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Session 7 - Molecular Clouds.
Display session, Wednesday, January 07
In photodissociation regions (PDRs), far ultraviolet (FUV; h\nu=6\--13.6 eV) radiation flux from nearby hot bright stars drives the dominant chemical and thermal processes in the interstellar medium. In this poster, a self-consistent picture of the physical conditions in the Orion Bar PDR is developed from multiwavelength observations. New high angular resolution (\sim9\arcsec) radio maps in the J=1\--0 emission lines of HCO^+ and HCN, made by combining NRAO 12-m single-dish observations with interferometric observations from Berkeley-Illinois-Maryland Association (BIMA) millimeter array, have been compared with previously published data in several wavelength ranges. The findings indicate that although chemical stratification occurs as predicted by the standard PDR model, with 3.3-\mum PAH (polycyclic aromatic hydrocarbon) emission and 2.122\mum emission from FUV-pumped molecular hydrogen (H_2) appearing closest to the ionization front, there is a general spatial coincidence of J=1\--0 emission from HCO^+, HCN, ^12CO, ^13CO, from clumps deeper in the molecular cloud. HCN appears to be tightly confined to these clumps, whereas HCO^+ is more extended toward clump surfaces. A new PDR model incorporating nitrogen chemistry is evaluated using these observations. In addition, far-infrared (FIR) fine-structure line observations of low-excitation PDRs in several reflection nebulae ([OI] (63\mum) and [CII] (158\mum) data from the Cryogenic Grating Spectrometer (CGS) on the Kuiper Airborne Observatory (KAO)) are compared with published FIR observations of the high-excitation Orion Bar PDR.