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

TL;DR

This study analyzes the infrared spectrum of Herbig-Haro 7, identifying new molecular hydrogen emission lines from highly excited levels, revealing a hot thermal component and ongoing dissociation processes in the shocked gas environment.

Contribution

The paper provides the first identification of the 1-0 S(29) H2 emission line in HH7, confirming the presence of quasi-bound states and a hot 5000 K thermal component.

Findings

Identification of the 1-0 S(29) line at 2.1042 μm.

Confirmation of a hot 5000 K thermal component.

Evidence of ongoing H2 dissociation in HH7.

Abstract

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 $\mu$m identified as arising from the $v$=2 $J$=29 level, which lies above the dissociation limit of H2. 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 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.} Conclusion. 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 levels with energies from 20,000 K to 53,000 K is maintained in local thermodynamic equilibrium at a single temperature of $\sim$5,000 K remains to be understood.