POLAR ACCRETION AND LIGHT VARIABILITY ON T-TAURI STARS

The proposition that some T Tauri stars are accreting material magnetically from a circumstellar disk is examined. The model developed by Ghosh & Lamb (1979a,b) is applied to the star-disk system, and the general features of the spectral energy distribution of the disk are discussed. It is found that accretion along magnetic field lines in a dipole stellar magnetic field affects the continuous spectra over a large wavelength range. The implications concerning light variability due to an inhomogeneous accretion flow are discussed. High speed photometric observations of BP Tauri, where a number of flare-like events have been recorded, are compared with the results from the model calculations. It is found that the changes in the black-body temperature of the excess emission during these events is in quantitative and qualitative agreement with these calculations. The emission from the plasma column of accreting material close to the stellar surface, where the kinetic energy is released, is analysed. It is found that soft X-ray radiation could emerge from the post-shock region in a bimodal way dependent on the optical depth in the accretion column.