ACCRETION DISK BOUNDARY-LAYERS IN CLASSICAL T-TAURI STARS

Results of one-dimensional time-dependent calculations of geometricall y thin accretion disk boundary layers are presented for classical T Ta uri stars for various values of M, (0.8 and 1.0 M.), R* (1.6, 2.15, a nd 4.3 R.), and M (5 x 10(-9)-5 x 10(-7) M. yr(-1)). The results exhib it a thermal boundary layer (delta(BL)(th) approximate to 0.1-0.2R) m uch larger than the dynamical one (delta(BL)(dyn) approximate to few p ercent), with characteristic low temperatures (T-BL(eff) approximate t o 5-6 x 10(3) K). In the limit of very low mass accretion rates charac teristic of T Tauri systems, the mid-plane temperature in the inner di sk drops well below 10(4) K (a few approximate to 10(3) K at most). A very sharp transition region separates the hot ionized thermal boundar y layer region from the cool neutral inner disk. For low values of the viscosity parameter (alpha approximate to 0.01), the medium is optica lly thin in the cool inner disk and slightly optically thick in the ho t thermal boundary layer. This result explains the absence of opticall y thick material (the ''inner hole'') as observed in the central part of circumstellar disks around young stellar objects. For higher values of the viscosity parameter (alpha approximate to 0.1), the optical de pth in the boundary layer region decreases (tau approximate to 1) and its temperature increases slightly, while the inner disk becomes parti ally ionized and optically more thick.