is described based on structure functions for determining whether a region of gas, such as a molecular cloud, is fractal or contains structure with characteristic scale sizes. Methods. Using artificial data containing structure it is shown that derivatives of higher order structure functions provide a powerful way to detect the presence of characteristic scales should any be present and to estimate the size of such structures. The method is applied to observations of hot H 2 in the Kleinman-Low nebula, north of the Trapezium stars in the Orion Molecular Cloud, including both brightness and velocity data. The method is compared with other techniques such as Fourier transform and histogram techniques. Results. It is found that the density structure, represented by H 2 emission brightness in the K-band (2-2.5 μm), exhibits mean characteristic sizes of 110, 550, 1700 and 2700 AU. The velocity data show the presence of structure at 140, 1500 and 3500 AU. Compared with other techniques such as Fourier transform or histogram, the method appears both more sensitive to characteristic scales and easier to interpret.