HCN and HCO+ images of the Orion Bar photodissociation region

The Orion Bar is an ideal astrophysical laboratory for studying photodissoc iation regions because of its nearly edge-on orientation in the observer's line of sight. High angular resolution (similar to 9") maps of the Orion Ba r in the J = 1-0 emission lines of HCO+ and HCN have been made by combining single-dish millimeter observations with interferometric data. This mappin g technique provides both large-scale structural information and high resol ution. The new maps show that HCO+ and HCN have globally similar spatial di stributions in the Orion Bar. Both molecular species show the same clumpy N E to SW bar seen by previous observers in molecular line emission from the Orion Bar. However, our maps show HCN emission to be more confined to the b ar structure and to clump cores than HCO+ emission. We do a crosscut compar ison of our full-synthesis maps with previously published observations of t he Orion Bar in: (1) the rotational transitions of (CO)-C-12 J = 1-0, (CO)- C-13 J = 1-0, CN N = 3-2, and CS J = 7-6; (2) the UV-pumped rovibrational t ransition of H-2 at 2.122 mu m; (3) 3.3 mu m emission attributed to the aro matic C-H bond stretching of polycyclic aromatic hydrocarbons (PAH); and (4 ) the atomic finestructure transitions of C I (609 mu m), O I (63 mu m), an d C II (158 mu m). The crosscuts show the same chemical stratification seen by previous observers as expected from an edge-on photodissociation region . In:addition, we see that the HCN peak profile is relatively narrow and sy mmetrical compared to the broader asymmetrical HCO+ peak. We argue that thi s difference in peak shape supports a previously published suggestion that HCO+ production is enhanced in warm gas at the surface of the photodissocia tion region. We explain these observations using a nonhomogeneous photodiss ociation region model to which we have added nitrogen chemistry and the the rmal chemical effects of polycyclic aromatic hydrocarbons. Instead of using a homogeneous model, we follow more recent models employing two components because the clumpiness seen in all the recent observations suggests at lea st two density components in the Orion Bar. From our model calculations, we have found that a ridge of dense clumps (3 x 10(6) cm(-3)) embedded in a l ower density interclump medium (5 x 10(4) cm(-3)) explains our observations very well. Although some of the observations (e.g., emissions from H-2, CO , O I, C I and C II) arise from the interclump medium, we show that HCN and HCO+ J = 1-0 emission must come from a ridge of dense clumps near the ioni zation front. This result agrees with the findings of previous observers, w ho have suggested the presence of dense clumps in the Orion Bar.