Occultation of young stellar objects by circumstellar disks I. Theoreticalexpectations and preliminary comparison with observations

The hypothesis that partial occultation of young stars by their circumstell ar disks is responsible for UX Ori and T Tauri Type III photometric variabi lity is quantitatively studied. We construct accretion disk models in which reprocessing of stellar and accretion luminosity leads to flaring of the d isk (assumed to be in vertical hydrostatic equilibrium), and compute the pr obability to observe the star through the disk atmosphere. The mass accreti on rate is found to primarily determine the range of view angles over which the star is fully or partially obscured by the disk. For average disk/star parameters relevant to active T Tauri stars, the probability of observing occultation events is similar to0.15, while it is typically 0.2 for paramet ers relevant to UX Ori. A preliminary analysis of available photometric dat a confirms these estimates and allows us to uncover in some stars cyclic va riability on a time-scale of weeks to years that may tentatively be attribu ted to disk inhomogeneities or warps. These results suggest that direct obs ervational study of the disk structure may be possible for a sizable fracti on of young stellar objects.