Debris disks in polarized light

James Graham
UC Berkeley


Debris disks are hosted by stars with ages of about ten to a few
hundred million years, and therefore they provide the principal window
on the transition from the oligarchic to late heavy bombardment phases
of solar system formation. Observing this era provides insight into
the evolution of our solar system relevant to the migration of the gas
and ice giants, the formation of the Kuiper belt and the Oort cloud,
and the delivery of water to the terrestrial planets. Debris disks are
comprised of transient grains generated in a collisional cascade that
spans planetesimal to sub-micron to dimensions. The total mass of
sub-micron grains that dominate visible images may seem insignificant;
however, these tiny grains are linked to the largest bodies in two
ways: the first is via the aforementioned cascade; the second is
through gravitational perturbations. Grains are subject to
size-dependent non-gravitational forces, and disk morphology reflects
the balance between stellar gravity, radiation pressure, planetary
perturbations, and corpuscular drag. Planets reveal their existence by
gravitationally sculpting disks, and evidence for them can be
established by matching the observed dust distribution with models
that predict their perturbations. Debris disks are seen in projection;
therefore, their appearance is determined jointly by the 3-d
distribution of dust and the optics of scattering. This is evident for
edge-on systems like b Pic where disentangling these two factors is
challenging. The emerging field of debris disk polarimetry shows how
disk structure can be revealed, while providing novel insights into
the physics structure of the dust, e.g., measurements of its porosity.