Adaptive Optics Imaging of the AU Microscopii Circumstellar Disk: Evidence for Dynamical Evolution

We present an H-band image of the light scattered from circumstellar dustaround the nearby (10 pc) young M star AU Microscopii (AU Mic, GJ 803, HD197481), obtained with the Keck adaptive optics system. We resolve the diskboth vertically and radially, tracing it over 17-60 AU from the star. Our AUMic observations thus offer the possibility to probe at high spatial resolution(0.04" or 0.4 AU per resolution element) for morphological signatures of thedebris disk on Solar-System scales. Various sub-structures (dust clumps andgaps) in the AU Mic disk may point to the existence of orbiting planets. Noplanets are seen in our H-band image down to a limiting mass of 1 M_Jup at >20AU, although the existence of smaller planets can not be excluded from thecurrent data. Modeling of the disk surface brightness distribution at H-bandand R-band, in conjunction with the optical to sub-millimeter spectral energydistribution, allows us to constrain the disk geometry and the dust grainproperties. We confirm the nearly edge-on orientation of the disk inferred fromprevious observations, and deduce an inner clearing radius <=10 AU. We findevidence for a lack of small grains in the inner (<60 AU) disk, either as aresult of primordial disk evolution, or because of destruction byPoynting-Robertson and/or corpuscular drag. A change in the power-law index ofthe surface brightness profile is observed near 33 AU, similar to a featureknown in the profile of the beta Pic circumstellar debris disk. By comparingthe time scales for inter-particle collisions and Poynting-Robertson dragbetween the two systems, we argue that the breaks are linked to one of thesetwo processes.Comment: 17 pages, 7 figures, 1 table; accepted by Ap