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R. F. Coker, J.G.L. Rae, T.W. Hartquist, S.A.E.G. Falle (University of Leeds)
We present further equilibrium models of molecular clumps that are supported by Alfvén waves damped by the linear process of ion-neutral friction. We used a WKB approximation to treat the inward propagation of externally generated waves and adopted a realistic ionization structure influenced by dissociation and ionization due to photons of external origin. The improved ionization structure results in an ionization fraction in the envelopes of the model clumps which is ~100 times greater than in other studies, in which ionization structure that is more appropriate for dark regions was assumed; thus, the waves can penetrate more deeply. In cases where significant internal dissipation occurred, the model clumps were found to contain central condensations surrounded by flat, extended, low density envelopes. The structures, and in particular the central density, of model clumps, while being fairly insensitive to the outer boundary density, depend sensitively on the assumed depletions of sulphur and metals as well as the frequency of the Alfvén wave and the strength of the large-scale magnetic field. The model clumps have central densities of ~104 cm-3, a large-scale field of ~100 \muG, and a velocity amplitude of a few km s-1 at a column depth corresponding to 2 magnitudes of visual extinction.
For models which satisfy observational constraints on the velocity amplitude of the waves, it is found that only a narrow band of frequencies can contribute wholly to the support of the clump. Specifically, we found that the ratio of the strong coupling frequency limit to 2 \pi over the wave crossing time is less than ~10. At higher frequencies, the Alfvén waves are overdamped; at lower frequencies, the clumps are smaller than the wavelength. The resulting frequency range is far smaller than that normally required for the establishment of an inertial power range spectrum.
We will also discuss some observational consequences of our clump models.
This work was supported by UK PPARC.
The author(s) of this abstract have provided an email address for comments about the abstract: robc@ast.leeds.ac.uk