Other
Scientific paper
Dec 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.247..353c&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 247, NO. 3/DEC1, P. 353, 1990
Other
47
Scientific paper
Molecular clouds contain magnetic fields with energies comparable to their gravitational binding energies. In the dynamic environment of the interstellar medium, strong hydromagnetic waves are excited in this field on wavelengths longer than the ion-neutral mean free path. In a typical molecular cloud this length-scale, λmin, is of the order of 10-1 of the cloud size. On shorter length-scales the gas is without wave pressure support, and can flow down field lines. The joint effects of excess gravity and flux leakage causes a local dynamic collapse. We test our ideas with a detailed -body calculation in which we impose MHD waves on an initially uniform isothermal gas cloud. The effect of magnetic fields is included in our calculation by the frictional drag on the dominant, neutral population. In the absence of MHD waves the cloud fragments slightly while collapsing, then merges together at the centre into a single, pressure-supported, flattened object. We impose a spectrum of large amplitude Alfvén waves whose velocity amplitude varies as k-3/2, where k is the wavenumber. The initial background magnetic field is chosen to have an energy density slightly larger than the gravitational energy density. The damping is assumed to be balanced by a continuous external supply of wave energy. The simulation shows that the magnetic field and hydromagnetic waves provide sufficient support against gravity so that the cloud undergoes a global, isotropic contraction at a quarter the free-fall rate. The shortest wave present, λ≍λmin, sets the minimum fragment mass, for small Jeans masses. We follow the evolution of fragments having a minimum overdensity of 30 (corresponding to a mass m ≥ 0.4 × 10-3 Mcloud). The fragments appear quickly, and then agglomerate together, yielding an evolving mass spectrum that remains approximately a power law, dN/dm ∝ m-α, where a is 2.5 ± 0.5. Several specific tests of this theory are proposed: (i) that a short wavelength cut-off restricts the MHD oscillations to about a single decade of wavelengths, (ii) that the densest regions occur at nodes in the wave velocity and (iii) that the velocity field in a cloud is not random but is the superposition of a number of waves, with the longest wave present having the largest amplitude.
Carlberg Ray G.
Pudritz Ralph E.
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