Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry

We have simultaneously monitored the photometric and polarimetric variations of the Classical T Tauri star AA Tau during the fall of 2002. We combine these data with previously published polarimetric data covering two earlier epochs. The phase coverage is complete, although not contiguous. AA Tau clearly shows cyclic variations coupled with the rotation of the system. The star-disk system produces a repeatable polarisation curve where the polarisation increases with decreasing brightness. The data fit well with the model put forward by Bouvier et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively dumping material onto the central star via magnetospheric accretion. The inner edge of the disk is deformed by its interaction with the tilted magnetosphere, producing “eclipses” as it rotates and occults the photosphere periodically. From the shape of the polarisation curve in the QU-plane we confirm that the accretion disk is seen at a large inclination, almost edge-on, and predict that its position angle is PA $\sim90^\circ$, i.e., that the disk's major axis is oriented in the East–West direction