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arXiv · 1905.10381

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

David A. Neufeld·Curtis DeWitt·Pierre Lesaffre·Sylvie Cabrit·Antoine Gusdorf·Le Ngoc Tram·Matthew Richter
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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.

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David A. Neufeld, Curtis DeWitt, Pierre Lesaffre, Sylvie Cabrit, Antoine Gusdorf, Le Ngoc Tram, Matthew Richter. 2019-05-24. SOFIA/EXES observations of warm H$_2$ at high spectral resolution: witnessing para-to-ortho conversion behind a molecular shock wave in HH7. https://doi.org/10.3847/2041-8213%2Fab2249

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