SOFIA/EXES Observations of Warm H2 at High Spectral Resolution: Witnessing Para-to-Ortho Conversion Behind a Molecular Shock Wave in HH7

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, which we observed 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 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-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.