The Evolution of Dust in the Terrestrial Planet Region of
Circumstellar Disks Around Young Stars
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
Department Chair: John F. Dubach
- Committee Members:
- Stephen E. Strom
James F. Walker
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 yr.
Using a model to predict the infrared emission from an optically thin
dust disk, we find our measurements are sensitive to
10 - 10 g of micron-radius dust grains
( = 2 g
cm) 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 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
The Postscript files are also available.
- Chapter 1. Introduction
- Chapter 2. Method
- Chapter 3. Observations
- Chapter 4. Results
- Chapter 5. Discussion
- Chapter 6. Conclusions
- Appendix A