MULTIWAVELENGTH STUDY OF THE MAGNETICALLY ACTIVE T-TAURI STAR HD-283447

We observed the luminous T Tauri star HD 283447 = V773 Tauri simultane ously at X-ray, ultraviolet, optical photometric and spectroscopic, an d radio wavelengths of several hours on UT 1992 September 11. ROSAT, I UE, VLA and an intercontinental VLBI network, and three optical observ atories participated in the campaign. The star is known for its unusua lly high and variable nonthermal radio continuum emission. High levels of soft X-ray and Mg II line emission are discovered, with luminosity L(x) = 5.5 x 10(30) ergs s-1 (0.2-2 keV) and L(Mg II) = 1 X 10(29) er gs s-1, respectively. Optically, the spectrum exhibits rather weak cha racteristics of ''classical'' T Tauri stars. A faint, broad emission l ine component, probably due to a collimated wind or infall, is present . During the campaign, the radio luminosity decreased by a factor of 4 , while optical/UV lines and X-ray emission remained strong but consta nt. The large gyrosynchrotron-emitting regions are therefore decoupled from the chromospheric and coronal emission. Five models for the magn etic geometry around the star are discussed: solar-type activity, dipo le magnetosphere, star-disk magnetic coupling, disk magnetic fields, a nd close binary interaction. The data suggest that two magnetic geomet ries are simultaneously present: complex multipolar fields like those on the Sun and a large-scale field possibly associated with the circum stellar disk.