5. Water absorption toward Orion

Wright (UNSW), van Dishoeck, Black (Onsala, Sweden), de Graauw (SRON Groningen) and collaborators obtained the first detections of thermal water vapor absorption lines toward Orion IRc2 using the ISO-SWS. Grating spectra covering wavelengths 25-45 $\mu$m yield 19 pure rotational lines, originating from levels 200-750 K above ground. Fabry-Pérot spectra of 5 transitions resolve the line profiles, which reveal the H₂O gas kinematics, see Fig. 2.8. The fact that all lines are seen in absorption is in striking contrast with data from the ISO-LWS, where the H₂O lines appear in emission. At least one line displays a P-Cygni 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. 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, generalized curve-of-growth method has been developed to analyze the data. Combined with the H₂ column density derived from ISO observations of the pure rotational H₂ lines and assuming a temperature of 200-350 K, the inferred H₂O abundance is $\sim$(2-5) $\times 10^{-4}$ in the warm shocked gas, similar to that derived from the LWS data, but higher than that found for most other shocked regions.

  

Figure 2.8: ISO-SWS Fabry-Pérot spectra toward Orion IRc2 of a selection of highly-excited H₂O lines. The P-Cygni-type profile seen for some lines indicates that the H₂O is located in an expanding shell with a very high water abundance. The panel in the lower right-hand corner displays the 5.3-7.0 $\mu$m grating spectrum of the H₂O $\nu _2$=1-0 vibration-rotation band (from: Wright et al.).