Analysis of the first infrared spectrum of quasi-bound H<SUB>2</SUB> line emission in Herbig-Haro 7

Abstract

Context. Highly excited molecular hydrogen (H<SUB>2</SUB>) 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<SUB>2</SUB> 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 H<SUB>2</SUB>. One emission line at 2.104 μm in this spectrum was unidentified. <BR /> Aims: We aim to complete the analysis of the spectrum of HH7 by including previously missing molecular data that have been recently computed. <BR /> 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. <BR /> 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 H<SUB>2</SUB>, 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. <BR /> 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<SUB>2</SUB> is occurring. The mechanism by which virtually all of the H<SUB>2</SUB> 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.