Kinematics of the Horsehead Nebula and IC 434 Ionization Front in CO and C+
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Stratospheric Observatory for Infrared Astronomy [C ii] 157 μm, APEX 860 μm J = 3-2 CO, and archival James Clerk Maxwell Telescope J = 2-1 CO and 13CO observations of the Horsehead Nebula are presented. The photon-dominated region (PDR) between the Orion B molecular cloud and the adjacent IC 434 H ii region is used to study the radial velocity structure of the region and the feedback impacts of UV radiation. Multiple west-facing cloud edges are superimposed along the line of sight with radial velocities that differ by a few kilometers per second. The Horsehead lies in the foreground blueshifted portion of the Orion B molecular cloud and is predominantly illuminated from the rear. The mean H2 density of the Horsehead, ∼ 6 × 103 cm-3, results in a spatially thin PDR where the photoablation flow has compressed the western cloud edge to an H2 density of (2-6) × 104 cm-3. The associated [C ii] 157 μm layer has a width L < 0.05 pc. The background parts of the Orion B cloud in the imaged field consist of a clumpy medium surrounded by molecular gas with H2 densities lower by one to two orders of magnitude. Along the straight part of the IC 434 ionization front, the PDR layer probed by [C ii] 157 μm emission is much thicker with L ∼ 0.5 pc. A possible model for the formation and evolution of this edge-on ionization front and PDR is presented. The [C ii] data were independently analyzed and published by Pabst et al. © 2018. The American Astronomical Society. All rights reserved.
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