What influences the structure of frog assemblages at forest streams?

We present the analysis of FUSE observations of two Herbig AeBe stars known to harbor young circumstellar disks: HD 100546 and HD 163296. In both cases we detect absorption lines from warm and dense H2. The thermalization of the rotational levels up to J ~ 4 allows evaluation of the temperature, density and typical size of the absorbing layer. These quantities are consistent with absorption of the light of the central star by a thin layer of a circumstellar disk seen at an intermediate inclination.