Eclipses by circumstellar material in the T Tauri star AA Tau -: II.: Evidence for non-stationary magnetospheric accretion

We report the results of a synoptic study of the photometric and spectroscopic variability of the classical T Tauri star AA Tau on timescales ranging from a few hours to several weeks. The AA Tau light curve had been previously shown to vary with a 8.2 d period, exhibiting a roughly constant brightness level, interrupted by quasi-cyclic fading episodes, which we interpreted as recurrent eclipses of the central star by the warped inner edge of its accretion disk (Bouvier et al. 1999). Our observations show the system is dynamic and presents non-stationary variability both in the photometry and spectroscopy. The star exhibits strong emission lines that show substantial variety and variability in their profile shapes and fluxes. Emission lines such as Halpha and Hbeta show both infall and outflow signatures and are well reproduced by magnetospheric accretion models with moderate mass accretion rates (10(-8)- 10(-9) M-. yr(-1)) and high inclinations (i greater than or equal to 60degrees). The veiling shows variations that indicate the presence of 2 rotationally modulated hot spots corresponding to the two magnetosphere poles. It correlates well with the He i line flux, with B - V and the V excess flux. We have indications of a time delay between the main emission lines (Halpha, Hbeta and He I) and veiling, the lines formed farther away preceding the veiling changes. The time delay we measure is consistent with accreted material propagating downwards the accretion columns at free fall velocity from a distance of about 8 R-star. In addition, we report periodic radial velocity variations of the photospheric spectrum which might point to the existence of a 0.02 M-. object orbiting the star at a distance of 0.08 AU. During a few days, the eclipses disappeared, the variability of the system was strongly reduced and the line fluxes and veiling severely depressed. We argue that this episode of quiescence corresponds to the temporary disruption of the magnetic configuration at the disk inner edge. The smooth radial velocity variations of inflow and outflow diagnostics in the Halpha profile yield further evidence for large scale variations of the magnetic configuration on a timescale of a month. These results may provide the first clear evidence for large scale instabilities developping in T Tauri magnetospheres as the magnetic field lines are twisted by differential rotation between the star and the inner disk. The interaction between the inner accretion disk and the stellar magnetosphere thus appears to be a highly dynamical and time dependent process.