Infrared Line Emission in the Herbig-Haro 7 Bowshock from 5,000 K Molecular Hydrogen in Quasi-bound States

Abstract

Previous spectroscopy of shock-excited H2 line emission in the Herbig-Haro 7 (HH7) bow shock and in the Orion Molecular Cloud (OMC-1) outflow has demonstrated that, although the vast majority of the line emission in those regions comes from gas at T ≈ 2,000 K, as expected for continuous shocks, 1-2% of the H2 is in LTE at a temperature of ~5,000 K. The existence of this latter component of the H2 is not explained by current shock models. One H2 line detected in HH7 is emitted from the quasi-bound v=2 J=29 level, whose energy is ~700 K above the dissociation limit of the ground state of H2. New calculations of the properties of quasi-bound H2 levels have led to the identification of a previously unidentified line in the HH7 spectrum as originating in a second quasi-bound state, v=1, J=31, ~1500 K above the dissociation limit. The existence of this 5,000 K gas and the confirmation that shock-excited H2 in molecular clouds exists in the above highly excited states offer new opportunities to study and more fully understand shock-excitation in molecular clouds.