Exozodiacal Dust

Citation

Kuchner, Marc Jason (2001) Exozodiacal Dust. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/73cv-y270. https://resolver.caltech.edu/CaltechETD:etd-09202008-105312

Abstract

Besides the sun, the most luminous feature of the solar system is a cloud of "zodiacal" dust released by asteroids and comets that pervades the region interior to the asteroid belt. Similar clouds of dust around other stars—exozodiacal clouds—may be the best tracers of the habitable zones of extra-solar planetary systems. This thesis discusses three searches for exozodiacal dust:

1. We observed six nearby main-sequence stars with the Keck telescope at 11.6 microns, correcting for atmosphere-induced wavefront aberrations and deconvolving the point spread function via classical speckle analysis. We compare our data to a simple model of the zodiacal dust in our own system based on COBE DIRBE observations and place upper limits on the density of exozodiacal dust in these systems.
2. We observed Sirius, Altair, and Procyon with the NICMOS Coronagraph on the Hubble Space Telescope to look for scattered light from exozodiacal dust and faint companions within 10 AU from these stars. We did not achieve enough dynamic range to surpass the upper limits set by IRAS on the amount of exozodiacal dust in these systems, but we did set strong upper limits on the presence of nearby late-type and sub-stellar companions. We explain the technique of coronagraphy with a discussion of the Fourier optics of a one-dimensional coronagraph.
3. The planned nulling capability of the Keck Interferometer should allow it to probe the region < 200 milliarcsecond from a bright star and to suppress on-axis starlight by factors of 10⁻³ to reveal faint circumstellar material. We model the response of the Keck Interferometer to hypothetical exozodiacal clouds to derive detection limits that account for the effects of stellar leakage, photon noise, noise from null depth fluctuations, and the fact that the cloud's shape is not known a priori. Our models show that the Keck Interferometer can detect an exozodiacal cloud with as little as 10 times the optical depth of the solar zodiacal cloud, even when the transmitted stellar signal is stronger than the signal from the dust cloud.

We also discuss the interaction of dust with planets. We used the COBE DIRBE Sky and Zodi Atlas and the IRAS Sky Survey Atlas to search for dynamical signatures of three different planets in the solar system dust complex:

1. We searched the COBE DIRBE Sky and Zodi Atlas for a wake of dust trailing Mars. We compare the DIRBE images to a model Mars wake based on the empirical model of the Earth's wake as seen by the DIRBE and place a 3-σ upper limit on the fractional overdensity of particles in the Mars wake of 18% of the fractional overdensity trailing the Earth.
2. We searched the COBE DIRBE Sky and Zodi Atlas for Trojan dust near Jupiter's L5 Lagrange point. We place a 3-a upper limit on the effective emitting area of large (10-100 micron diameter) particles trapped at Jupiter's L5 Lagrange point of 6 x 10¹⁷ cm², assuming that these large dust grains are distributed in space like the Trojan asteroids. We would have detected the Mars wake if the surface area of dust in the wake scaled simply as the mass of the planet times the Poynting-Robertson time scale.
3. We compared the COBE DIRBE Sky and Zodi Atlas and the IRAS Sky Survey Atlas to search for dust created in the Kuiper Belt and trapped in mean-motion resonances with Neptune. We place a model-dependent upper limit of 0.6 MJy steradian⁻¹ on the brightness of dust trapped in Neptune's 2:3 resonance.

Thesis Files