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

## TL;DR

This study uses high-resolution SOFIA/EXES observations of H₂ lines in HH7 to detect para-to-ortho conversion within a shock wave, providing evidence for C-type shocks and chemical changes in interstellar environments.

## Contribution

First spectrally-resolved detection of velocity shifts between ortho and para H₂ lines in a shock, confirming para-to-ortho conversion and supporting C-type shock models.

## Key findings

- Detected velocity shifts indicating para-to-ortho H₂ conversion.
- Observed chemical changes consistent with non-dissociative C-type shocks.
- Provided evidence for continuous flow velocity changes in shock waves.

## 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.