HCN and HCO+ Images of the Orion Bar Photodissociation Region

Details HCN and HCO+ Images of the Orion Bar Photodissociation Region HCN and HCO+ Images of the Orion Bar Photodissociation Region

Nov 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998adil...ry...01y&link_type=abstract

NCSA Astronomy Digital Image Library

Astronomy and Astrophysics

Astronomy

Scientific paper

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 arcsec) 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, with both molecular species displaying a clumpy structure organized in a characteristic NE to SW bar. However, HCN emission appears to be more confined to the bar structure and to clump cores than HCO+ emission. The new maps are compared with previous observations of the Orion Bar in several molecular line transitions: (1) 12^CO and 13^CO J= 1-0, (2) the UV-pumped rovibrational transition of H_2 at 2.122 microns, (3) 3.3-micron emission attributed to the aromatic C-H bond stretching of polycyclic aromatic hydrocabons (PAH), and (4) the fine structure transition of neutral carbon (CI) at 609 microns. Comparing crosscut scans from all of the maps shows the chemical stratification expected from an edge-on photodissociation region. We find that two components are necessary to explain the observations: a ridge of dense (3 x 10^6 cm^-3) embedded in a lower density(5 x 10^4 cm^-3) interclump medium. Most of the previous observations can be explained by emission from the interclump medium. The HCN and HCO+ observations, however, must arise from a dense ridge of clumps. In fact, the bulk of the HCN and HCO+ J=1-0 emission appears to arise deep within the clump, indicating that these low-J transition lines do not come from products of PDR chemistry.

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