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 Hα and Hβ 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 ≥ 60 • ). 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  line flux, with B − V and the V excess flux. We have indications of a time delay between the main emission lines (Hα, Hβ and He ) 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 . 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 Hα 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.

We present the first comprehensive study of the narrow emission lines of T Tauri stars (TTS). These narrow lines have been reported in the literature as originating in the stellar atmosphere and having Gaussian-type profiles centered at the stellar rest velocity, with a base width not larger than 50 km s-1. Here, we concentrate on the Ca n lines XX8498, 8542, and 8662 and the helium line X5876. After applying veiling corrections, the average narrow component line emission is found to be larger than that found in active main-sequence stars: up to several times larger for classical T Tauri stars with strong rates of disk accretion. More striking is the finding that the resulting line emission strengths of these lines correlate with veiling. The correlation is confirmed on individual stars for which observations at several epochs exist and for which veiling varies widely on relatively short timescales. We also find a correlation between the narrow emission fluxes and the near-infrared excesses for stars with low levels of veiling, which includes the few weak-lined TTS of the sample. We discuss possible formation sites for the narrow emission lines in the classical TTS, and we present simple models to explain the observations. In these models, the excess line emission found for the stars with higher accretion rates is assumed to originate in localized regions near the magnetic footpoints of the accretion column. We refer to these hypothetical regions in the atmosphere collectively as the "hot chromosphere" since we assume they are additionally heated by the reprocessed energy of the colliding gas in the accretion process. Computing two chromospheric models, one representing the typical weak TTS chromosphere and the other representing the best guess at the "hot chromosphere," we find the following. The "hot chromosphere" is characterized by a steep temperature gradient beginning at low continuum optical depths in order to give simultaneously the large observed central flux and the relatively narrow baselines (50-60 km s ~1). The chromosphere temperature rise is not similar to the earlier deep chromosphere models in which a sudden chromospheric temperature rise is appended to the photosphere at relatively large mass column. For the most extreme cases (i.e., largest line fluxes), 20%, at most, of the star's surface must be covered by "hot chromospheric" regions.

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