HCN and HCO+Images of the Orion Bar Photodissociation Region

Abstract

The Orion Bar is an ideal astrophysical laboratory for studying photodissociation regions because of its nearly edge-on orientation in the observer's line of sight. High angular resolution (~9'') maps of the Orion Bar in the J = 1-0 emission lines of HCO+ and HCN have been made by combining single-dish millimeter observations with interferometric data. This mapping technique provides both large-scale structural information and high resolution. The new maps show that HCO+ and HCN have globally similar spatial distributions in the Orion Bar. Both molecular species show the same clumpy NE 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 bar structure and to clump cores than HCO+ emission. We do a crosscut comparison of our full-synthesis maps with previously published observations of the Orion Bar in: (1) the rotational transitions of 12CO J = 1-0, 13CO J = 1-0, CN N = 3-2, and CS J = 7-6; (2) the UV-pumped rovibrational transition of H2 at 2.122 μm; (3) 3.3 μm emission attributed to the aromatic C–H bond stretching of polycyclic aromatic hydrocarbons (PAH); and (4) the atomic fine-structure transitions of C I (609 μm), O I (63 μm), and C II (158 μm). 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 symmetrical compared to the broader asymmetrical HCO+ peak. We argue that this difference in peak shape supports a previously published suggestion that HCO+ production is enhanced in warm gas at the surface of the photodissociation region. We explain these observations using a nonhomogeneous photodissociation region model to which we have added nitrogen chemistry and the thermal 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 least two density components in the Orion Bar. From our model calculations, we have found that a ridge of dense clumps (3 × 106 cm-3) embedded in a lower density interclump medium (5 × 104 cm-3) explains our observations very well. Although some of the observations (e.g., emissions from H2, 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 ionization front. This result agrees with the findings of previous observers, who have suggested the presence of dense clumps in the Orion Bar.