We propose to use the space-based capability of ISO + SWS to study the
physical conditions in pre-main-sequence protoplanetary disks. Given its
high abundance and the large number of transitions that sample a wide range
in temperature, water vapor is expected to be an excellent probe of the
physical properties of disks over a range of radii (~ 0.1-5 AU). Although
strong absorption in the Earth's atmosphere generally precludes ground-based
studies of water vapor, our discovery of hot (~ 2000K) water emission from
pre-main-sequence disks, made in the wings of the strong telluric bands,
demonstrates the viability of water as a probe of the physical conditions at
small disk radii. We will use the SWS to obtain spectra of the much stronger
fundamental H2O bands centered near 2.7 and 6 microns. These bands will
sample cooler disk gas at larger radii (r > 0.3 AU). The data will be modeled
in order to extract physical properties, gas temperature and column density,
from the observed spectra.