The following document lists the file abstract/MGIARD_H2OMAPS.abs
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===================================================================== ==> This proposal requests an upgrade from Priority 3 for MGIARD.H2OMAP ===================================================================== We propose to use ISO SWS to measure the abundance of water vapor in photo- dissociation regions and the molecular cloud in their vicinity. Such regions are expected to be chemically very active, particularly due to the release of gaseous species from the grain mantles. Because of the high dipole moment of H2O, 1.88 Debye, the gas density in these regions, 1e4 to 1e6 cm-3, allows little collisional excitation. However, radiative pumping by the strong dust continuum is possible. The case of H2O is particularly favorable, since the fundamental vibrational transition of the molecule, nu2, happens to be coin- cident with the 6.2 mu emission feature. The de-excitation produces emission lines between 6.4 and 6.7 mu, in a spectral region of minimum continuum flux density. Consequently, these lines show a high contrast on the dust continuum, and can be detected with SWS, despite the low resolution of the grating. The intensity of the lines depends only on the H2O column density and the 6.2 mu exciting flux density, so that H2O abundances can be derived. Using the lines of H2O and H218O allows to measure water vapor abundances ranging from 1e-9 to 1e-4. Both the ortho and para H2O will be probed with this method, and the ortho-to-para abundance ratio will be determined. 2000 seconds have been allocated to this program in the first call. We have programmed a reduced observation set toward 3 positions toward the M17 south-west interface. These observations have not yet been executed. However, a coarse SWS01 spectrum obtained toward the M17 interface (Program MPEWARM, JL. Puget observing time, see Fig. 1 in the paper version of the proposal) shows evidence for a possible marginal detection of the stronger predicted emission line : ortho H2O at 6.64 micron. The method we propose is able to inform on the spatial distribution of water vapor in photo-dissociation regions at a scale which resolves the larger molecular clumps of these very inhomogeneous regions.