HH 110 Jet Near-Infrared Imaging: The Outflow Mixing Layer

Abstract

We present near-infrared images of the Herbig-Haro 110 jet centered at the molecular hydrogen lines v = 1-0 (2.121 μm) and v = 2-1(2.248 μm). In young stellar objects these lines are mostly excited by low-velocity shocks and the energy released by turbulent processes. The ratio of these lines provides us with a preliminary diagnostic of the molecular gas excitation. The HH 110 jet was selected for four reasons: (1) it has a complicated optical "turbulent" morphology, (2) it is close to a high-density circumstellar environment (a molecular core), (3) it does not have an obvious driving source, and (4) theoretical models suggest that this object corresponds to the early stages of a jet-cloud collision. <P />We find that the molecular hydrogen emission follows the optical Hα and [S II] emission in a "straight" section of the jet (the northern ∼1'), but it becomes shifted westward and separated from the optical emission afterward. We suggest that these morphological properties of the molecular hydrogen emission are consistent with that of a boundary layer. <P />The 2.248 μm emission is faint in most condensations, except for the B1, E1, H1, and P1 knots, where it is clearly detected. The ratio of the 2.121 μm to the 2.248 μm lines for these knots ranges from 4.5 to 7.5, which implies excitation temperatures of ∼2400-3100 K. The brightest condensation, H, however, has a smaller ratio (∼1.9) and a higher temperature (∼5900 K), which suggests a nonisothermal jet structure. In those cases where the H<SUB>2</SUB> emission is likely to be due to shocks, the line ratios are small enough to be explained by either C- or J-type shocks.