ISO - SWS observations of pure rotational H2O absorption lines toward Orion - IRc2

First detections of thermal water vapor absorption lines have been made toward Orion IRc2 using the Short Wave- length Spectrometer(SWS) on board the Infrared Space Obser- vatory (ISO). Grating spectra covering wavelengths 25-45 m 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 H 2O 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 H2O lines appear in emission. At least one line dis- plays 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 s 1 , in agreement with the value inferred from H2O 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 transi- tions 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 H2O column density of 1:5 10 18 cm 2 are inferred, each with an estimated maximum uncertainty of 20%. Combined with the H2 column density derived from ISO observations of the pure rotational H2 lines, and an assumed temperature of 200-350 K, the inferred H2O abundance is 2-510 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).