Analysis of the first infrared spectrum of quasi-bound H2 line emission in Herbig-Haro 7

Context. Highly excited molecular hydrogen (H-2) has been observed in many regions of shocked molecular gas. A recently published K-band spectrum of Herbig-Haro 7 (HH7) contains several vibration-rotation lines of H-2 from highly excited energy levels that have not been detected elsewhere, including a line at 2.179 mu m identified as arising from the v = 2; J = 29 level, which lies above the dissociation limit of H-2. One emission line at 2.104 mu m in this spectrum was unidentified. Aims. We aim to complete the analysis of the spectrum of HH7 by including previously missing molecular data that have been recently computed. Methods. We re-analysed the K-band spectrum, emphasising the physics of quasi-bound upper levels that can produce infrared emission lines in the K band. Results. We confirm the identification of the 2 1 S (27) line at 2.1785 mu m and identify the line at 2.1042 mu m as due to the 1 0 S (29) transition of H-2, whose upper level energy is also higher than the dissociation limit. This latter identification, its column density, and the energy of its upper level further substantiate the existence of a hot thermal component at 5000K in the HH7 environment. Conclusions. The presence of the newly identified 1-0 S (29) line, whose quasi-bound upper level (v = 1, J = 31) has a significant spontaneous dissociation probability, shows that dissociation of H-2 is occurring. The mechanism by which virtually all of the H2 in levels with energies from 20 000K to 53 000K is maintained in local thermodynamic equilibrium at a single temperature of similar to 5000K remains to be understood.