|Testing the Water Predictions of Thermo-Chemical Models of Externally-Illuminated Molecular Gas
|Models that include the key radiative and chemical processes necessary to understand the emission from externally-illuminated molecular gas - so called thermo-chemical or PDR models - are now commonly invoked to account for the line flux from far-ultraviolet illuminated galactic and extragalactic molecular clouds as well as protoplanetary disk surfaces irradiated by their central stars. Whether these models are correct in general, and for water in particular, remains an open question; using present observations to test these models has proven difficult because of the complexity of the regions observed and the resulting uncertainty in many of the model input parameters. Using HIFI, we will obtain scan maps in the ground-state 1(10)-1(01) ortho-water transition at 557 GHz across the ionization front-molecular cloud interface toward two well-studied, uncomplicated regions with favorable geometry - Cepheus B and the starless dark globule DC 267.4-7.5. The proposed observations will produce measures of the gas-phase water abundance across these two atomic-to-molecular transition zones, which will be compared to the model-predicted profiles of water emission versus Av into the clouds. Because of the edge-on geometry and the absence of known embedded sources along the scan paths, the proposed observations provide the best, most controlled test of these models to date. Both regions have been successfully observed previously with SWAS, but SWAS.s much larger beam size made the kind of study proposed here impossible. Neither the Herschel WISH nor PRISMAS Key Programs contains observations that come close to offering as rigorous a test of the models as constructed here. Because these observations require a space-borne telescope with both high spatial and spectral resolution, they cannot be carried out from any other observatory in the foreseeable future.
|Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
|Publisher And Registrant
|European Space Agency
|European Space Agency, 2012, OT2_gmelnick_2, SPG v14.1.0. https://doi.org/10.5270/esa-qutg27o