Abstract
We have computed extensive grids of models of both C- and J-type planar shock waves, propagating in dark, cold molecular clouds, in order to study systematically the be- haviour of the ortho:para-H2 ratio. Careful attention was paid to both macroscopic (dynamical) and microscopic (chemical re- actions and collisional population transfer in H2) aspects. We relate the predictions of the models to observational determina- tions of the ortho:para-H2 ratio using both pure rotational lines and rovibrational lines. As an illustration, we consider ISO and ground-based H2 observations of HH 54. Neither planar C-type nor planar J-type shocks appear able to account fully for these observations. Given the additional constraints provided by the observed ortho:para H2 ratios, a C-type bowshock, or a C-type precursor followed by a J-type shock, remain as plausible mod- els.
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