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Astron. Astrophys. 326, 1176-1186

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Alfvén wave support of a dwarf molecular cloud

I. An isothermal model

C.E. Martin1, J. Heyvaerts2, and E.R. Priest1

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

Received 14 April 1997 / Accepted 13 May 1997


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

© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998