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Astron. Astrophys. 326, 1176-1186 (1997)

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Alfvén wave support of a dwarf molecular cloud

I. An isothermal model

C.E. Martin 1, J. Heyvaerts 2 and E.R. Priest 1

1 School of Mathematical and Computational Sciences, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, Scotland
2 Observatoire de Strasbourg et Université Louis Pasteur, 11 rue de l'Université, F-6700 Strasbourg, France

Received 14 April 1997 / Accepted 13 May 1997

Abstract

Dwarf (or dark) molecular clouds and molecular clumps have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. Equilibrium perpendicular to a global magnetic field is by magnetic forces and it is proposed that along the field the gas is supported by an Alfvén wave pressure force. A self-consistent analytical model, utilising a WKB approximation, is developed for such support. It is found that Alfvén waves are indeed a good candidate for this support, generating model cloud thicknesses consistent with observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although weak damping is an advantage. The possible sources of these waves are discussed.

Key words: ISM: clouds – ISM: magnetic fields – ISM: structure – MHD – waves

Send offprint requests to: clarem@dcs.st-and.ac.uk

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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