Magnetic accretion and photopolarimetric variability in T Tauri stars

We investigate the photopolarimetric variability of a magnetic accretion disk model for pre-main-sequence T Tauri stars. In these dynamical models, material from a magnetically truncated Keplerian disk accretes along magnetic field lines onto the stellar surface at high latitudes, producing ''hot spots'' on the stellar photosphere. These spots have been proposed as the origin of the UV excesses in T Tauri stars. As the star rotates, the observed UV excess varies, as will the polarization arising from the intrinsically polarized spot the radiation of which is also scattered in the circumstellar disk. We calculate the photopolarimetric variability for a simple magnetic accretion model in which the hot spots are two diametrically opposed, circular regions with temperatures much larger than the surrounding photosphere. Our Monte Carlo calculations show that the predicted amplitudes of the photopolarimetric variability are consistent with those observed in T Tauri stars. This model predicts correlations between the brightness and polarization that can be tested with synoptic observations. The model also predicts quasi-periodic variations if the hot spots remain relatively fixed on the stellar surface on timescales comparable to the rotation period.