Degree of polarization in debris disks

All four debris disks are detected in polarized intensity, and three are also recovered in total intensity. We measured a peak degree of polarization of <~40% for all three disks. For the disk around HD 129590, we are also able to determine the degree of polarization in the radiation-pressure-driven halo.

To reproduce the observed polarization fractions, we find that the particles must consist of highly refractive and absorbing material. For HD 129590, by measuring the polarization fraction beyond the birth ring, we constrain the width of the size distribution to be increasingly small toward greater radii, which is compatible with the effect of radiation pressure. We put these findings to the test and present a self-consistent approach to producing synthetic images, assuming different profiles for the radiation pressure strength, and accounting for the presence of unbound grains.

We find the contribution of these grains to be especially critical in order to reproduce the increasing degree of polarization with stellocentric distance. Some of our results - namely a very small blow-out size and very large (n, k) values for the optical constants, which are required to reproduce the observed degree of polarization - might seem difficult to reconcile with our understanding of cosmic dust. Similar results have been obtained for other disks and we discuss the current limitation of available light-scattering models as well as possible avenues to alleviate these limitations.