  First detections of thermal water vapor absorption lines have been made
toward Orion IRc2 using the Short Wavelength Spectrometer (SWS) on board the
Infrared Space Observatory (ISO). Grating spectra covering wavelengths 25-45
micron yield 19 pure rotational lines, originating from energy levels 200-750 K
above ground. Fabry-Perot spectra of 5 transitions resolve the line profiles
and reveal the water gas kinematics. The fact that all lines are seen in
absorption is in striking contrast with data from the ISO Long Wavelength
Spectrometer (LWS), where the water lines appear in emission. At least one line
displays a P-Cygni type profile, which suggests that the water is located in an
expanding shell centered on or near IRc2. The expansion velocity is 18 km per
second, in agreement with the value inferred from water maser observations by
Genzel et al. (1981). Because the continuum is intense and likely formed in or
near the water-containing gas, the excitation of the observed transitions is
dominated by radiative processes. A simple, generalised curve-of-growth method
is presented and used to analyze the data. A mean excitation temperature of 72
K and a total water column density of 1.5e18 molecules per cm squared are
inferred, each with an estimated maximum uncertainty of 20%. Combined with the
molecular hydrogen column density derived from ISO observations of the pure
rotational lines, and an assumed temperature of 200-350 K, the inferred water
abundance is 2e-4 to 5e-4 in the warm shocked gas. This abundance is similar to
that found recently by Harwit et al. (1998) toward Orion using data from the
LWS, but higher than that found for most other shocked regions by, for example,
Liseau et al. (1996).
