Radiative Transfer Models of Emission and Absorption in the H2o 6 Micron Vibration-rotation Band toward Orion-bn-kl

We report the spectrum of Orion-BN/KL between 5.3 and 7.2 mm observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory. H2O lines of the –0 bending mode with l ! n 5 1 2 6.3 mm (the R-branch) are observed in absorption, while lines with l 1 6.3 mm (the P-branch) are observed in emission. Radiative transfer models including the (0, 0, 0), (0, 1, 0), and (0, 0, 1) vibrational levels show that this effect is produced in a natural way by H2O absorption of 6 mm continuum photons followed by spontaneous de-excitation to the ground state (resonant scattering). The intensities of the absorption lines can be explained with an H2O column density of a few 10 17 cm in gas with a temperature of ∼150 K and cm, 6 n(H ) ∼ 10 2 although these parameters depend on the assumed location of the absorbing shell. Since the observed intensity of the H2O emission lines are larger than those seen in absorption, we suggest that rovibrational excitation of H2O by collisions in denser and hotter gas, such as found in shocks, also contribute to the P-branch emission of the 6 mm H2O band. Subject headings: infrared: ISM: lines and bands — ISM: abundances — ISM: individual (Orion-BN) — ISM: individual (Orion Kleinmann-Low) — ISM: molecules — radiative transfer