Highly Excited H2 in Herbig-Haro 7: Formation Pumping in Shocked
  Molecular Gas?
  We have obtained K-band spectra at R~5,000 and angular resolution 0.3" of a
section of the Herbig-Haro 7 (HH7) bow shock, using the Near-Infrared Integral
Field Spectrograph at Gemini North. Present in the portion of the data cube
corresponding to the brightest part of the bow shock are emission lines of H2
with upper state energies ranging from ~6,000 K up to the dissociation energy
of H2, ~50,000 K. Because of low signal-to-noise ratios, the highest excitation
lines cannot be easily seen elsewhere in the observed region. However,
excitation temperatures, measured throughout much of the observed region using
lines from levels as high as 25,000 K, are a strong function of upper level
energy, indicating that the very highest levels are populated throughout. The
level populations in the brightest region are well fit by a two-temperature
model, with 98.5% of the emitting gas at T=1800 K and 1.5% at T=5200 K. The
bulk of the H2 line emission in HH7, from the 1,800 K gas, has previously been
well modeled by a continuous shock, but the 5,200 K component is inconsistent
with standalone standard continuous shock models. We discuss various possible
origins for the hot component and suggest that this component is H2 newly
reformed on dust grains and then ejected from them, presumably following
dissociation of some of the H2 by the shock.
