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

We observed the luminous T Tauri star HD 283447 = V773 Tauri simultaneously at X-ray, ultraviolet, optical photometric and spectroscopic, and radio wavelengths of several hours on UT 1992 September 11. ROSAT, IUE, VLA and an intercontinental VLBI network, and three optical observatories participated in the campaign. The star is known for its unusually 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) ergs s-1, respectively. Optically, the spectrum exhibits rather weak characteristics of ''classical'' T Tauri stars. A faint, broad emission line 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 constant. The large gyrosynchrotron-emitting regions are therefore decoupled from the chromospheric and coronal emission. Five models for the magnetic geometry around the star are discussed: solar-type activity, dipole magnetosphere, star-disk magnetic coupling, disk magnetic fields, and close binary interaction. The data suggest that two magnetic geometries are simultaneously present: complex multipolar fields like those on the Sun and a large-scale field possibly associated with the circumstellar disk.