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Ambipolar diffusion is important in redistributing magnetic flux and indamping Alfven waves in molecular clouds. The importance of ambipolar diffusionon a length scale $\ell$ is governed by the ambipolar diffusion Reynoldsnumber, $\rad=\ell/\lad$, where $\lad$ is the characteristic length scale forambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sampleof 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,comparable to the theoretically expected value. We identify several regimes ofambipolar diffusion in a turbulent medium, depending on the ratio of the flowtime to collision times between ions and neutrals; the clumps observed byCrutcher (1999) are all in the standard regime of ambipolar diffusion, in whichthe neutrals and ions are coupled over a flow time. We have carried outtwo-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes fora range of values of $\rad$. The mean Mach numbers were fixed at $\calm=3$ and$\ma=0.67$; self-gravity was not included. We study the properties ofoverdensities--i.e., clumps--in the simulation and show that the slope of thehigher-mass portion of the clump mass spectrum increases as $\rad$ decreases,which is qualitatively consistent with Padoan et al. (2007)'s finding that themass spectrum in hydrodynamic turbulence is significantly steeper than in idealMHD turbulence. For a value of $\rad$ similar to the observed value, we find aslope that is consistent with that of the high-mass end of the Initial MassFunction for stars. However, the value we find for the spectral index in ourideal MHD simulation differs from theirs, presumably because our simulationshave different initial conditions. This suggests that the mass spectrum of theclumps in the Padoan et al. (2007) turbulent fragmentation model for the IMFdepends on the environment, which would conflict with evidence ...

astrophysical journal/the astrophysical journal

Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar  Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to  Physical Units