2000A&A...356.1010W

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 behaviour of the ortho:para-Htwo ratio. Careful attention was paid to both macroscopic (dynamical) and microscopic (chemical reactions and collisional population transfer in Htwo) aspects. We relate the predictions of the models to observational determinations of the ortho:para-Htwo ratio using both pure rotational lines and rovibrational lines. As an illustration, we consider ISO and ground-based Htwo 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 Htwo ratios, a C-type bowshock, or a C-type precursor followed by a J-type shock, remain as plausible models.