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Molecular H 2 Emission in HH 47A: Hubble Space Telescope GHRS and FOC Observations
Curiel, Salvador; Raymond, John C.; Wolfire, Mark; Hartigan, Patrick; Morse, Jon; Schwartz, Richard D.; Nisenson, Peter
AA(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AB(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138), AC(Astronomy Department, University of California at Berkeley, NASA-Ames (MS 245-3), Moffett Field, CA 94035), AD(Rice University, Houston, TX 77251), AE(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218), AF(Department of Physics and Astronomy, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121), AG(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138)
Astrophysical Journal v.453, p.322 (ApJ Homepage)
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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 (FOC). The GHRS spectrum shows three prominent emission lines of H2 which are produced by Lyalpha fluorescence and one line that we cannot identify. Fluorescence from Lyman alpha generated in the bow shock and Mach disk of HH 47A can account for the observed H2 line fluxes provided that the H2 absorbs about one-third of these Lyman 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 H2 emission. This image closely resembles optical [S II] and Halpha images of HH 47A but differs significantly from H2 images of the region taken at near-infrared wavelengths. This is because the two-photon continuum and the Lyalpha photons which drive H2 fluorescence both originate in the Halpha-emitting gas, and the Lyalpha mean free path is small.

The presence of molecular hydrogen in HH 47A is difficult to understand. If the H2 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 H2 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 H2 in this region.

Numerical calculations indicate that the UV H2 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, while the near-infrared H2 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.

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