The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars


Diane Dutkevitch

Department of Physics and Astronomy
B.A., Wells College
M.A., University of Rochester
Ph.D., University of Massachusetts, Amherst

submited May 1995 © 1995

Committee Chair: Michael F. Skrutskie

Committee Members:
Stephen E. Strom
Suzan Edwards
James F. Walker
Department Chair: John F. Dubach

To my Lord, Jesus, and those who showed me His love.


Circumstellar disks with masses comparable to the primeval solar nebula have been discovered around numerous pre-main sequence stars; it is believed the disks are a natural byproduct of star formation. If most stars originally have massive circumstellar disks, it is very likely planetary systems are common. Orbiting planets are not directly observable owing to their relatively cool temperatures and meager surface area. However, in the early stages of planetary formation, the surface area of debris in the disk may exceed the surface area of the star by many orders of magnitude. Material in the terrestrial zone emits primarily at near-infrared wavelengths; sufficient disk debris may produce detectable excess emission at these wavelengths. As clearing mechanisms, including possible planetary formation, remove the small particles in the disk, the strong infrared emission diminishes. By observing the excess infrared emission as a function of stellar age and spectral type, timescales for inner disk processes which create or remove small particles can be established.

This dissertation presents sensitive, simultaneous, near-infrared broadband continuum observations of old pre-main sequence and young main-sequence cluster stars. The stellar ages range from 1-600 Myr, spanning the predicted epoch of planetary formation for solar-type stars. A wide range of spectral types were observed. We detect no excess emission after an age of about 3 ×10^6 yr.

Using a model to predict the infrared emission from an optically thin dust disk, we find our measurements are sensitive to 10^20 - 10^21 g of micron-radius dust grains (rho = 2 g cm^-3) distributed within the terrestrial zone. Adapting this result to a more realistic particle size distribution, we believe we can detect debris in extra-solar systems until the terrestrial planets are 90-95% complete.

Older models of the formation of the Earth which assume orderly growth predict the Earth is 90% complete after about 80 Myr. Newer models allow runaway growth, which shortens the timescale to ~10^5 yr. If the observed clearing in the inner disk reflects the formation of terrestrial planets, our results strongly support models of planetary formation which incorporate runaway growth. Implications are discussed.


Chapter 1. Introduction
Chapter 2. Method
Chapter 3. Observations
Chapter 4. Results
Chapter 5. Discussion
Chapter 6. Conclusions
Appendix A
Appendix B
Appendix C
The Postscript files are also available.