2023A&A...669L...7R

Context. Highly excited molecular hydrogen (H₂) 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 H₂ from highly excited energy levels that have not been detected elsewhere, including a line at 2.179 μm identified as arising from the v = 2, J = 29 level, which lies above the dissociation limit of H₂. One emission line at 2.104 μm 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 μm and identify the line at 2.1042 μm as due to the 1-0 S(29) transition of H₂, 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 5000 K 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 H₂ is occurring. The mechanism by which virtually all of the H₂ in levels with energies from 20 000 K to 53 000 K is maintained in local thermodynamic equilibrium at a single temperature of ∼5000 K remains to be understood.