  We have used the Hubble Space Telescope/ACS coronagraph to make polarization
maps of the AU Mic debris disk. The fractional linear polarization rises
monotonically from about 0.05 to 0.4 between 20 and 80 AU. The polarization is
perpendicular to the disk, indicating that the scattered light originates from
micron sized grains in an optically thin disk. Disk models, which
simultaneously fit the surface brightness and polarization, show that the inner
disk (< 40-50 AU) is depleted of micron-sized dust by a factor of more than
300, which means that the disk is collision dominated. The grains have high
maximum linear polarization and strong forward scattering. Spherical grains
composed of conventional materials cannot reproduce these optical properties. A
Mie/Maxwell-Garnett analysis implicates highly porous (91-94%) particles. In
the inner Solar System, porous particles form in cometary dust, where the
sublimation of ices leaves a "bird's nest" of refractory organic and silicate
material. In AU Mic, the grain porosity may be primordial, because the dust
"birth ring" lies beyond the ice sublimation point. The observed porosities
span the range of values implied by laboratory studies of particle coagulation
by ballistic cluster-cluster aggregation. To avoid compactification, the upper
size limit for the parent bodies is in the decimeter range, in agreement with
theoretical predictions based on collisional lifetime arguments. Consequently,
AU Mic may exhibit the signature of the primordial agglomeration process
whereby interstellar grains first assembled to form macroscopic objects.
