Dust Dynamics, Surface Brightness Profiles, and Thermal Spectra of Debris Disks: The Case of AU Microscopii

AU Microscopii is a 12 Myr old M dwarf that harbors an optically thin, edge-on disk of dust. The scattered light surface brightness falls with projected distance b from the star as b^-a; within b = 43 AU, a = 1-2, while outside 43 AU, a = 4-5. We devise a theory to explain this profile. At a stellocentric distance r = rBR = 43 AU, we posit a ring of parent bodies on circular orbits: the "birth ring, " wherein micron-sized grains are born from the collisional attrition of parent bodies. The "inner disk" at r rBR comprises grains just large enough to remain bound to the star, on orbits rendered highly eccentric by stellar wind and radiation pressure. How the vertical optical depth tau scales with r depends on the fraction of grains that migrate inward by CPR drag without suffering a collision. If this fraction is large, the inner disk and birth ring share the same optical depth, and tau scales as r^-5/2 in the outer disk. By contrast, under collision-dominated conditions, the inner disk is empty, and tau scales as r^-3/2 outside. These scaling relations, which we derive analytically and confirm numerically, are robust against uncertainties in the grain size distribution. By simultaneously modeling the surface brightness and thermal spectrum, we break model degeneracies to establish that the AU Mic system is collision-dominated, and that its narrow birth ring contains a lunar mass of decimeter-sized bodies. The inner disk is devoid of micron-sized grains; the surface brightness at b < 43 AU arises from light forward scattered by the birth ring. Inside b = 43 AU, the disk's V-H color should not vary with b; outside, the disk must become bluer as ever smaller grains are probed.