2019ApJ...878L..18N

Spectrally resolved observations of three pure rotational lines of H₂, conducted with the EXES instrument on SOFIA toward the classic bow shock HH7, reveal systematic velocity shifts between the S(5) line of ortho-H₂ and the two para-H₂ 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-H₂ to ortho-H₂ 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 nondissociative, 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-H₂ to ortho-H₂ 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.