Infrared [Fe II] Emission Lines from Radiative Atomic Shocks
  [Fe II] emission lines are prominent in the infrared (IR), and they are
important diagnostic tools for radiative atomic shocks. We investigate the
emission characteristics of [Fe II] lines using a shock code developed by
Raymond (1979) with updated atomic parameters. We first review general
characteristics of IR [Fe II] emission lines from shocked gas, and derive [Fe
II] line fluxes as a function of shock speed and ambient density. We have
compiled the available IR [Fe II] line observations of interstellar shocks and
compare them to the ratios predicted from our model. The sample includes both
young and old supernova remnants in the Galaxy and the Large Magellanic Cloud
and several Herbig-Haro objects. We find that the observed ratios of IR [Fe II]
lines generally fall on our grid of shock models, but the ratios of some
mid-infrared lines, e.g., [Fe II] 35.35 um/[Fe II] 25.99 um, [Fe II] 5.340
um/[Fe II] 25.99 um, and [Fe II] 5.340 um/[Fe II] 17.94 um, are significantly
offset from our model grid. We discuss possible explanations and conclude that
the uncertainty in atomic rates might be the major source of uncertainty, while
uncertainties in the shock modeling and the observations certainly exist.
