MOLECULAR H-2 EMISSION IN HH 47A - HUBBLE-SPACE-TELESCOPE GHRS AND FOC OBSERVATIONS

We present Hubble Space Telescope ultraviolet observations of the bow shock at the end of the HH 47 stellar jet obtained with the Goddard High Resolution Spectrograph (GHRS) and the Faint Object Camera (FOG). 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 alpha generated in the bow shock and Mach disk of HH 47A can account for the observed H-2 line fluxes provided that the H, absorbs about one-third of these Ly alpha 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. This image closely resembles optical [S II] and H alpha images of HH 47A but differs significantly from H, images of the region taken at near-infrared wavelengths. This is because the two-photon continuum and the Ly alpha photons which drive H-2 fluorescence both originate in the Hat-emitting gas, and the Ly alpha mean free path is small. 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 H-2 molecules in the region between the bow shock and the Mach disk or from a C-shock or magnetic precursor, while the near-infrared H-2 emission observed in HH 47A arises mainly from 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 in which the HH 47A bow shock is running into a clump of molecular gas which could be either comoving with the preshock gas or nearly stationary with respect to the ambient cloud, primary on its more distant face.