Optical and infrared images and spectroscopy of the HH 168 bubble in Cepheus A

We present new optical emission-line and continuum images taken with the Hu bble Space Telescope of the shocked bubble HH 168 (GGD 37) associated with the massive star formation region Cepheus A. The new images clearly resolve the interface between the molecular flow, defined by near-IR images of H-2 , and the shocked, optically emitting gas. The H-2 emits in a clumpy sheath that surrounds the optical emission, in what appears to be a precursor or a C-type shock. This region also emits in [S II]. Hence, unlike most other Herbig-Haro (HH) regions, where [S II] radiates only behind Ha in a cooling zone, in HH 168 the [S II] emission leads H alpha when H-2 emission is pre sent. The [S II] in the precursor C-shock separates spatially from the H-2; hence, this region is not isothermal. H-2 emission is absent and [O III] i s bright near the apices of the high excitation bow shocks in the flow, as expected from theory. Radial velocities of H-2 derived from high spectral r esolution slit maps of the two highest excitation HH objects lie within sim ilar to 30 km s(-1) of the molecular cloud velocity, consistent with the pr ecursor scenario. Some of the H-2 emission in the region is redshifted and apparently unrelated to the HH 168 bubble. Additional discoveries include n umerous bright pointlike H alpha emission objects, some of which may be T T auri stars, a bright jet unrelated to the bubble how, a bumpy morphology to the bright bow shock S, and several locations where "fingers" of H-2 termi nate in optically visible HH emission, similar to what is seen in the Orion Nebula.