The Structure of Molecular Clouds: are they Fractal?

Summary

Molecular clouds are observed to be highly structured and fragmented but also follow simple power law relationships between, for example, their size and linewidth as first described by Larson. This self-similarity has led to a fractal description of cloud structure, but in recent years there have been a number of observations that indicate the existence of characteristic scales in molecular cloud cores and clusters of young stars. I present some observations of molecular clouds from large (1-10 pc) to small (0.1 pc) scales, and discuss whether a fractal description of cloud structure is universally appropriate.

Introduction

The density and velocity structure within a molecular cloud is a remnant of its formation environment and the starting point for the creation of stars. It determines how deeply radiation can propagate through the cloud, and is a critical parameter for understanding the evolution of the ISM. How is it best described?

Beginning with Larson (1981), correlations between cloud properties such as linewidth and size have been fit by power laws. Since a power law does not have a characteristic scale, the implication is that clouds are scale-free and self-similar. This has led to statements in the literature that clouds are best described as fractals (e.g. Falgarone, Phillips, & Walker 1991; Elmegreen 1997). On the other hand, other recent studies (Larson 1995; Simon 1997; Goodman et al. 1998; Blitz & Williams 1997) suggest that there are characteristic size and velocity scales in star-forming regions.