SOFIA/EXES observations of warm H${}_2$ at high spectral resolution: witnessing para-to-ortho conversion behind a molecular shock wave in HH7

Abstract

Spectrally-resolved observations of three pure rotational lines of H${}_2$, conducted with the EXES instrument on SOFIA toward the classic bow shock HH7, reveal systematic velocity shifts between the S(5) line of ortho-H${}_2$ and the two para-H${}_2$ lines [S(4) and S(6)] lying immediately above and below it on the rotational ladder. These shifts, reported here for the first time, imply that we are witnessing the conversion of para-H${}_2$ to ortho-H${}_2$ within a shock wave driven by an outflow from a young stellar object. The observations are in good agreement with the predictions of models for non-dissociative, C-type molecular shocks. They provide a clear demonstration of the chemical changes wrought by interstellar shock waves, in this case the conversion of para-H${}_2$ to ortho-H${}_2$ in reactive collisions with atomic hydrogen, and provide among the most compelling evidence yet obtained for C-type shocks in which the flow velocity changes continuously.

arXiv comment19 pages, 4 figures. Accepted for publication in the Astrophysical Journal Letters

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Cited by in corpus (6)

• Shock excitation of H${}_2$ in the James Webb Space Telescope era
• Probing the hidden atomic gas in Class I jets with SOFIA
• A detailed temperature map of the archetypal protostellar shocks in L1157
• C-type shock modelling -- the effect of new H${}_2$-H collisional rate coefficients
• Shocks and Molecules in Diffuse Interstellar Cloud Pairs
• Analysis of the first infrared spectrum of quasi-bound H2 line emission in Herbig-Haro 7