ISO – SWS observations of pure rotational H 2 O absorption lines toward Orion – IRc 2 ⋆

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 µm 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 2 O 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 H 2 O 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 s −1 , in agreement with the value inferred from H 2 O 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 ra-diative 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 H 2 O column density of 1.5 × 10 18 cm −2 are inferred, each with an estimated maximum uncertainty of 20%. Combined with the H 2 column density derived from ISO observations of the pure rotational H 2 lines, and an assumed temperature of 200–350 K, the inferred H 2 O abundance is 2–5×10 −4 in the warm shocked gas. This abundance is similar to that found recently by Harwit et al. (1998) toward Orion using Send offprint requests to: ⋆ Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASA. data from the LWS, but higher than that found for most other shocked regions by, for example, Liseau et al. (1996).