Kinematics of the Horsehead Nebula and IC 434 Ionization Front in CO and C

Stratospheric Observatory for Infrared Astronomy [C II] 157 mu m, APEX 860 mu m J = 3-2 CO, and archival James Clerk Maxwell Telescope J = 2-1 CO and (CO)-C-13 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 H-2 density of the Horsehead, similar to 6 x 10(3) cm(-3), results in a spatially thin PDR where the photoablation flow has compressed the western cloud edge to an H-2 density of (2-6) x 10(4) cm(-3). The associated [C II] 157 mu m 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 H-2 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 mu m emission is much thicker with L similar to 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.