Steve V. W. Beckwith 1, M. R. Meyer 2, A. Natta 3, & M. Robberto 4,5
1 Space Telescope Science Institute, Baltimore MD, USA
2 Steward Observatory, Tucson AZ, USA
3 Osservatorio Astrofisico di Arcetri, Firenze, Italy
4 Max-Planck-Institut für Astronomie, Heidelberg, Germany
5 Osservatorio Astronomico di Torino, Torino, Italy
Using ISO-PHOT, we have investigated the nature and evolution of circumstellar
disks
around young, low-mass stars. In order to determine the evolutionary timescales
of
circumstellar disks, a large sample ( 100) of stars between the ages of 106
and 109
years has been surveyed at 20 and 60 
.
The detection rate of IR excess emission
allows us to relate the timescale for dissipation or coagulation of small
particles in
the disks to the timescale for cessation of accretion onto the disks and the
stars. It
turns out that disk dissipation occurs soon after the termination of accretion,
suggesting that planetesimal formation can be a rather rapid phenomenon.
To analyse the detailed structure of the disks, broad band photometry and
higher
resolution spectra have been taken between 2.5 and 100
for
20 stars in Taurus
(1.5 Myr old) and Chameleon (3 MYr old) dark clouds. The resulting spectral
energy
distributions allow us to test the current theories of disk structure and
heating,
and to search for evolutionary changes brought about by the formation of
planetary
systems.