Formation of the structure of interstellar molecular clouds

The problem of how interstellar molecular clouds are formed has not yet been solved. In this case, the standard model that attributes the formation of clouds to thermal instability in a homogeneous medium is inapplicable. Observations show that the clouds consist predominantly of the gas ejected from stars, mostly from red giants. The clouds are fragmented and have fractal structure. These features serve as a basis for the assumption that interstellar clouds are formed during the coalescence of cloudlets and the subsequent combination of the emerging objects into larger objects. For a fragmented cloud of considerable mass to be obtained, no gas mixing must take place during the coalescence. The fractal structure of interstellar molecular clouds can be explained in terms of this model for their formation. We consider a simple phenomenological cloud model and determine its fractal dimension D. Both analytic and numerical calculations yield D approximate to 2.5, which matches its observed value.