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

Abstract

Stratospheric Observatory for Infrared Astronomy [C ii ] 157 μ m, APEX 860 μ m J  = 3−2 CO, and archival James Clerk Maxwell Telescope J = 2−1 CO and 13 CO 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, , results in a spatially thin PDR where the photoablation flow has compressed the western cloud edge to an H 2 density of . The associated [C ii ] 157 μ 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 μ m 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.