Very High Excitation Lines of H2 in the Orion Molecular Cloud Outflow

Vibration–rotation lines of H2 from highly excited levels approaching the dissociation limit have been detected at a number of locations in the shocked gas of the Orion Molecular Cloud (OMC-1), including in a Herbig–Haro object near the tip of one of the OMC-1 “fingers.” Population diagrams show that, while the excited H2 is almost entirely at a kinetic temperature of ∼1800 K (typical for vibrationally shock-excited H2), as in the previously reported case of Herbig–Haro object HH 7 up to a few percent of the H2 is at a kinetic temperature of ∼5000 K. The location with the largest fraction of hot H2 is the Herbig–Haro object, where the outflowing material is moving at a higher speed than at the other locations. Although theoretical work is required for a better understanding of the 5000 K H2 (including how it cools), its existence and the apparent dependence of its abundance relative to that of the cooler component on the relative velocities of the outflow and the surrounding ambient gas appear broadly consistent with it having recently reformed. The existence of this high-temperature H2 appears to be a common characteristic of shock-excited molecular gas.