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Astron. Astrophys. 356, 1010-1022 (2000)


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The ortho:para-H2 ratio in C- and J-type shocks *

D. Wilgenbus 1, S. Cabrit 1, G. Pineau des Forêts 2,3 and D.R. Flower 4

1 Observatoire de Paris, DEMIRM, UMR 8540 du CNRS, 61 Avenue de l'Observatoire, 75014 Paris, France
2 Observatoire de Paris, DAEC, UMR 8631 du CNRS, 92195 Meudon Principal Cedex, France
3 Université Paris XI, Institut d'Astrophysique Spatiale, 91405 Orsay Cedex, France
4 The University, Physics Department, Durham DH1 3LE, UK

Received 16 November 1999 / Accepted 8 February 2000

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 behaviour of the ortho:para-H2 ratio. Careful attention was paid to both macroscopic (dynamical) and microscopic (chemical reactions and collisional population transfer in H2) aspects. We relate the predictions of the models to observational determinations 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 models.

Key words: Magnetohydrodynamics (MHD) – molecular processes – shock waves – ISM: individual objects: HH 54 – ISM: molecules – infrared: ISM: lines and bands

* Tables 2a-f and 4a-f are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: david.wilgenbus@obspm.fr

© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000

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