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P. C. Myers (Harvard-Smithsonian CfA), E. G. Zweibel (U.Colorado and JILA)
We model the structure and evolution of a turbulent, magnetized, flattened, self-gravitating molecular cloud. A uniform mean magnetic field threads an infinite horizontal layer, supported against self-gravity by the pressure of Alfven waves and thermal motions. The equilibrium density decreases asymptotically with height as z**(-2), declining more gradually than in the isothermal case. The layer is within a factor ~2 of magnetically critical. The dominant wave damping mechanism is nonlinear steepening into shocks rather than ion-neutral friction. Such a layer can condense quasistatically if its initial midplane turbulence has Mach number <~2. The layer "settles" rather than collapses, with speed proportional to the wave damping rate. If the layer has modest nonuniformity in its column density, it can develop "differential condensation," where regions of column density greater than average by a factor of ~2 produce midplane number density greater than average by a factor of ~10. For field strength, temperature, column density, and velocity dispersion observed in nearby clouds, this model predicts a temporal increase in midplane density by a factor ~4, to 2 x 10**(4) cm**(-3), with typical inward speed ~ 0.1 km/s, and with line width decreasing to 0.2 km/s, all in ~ 0.5 Myr. This idealized model of turbulent dissipation matches more observed features of star-forming dense cores and their environs than do models which assume purely static magnetic fields.