We have used the coronagraphic mode of the Advanced Camera for Surveys aboard the Hubble Space Telescope to make the first polarization maps of the debris disk surrounding the nearby M star AU Microscopii ( GJ 803 ) .
The linear polarization of the scattered light from the disk is unambiguously detected .
We find that the degree of polarization for the disk rises monotonically from about 0.05 to 0.35 between projected radii of 20 and 50 AU .
Polarized light is detectable out to about 80 AU , where the fractional polarization reaches a maximum observed value of 0.41 \pm 0.02 .
Polarization vectors are oriented perpendicular to the disk mid-plane , indicating that polarization originates from single scattering in an optically thin dust disk where the albedo is dominated by small ( x = 2 \pi a / \lambda \lesssim 1 ) particles .
We use simple , optically thin disk models to infer the spatial structure of the disk and the scattering properties of the constituent grains by simultaneously fitting the surface brightness profile and the degree of linear polarization .
The best fit models require that the dust grains exhibit high maximum linear polarization and strong forward scattering .
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 , i.e. , grains that are dragged inward by corpuscular and Poynting-Robertson drag undergo a destructive collision .
While the inferred optical properties are covariant with the radial distribution of dust , the only acceptable models have p _ { max } \geq 0.50 and g \geq 0.7 .
These constraints can not be met by spherical grains composed of conventional materials .
A Mie scattering analysis implicates grains where the real part of the refractive index is about 1.03 , which is a signature of highly porous ( 91–94 % ) media .
More reliable methods for calculating the scattering properties of aggregates confirm that the observed values of p _ { max } and g can be matched by high porosity , micron-sized clusters of small particles .
In the inner Solar System , porous particles form naturally 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 recent laboratory studies of particle coagulation in the proto-solar nebula 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 .