Decoding Images of Debris Disks

Abstract

Current observations of debris disks reveal a wealth of radial and azimuthal structures likely created by planet-disk interactions. Future images of exozodiacal disks may reveal similar structures. In this work I summarize my observations and modeling of the structure of exozodiacal dust clouds.

I present our observations of the 51 Ophiuchi circumstellar disk made with the Keck Interferometer operating in nulling mode at N-band. I modeled these data simultaneously with VLTI-MIDI visibility data and a <italic>Spitzer</italic> IRS spectrum and showed that the best-fit disk model is an optically thin disk with size-dependent radial structure. This model has two components, with an inner exozodiacal disk of blackbody grains extending to ~4 AU and an outer disk of small silicate grains extending out to ~1200 AU. This model is consistent with an inner "birth" disk of continually colliding parent bodies producing an extended envelope of ejected small grains and resembles the disks around Vega, AU Microscopii, and &beta; Pictoris.

I produced models of resonant ring structures created by planets in debris disks. I used a custom-tailored hybrid symplectic integrator to model 120 resonant ring structures created by terrestrial-mass planets on circular orbits interacting with collisionless steady-state dust clouds around a Sun-like star. I used these models to estimate the mass of the lowest-mass planet that can be detected through observations of a resonant ring, and showed that the resonant ring morphology is degenerate and depends on only two parameters: planet mass and a_p<super>1/2</super>/&beta;, where a_p is the planet's semi-major axis and &beta; is the ratio of radiation pressure force to gravitational force on a grain. I introduced a new computationally-efficient "collisional grooming" algorithm that enables us to model grain-grain collisions in structured debris disks and used this algorithm to show how collisions can alter the morphology of a resonant ring structure. My collisional models reveal that collisions act to remove azimuthal and radial asymmetries from the disk. I showed that the collision rate for background particles in a resonant ring structure is enhanced by a factor of a few compared to the rest of the disk, and dust grains in or near mean motion resonances have even higher collision rates. I also used this algorithm to model the 3-D structure of the Kuiper Belt's dust cloud at four different dust levels. I found that the Kuiper Belt dust would look like an azimuthally symmetric ring at 40-45 AU when viewed from afar at submillimeter wavelengths. At visible wavelengths, the Kuiper Belt dust cloud would reveal two resonant ring structures: one created by Saturn near 10 AU and one created by Neptune near 30 AU. A denser version of our Kuiper Belt dust cloud, with an optical depth 1000 times greater, would look qualitatively similar at submillimeter wavelengths, but would be void of Neptune's resonant ring structure at visible wavelengths. My simulations suggest that mean motion resonances with planets can play strong roles in the sculpting of debris disks even in the presence of collisions, though their roles are somewhat different than what has been anticipated.