Hubble Space Telescope faint object spectrograph optical and ultraviolet spectroscopy of the bow shock HH 47A

We present new spectra obtained with the Faint Object Spectrograph aboard t he Hubble Space Telescope of the HH 47A bow shock and Mach disk that cover the entire spectral range between lambda lambda 2220 and 6810. In addition to emission lines seen previously from HH objects, we uncover over a dozen weak Fe II transitions in the ultraviolet. The flux ratios between these pe rmitted lines can only be understood if transitions to the ground state are resonantly scattered within HH 47A. The expected column density of Fe rr w ithin HH 47A suffices to scatter these lines, although the scattering optic al depths imply that the Fe II line broadening must exceed that expected fr om thermal motions. Excitation of ultraviolet Fe II occurs locally within H H 47A, probably from collisions within the hot postshock gas and not from U V pumping from some nearby O stars. The data show no evidence for significa nt depletion of Fe within HH 47A. The emission line's fluxes and ratios ind icate that jet material currently enters the Mach disk with a density of si milar to 350 cm(-3) and a velocity of similar to 40 km s(-1). The mass-loss rate of the exciting star, as measured by the mass flux through the Mach d isk, is 1.6 x 10(-8) M. yr(-1). This mass-loss rate is considerably lower t han that closer to the star where the jet is brighter, probably because the density along the jet is highly nonuniform. A single-shock velocity does n ot match the bow shock spectrum well. We propose that secondary shocks rehe at the gas within the cooling zone of the HH 47A bow shock. Compression fro m the first shock will cause these secondary shocks to be strongly magnetiz ed, and the secondary shocks should emit strongly in low-excitation lines s uch as Mg II, C II], and [S II]. The weak blue continua seen at optical wav elengths in spectra of the Mach disk and bow shock extend into the ultravio let and have spectral energy distributions and total fluxes consistent with those expected from two-photon emission.