2000AJ....120.1436H

We present new optical emission-line and continuum images taken with the Hubble 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 H₂, and the shocked, optically emitting gas. The H₂ 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 Hα in a cooling zone, in HH 168 the [S II] emission leads Hα when H₂ emission is present. The [S II] in the precursor C-shock separates spatially from the H₂; hence, this region is not isothermal. H₂ emission is absent and [O III] is bright near the apices of the high excitation bow shocks in the flow, as expected from theory. Radial velocities of H₂ derived from high spectral resolution slit maps of the two highest excitation HH objects lie within ∼30 km.s^–1 of the molecular cloud velocity, consistent with the precursor scenario. Some of the H₂ emission in the region is redshifted and apparently unrelated to the HH 168 bubble. Additional discoveries include numerous bright pointlike Hα emission objects, some of which may be T Tauri stars, a bright jet unrelated to the bubble flow, a bumpy morphology to the bright bow shock S, and several locations where ``fingers'' of H₂ terminate in optically visible HH emission, similar to what is seen in the Orion Nebula.