MAGNETIC-STRUCTURE IN COOL STARS .18. UV-LINE EMISSIONS FROM T-TAURI STARS

The flux densities in the UV spectral emission lines from T Tauri stars are compared with those from other cool stars. In the flux-flux diagrams for pairs of UV emission lines the data points for T Tauri stars extend the relations as defined by main-sequence and evolved stars towards larger flux densities by a factor of 40. We discuss the large emission-line flux densities and the relations between flux densities from different lines in the light of magnetic activity and recent models of accretion disks in T Tauri stars. The very large emission fluxes in chromospheric lines may be explained by a large emitting volume associated with the warm (T approximately 10(4) K) inner disk boundary layer, but the fluxes and the line broadening in the high-excitation lines requires the presence of hotter material, with temperatures of about 2 10(5) K, that must be attached to the inner disks but that is not predicted by existing disk models.