An excitation study of bow shocks driven from protostars in S233IR

We present narrow-band near-infrared images of the high-mass star formation region S233IR. We detect several groups of molecular hydrogen knots aligned with known outflows. Images in vibrationally-excited H2 and [Fe II] reveal the excitation distributions across the main two conspicuous bow shocks. Emission from the [Fe II] line is strong and located at the apex of both bow shocks as well as other locations within the northern bow where we distinguish a compact Mach disk through its [Fell] emission. The H2 emission is not restricted to the bow wings but distinct components are detected near the bow apices. The data are broadly comparable to steady bow shocks of speed 60-80 km s-1 possessing dissociative caps and C-type flanks. Nevertheless, non-steady bow shock behavior is essential to interpret all aspects of the bow structures. In this respect, the S233 bow caps may be in the neutral transformation stage from J-type to C-type.