2018AJ....155...80B

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.