The Inability of Ambipolar Diffusion to Set a Characteristic Mass Scale in Molecular Clouds

We investigate the question of whether ambipolar diffusion (ion-neutraldrift) determines the smallest length and mass scale on which structure formsin a turbulent molecular cloud. We simulate magnetized turbulence in a mostlyneutral, uniformly driven, turbulent medium, using a three-dimensional,two-fluid, magnetohydrodynamics (MHD) code modified from Zeus-MP. We find thatsubstantial structure persists below the ambipolar diffusion scale because ofthe propagation of compressive slow MHD waves at smaller scales. Contrary tosimple scaling arguments, ambipolar diffusion thus does not suppress structurebelow its characteristic dissipation scale as would be expected for a classicaldiffusive process. We have found this to be true for the magnetic energy,velocity, and density. Correspondingly, ambipolar diffusion leaves the clumpmass spectrum unchanged. Ambipolar diffusion appears unable to set acharacteristic scale for gravitational collapse and star formation in turbulentmolecular clouds.Comment: 16 pages, 5 figures. ApJ accepte