Magnetospheric accretion onto the T Tauri star AA Tauri - I. Constraints from multisite spectrophotometric monitoring

We have monitored the photometric, spectroscopic and polarimetric variation s of the classical T Tauri star (CTTS) AA Tau over a period of a month. The light curve consists of more than 260 measurements in each of the B and V- bands over a continuous time period of 30 days and more than 180 measuremen ts in the R and I-bands. This provides unprecedented detail of the photomet ric variations of a CTTS on timescales ranging from hours to weeks. We find that AA Tan's light curve is quite unlike that of most other CTTS. It exhibits a roughly constant brightness level, interrupted by quasi-cycli c fading episodes with an amplitude of 1.4 mag in BVRI filters. We interpre t this behaviour as resulting from quasi-periodic occultations of the stell ar photosphere by opaque circumstellar material. The interpretation derives from the: lack of significant color variations associated with the fading of the system and is strengthened by the higher polarization level measured when the system is faint. We argue that the occultations are produced by a warp in AA Tau's inner dis k which presumably results from the dynamical interaction between the disk and the stellar magnetosphere. We present a model that accounts for the obs ervations quite naturally if we assume that the stellar magnetosphere is a large-scale dipole tilted onto the stellar rotational axis which disrupts t he disk at the corotation radius. We derive the geometrical properties of A A Tau's accretion zone in the framework of this model and constrain the loc ation of veiling and Balmer line emitting regions. Although AA Tau's light curve is atypical, the constraints derived here on the structure of its accretion zone may apply as well to other CTTS. It is probably only because AA Tau is seen at a peculiar inclination, close to ed ge-on, that occultations are conspicuous and its photometric behaviour so c learly reveals this phenomenon.