Interstellar extinction and its relation to the three-dimensional cloud structure

We present simulated extinction measurements using different three-dimensional models for the density structure of interstellar clouds. The distribution of the extinction values and the properties of the dispersion observed at small scales are studied. The results are compared with previous observations of real clouds. Our results show that the observed small scale extinction variations are very effective in limiting the free parameters of the cloud models. On the other hand, all the tested models (i.e. fractal and structure-tree models) were able to reproduce the observed extinction variations with some parameter values. This was found to be true even for a model consisting of randomly placed clumps of constant density. The extinction measurements can, however, be used to get crude estimates of the volume filling factor of the clouds. Comparison between the models and the observations of the cloud IC 5146 (Lada et al. 1994) indicates a volume filling factor in the range 0.1–0.2.