Jet Bow Shocks and Clumpy Shells of H_2 Emission in the Young Stellar Outflow Cepheus A

Hartigan, P. (Rice), Carpenter, J. M. (IfA), Dougados, C. (Grenoble), and Skrutskie, M. F. (UMass)
New narrow-band infrared H_2 and continuum images taken in subarcsecond seeing with the CFHT resolve the shells of molecular hydrogen discovered by Bally and 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.

Journal of publication: Astronomical Journal 111, 1278, 1996.