Molecular H2 Emission in HH47A: HST GHRS and FOC Observations

Curiel, S. and Raymond, J. (CfA), Wolfire, M. (UCB), Hartigan, P. (Rice), Morse, J. (STScI), Schwartz, R. (UMSL), and Nisenson, P. (CfA)
We present HST ultraviolet observations of the bow shock at the end of the HH 47 stellar jet obtained with the GHRS spectrograph and the FOC camera. The GHRS spectrum shows three prominent emission lines of H_2 which are produced by Ly-alpha fluorescence, and one line that we cannot identify. Fluorescence from Ly-a generated in the bow shock and Mach disk of HH 47A can account for the observed H_2 fluxes provided that the H_2 absorbs about one-third of these Ly-a photons. We find that our FOC image of HH 47A is made up of about 70% hydrogen two photon continuum and about 30% fluorescent H_2 emission. The FOC image closely resembles optical [S II] and H-alpha images of HH 47A, but differs significantly from H_2 images of the region taken at near-infrared wavelengths. This is because the two photon continuum and the Ly-a photons which drive H_2 fluorescence both originate in the H-alpha emitting gas, but the Ly-a mean free path is smaller than the region of infrared H_2 emission.

The presence of molecular hydrogen in HH 47A is difficult to understand. If the H_2 forms in a dense region between the bow shock and the Mach disk, then the emission should be significantly more blueshifted than observed. Models that excite H_2 using C-shocks or magnetic precursors assume that molecular hydrogen exists in the preshock gas of HH 47A. However, this gas lies within the wake of a previous high velocity ejection from the star and is exposed to ionizing radiation from the Gum nebula, so we would not expect to find any H_2 in this region.

Numerical calculations indicate that the UV H_2 line emission may be produced by either reformed H2 molecules in the region between the bow shock and the Mach disk or from a C-shock or magnetic precursor along the wings of the bow shock. We propose that the scenario that best explains the current results is one where the HH 47A bow shock runs into a clump of molecular gas that either moves along with the preshock gas, or is nearly stationary with respect to the ambient cloud and is located behind the jet.

Journal of publication: Ap J 453, 322, 1995