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

AA Tau has been monitored for more than 20 years since 1987, exhibiting a nearly constant brightness level of V=12.5 mag. We report here that in 2011 it suddenly faded away, becoming 2 magnitudes fainter in the V-band, and has remained in this deep state since then. We report new optical and near-IR photometry and spectroscopy obtained during the fading event. The system appears much redder and fainter than it was in the bright state. Also, the 8.2d photometric period continuously observed for more than 20 years is not seen during most of the deep state. The analysis of the system's brightness and colors suggests that the visual extinction on the line of sight has increased by about 3-4 magnitudes in the deep state. At optical wavelengths, the system appears to be dominated by scattered light, probably originating from the upper surface layers of a highly inclined circumstellar disk. The profiles of the Balmer lines have significantly changed as well, with the disappearance of a central absorption component regularly observed in the bright state. We ascribe this 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 star has not changed as the system faded away. We conclude that the deepening of the AA Tau system is due to a sudden increase of circumstellar dust extinction on the line of sight without concomitant change in the accretion rate. We suggest that the enhanced obscuration may be produced by a non-axisymmetric overdense region in the disk, located at a distance of 7.7 AU or more, that was recently brought on the line of sight by its keplerian motion around the central star.