Jet bow shocks and clumpy shells of H-2 emission in the young stellar outflow Cepheus A

New narrowband infrared H-2 and continuum images taken in subarcsecond seeing with the CFHT resolve the shells of molecular hydrogen discovered by Bally & Lane (1991) in Cepheus A into dozens of tiny clumps. The molecular gas lies exterior to HH objects that are visible in [S II] and H alpha images of the region, Additional wide field images show that the H-2 emission is distributed primarily to the east and west of the luminous cluster of massive stars that marks the center of Cepheus A. The molecular emission to the east appears as an irregular jet, while that to the west concentrates in shells. The various mechanisms proposed to heat molecular gas in young stellar outflows predict distinct spatial and kinematic signatures for the optical and H-2 emission that can be compared with observations. In Cepheus A, shocks appear to be responsible for most of the H-2 emission, though fluorescence could excite a diffuse bow-shaped feature that has a bright optical counterpart. We propose that wakes from the bow shocks in the HH objects heat and accelerate the H-2 into the observed shells, while the molecular clumps form as a result of cooling instabilities behind the bow shocks. (C) 1996 American Astronomical Society.