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AA Tau has been monitored for more than 20 years since 1987, exhibiting anearly constant brightness level of V=12.5 mag. We report here that in 2011 itsuddenly faded away, becoming 2 magnitudes fainter in the V-band, and hasremained in this deep state since then. We report new optical and near-IRphotometry and spectroscopy obtained during the fading event. The systemappears much redder and fainter than it was in the bright state. Also, the 8.2dphotometric period continuously observed for more than 20 years is not seenduring most of the deep state. The analysis of the system's brightness andcolors suggests that the visual extinction on the line of sight has increasedby about 3-4 magnitudes in the deep state. At optical wavelengths, the systemappears to be dominated by scattered light, probably originating from the uppersurface layers of a highly inclined circumstellar disk. The profiles of theBalmer lines have significantly changed as well, with the disappearance of acentral absorption component regularly observed in the bright state. We ascribethis change to the scattering of the system's spectrum by circumstellar dust.Noticeably, the mass accretion rate in the inner disk and onto the central starhas not changed as the system faded away. We conclude that the deepening of theAA Tau system is due to a sudden increase of circumstellar dust extinction onthe line of sight without concomitant change in the accretion rate. We suggestthat the enhanced obscuration may be produced by a non-axisymmetric overdenseregion in the disk, located at a distance of 7.7 AU or more, that was recentlybrought on the line of sight by its keplerian motion around the central star.

AA Tau's sudden and long-lasting deepening: enhanced extinction by its circumstellar disk