Hubble Space Telescope FOS Optical and Ultraviolet Spectroscopy of the Bow Shock HH 47A

Hartigan, P. (Rice), Morse, J., Tumlinson, J. (CASA), Raymond, J. (CfA) & Heathcote, S. (NOAO)

We present new spectra obtained with the Faint Object Spectrograph aboard the Hubble Space Telescope of the HH 47A bow shock and Mach disk that cover the entire spectral range between 2220 A and 6810 A. 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 permitted lines can only be understood if transitions to the ground state are resonantly scattered within HH 47A. The expected column density of Fe II within HH 47A suffices to scatter these lines, though the scattering optical depths imply that the Fe II line broadening must exceed that expected from thermal motions. Excitation of ultraviolet Fe II occurs locally within HH 47A, probably from collisions within the hot postshock gas, and not from UV pumping from some nearby O-stars. The data show no evidence for significant depletion of Fe within HH 47A.

The emission lines fluxes and ratios indicate that jet material currently enters the Mach disk with a density of ~ 350 cm-3 and a velocity of ~ 40 km/s . The mass loss rate of the exciting star, as measured by the mass flux through the Mach disk, is 1.6 x 10-8}M_sun yr-1. This mass loss rate is considerably lower than 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 not match the bow shock spectrum well. We propose that secondary shocks reheat the gas within the cooling zone of the HH 47A bow shock. Compression from the first shock will cause these secondary shocks to be strongly magnetized, and the secondary shocks should emit strongly in low-excitation lines such as Mg II, C II] and [S II]. The weak blue continua seen at optical wavelengths in spectra of the Mach disk and bow shock extend into the ultraviolet, and have spectral energy distributions and total fluxes consistent with those expected from 2-photon emission.

1999, ApJ 512, 901