  From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at
CFHT and TBL, we report the detection of Zeeman signatures on the prototypical
classical TTauri star AATau, both in photospheric lines and accretion-powered
emission lines. Using time series of unpolarized and circularly polarized
spectra, we reconstruct at two epochs maps of the magnetic field, surface
brightness and accretion-powered emission of AATau. We find that AATau hosts a
2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably
dynamo origin. We also show that the magnetic poles of AATau host large cool
spots at photospheric level and accretion regions at chromospheric level.
  The logarithmic accretion rate at the surface of AATau at the time of our
observations is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2
in average (in Msun/yr); this is an order of magnitude smaller than the disc
accretion rate at which the magnetic truncation radius (below which the disc is
disrupted by the stellar magnetic field) matches the corotation radius (where
the Keplerian period equals the stellar rotation period) - a necessary
condition for accretion to occur. It suggests that AATau is largely in the
propeller regime, with most of the accreting material in the inner disc regions
being expelled outwards and only a small fraction accreted towards the surface
of the star. The strong variability in the observed surface mass-accretion rate
and the systematic time-lag of optical occultations (by the warped accretion
disc) with respect to magnetic and accretion-powered emission maxima also
support this conclusion.
  Our results imply that AATau is being actively spun-down by the star-disc
magnetic coupling and appears as an ideal laboratory for studying angular
momentum losses of forming Suns in the propeller regime.

Solar and Stellar Astrophysics