Imaging of protoplanetary and debris disks

Most stars form surrounded by dusty disks of circumstellar matter. Initially gas rich, these disks give birth to planetary systems through complex physical processes which are still poorly understood. Spatially resolved images of protoplanetary disks are essential to measure their physical properties and constrain the processes of planet formation. As protoplanets grow in mass, they can begin carving out regions cleared of smaller bodies, leading to complex transitional disk structures as planetary assembly progresses. Once primordial gas has dissipated, dynamical stirring can increase the relative velocities of remaining planetesimals and collisional accretion gives way to collisional destruction. Dust particles produced through collisional cascades will eventually be driven from the system by radiative forces or stellar winds; in quasi-equilibrium, the transient populations of dust give rise to debris disks that persist for tens or hundreds of millions of years. Debris disks are analogous to the zodiacal dust in our own solar system produced from comets, asteroids, and Kuiper Belt objects, though currently observable examples are generally younger and have higher optical depth than our solar system today. Throughout this evolution of circumstellar disks from protoplanetary to transitional to debris, starlight scatters from their constituent dust particles, and in doing so becomes polarized. This induced linear polarization, which can easily exceed values of more than 50%, renders polarimetry an especially potent tool for studying circumstellar disks. In recent years such observations have been extended to high angular resolution via diffraction-limited observations made with adaptive optics (AO) or using the Hubble Space Telescope (HST). In this chapter we concern ourselves particularly with spatially resolved imaging linear polarimetry of circumstellar disks, mostly at optical and near infrared (IR) wavelengths (Fig. 15.1). Broadly speaking, applications of imaging polarimetry to circumstellar disks fall into two classes: (1) Using polarization to gain increased sensitivity to faint circumstellar material against the background halo of relatively weakly polarized starlight.