Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion. II. Comparison with Observation, Clump Properties, and Scaling to Physical Units
- Univ. of California, Berkeley, CA (United States); Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Paris (France)
- Univ. of California, Berkeley, CA (United States)
- Univ. of California, Berkeley, CA (United States); Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Ambipolar diffusion (AD) is important in redistributing magnetic flux and in damping Alfven waves in molecular clouds. The importance of AD on a length scale is governed by the AD Reynolds number, RAD = $$\ell/\ell$$AD, where $$\ell$$AD is the characteristic length scale for AD. The logarithmic mean of the AD Reynolds number in a sample of 15 molecular clumps with measured magnetic fields is 17, comparable to the theoretically expected value. Herein we identify several regimes of AD in a turbulent medium, depending on the ratio of the flow time to collision times between ions and neutrals; the clumps observed by Crutcher in 1999 are all in the standard regime of AD, in which the neutrals and ions are coupled over a flow time. We have carried out two-fluid simulations of AD in isothermal, turbulent boxes for a range of values of RAD. The mean Mach numbers were fixed at $$\mathcal{M} = 3$$ and $$\mathcal{M}_A$$ = 0.67; self-gravity was not included. We study the properties of overdensities—i.e., clumps—in the simulation and show that the slope of the higher-mass portion of the clump mass spectrum increases as RAD decreases, which is qualitatively consistent with Padoan et al.’s finding that the mass spectrum in hydrodynamic turbulence is significantly steeper than in ideal MHD turbulence. For a value of RAD similar to the observed value, we find a slope that is consistent with that of the high-mass end of the initial mass function (IMF) for stars. However, the value we find for the spectral index in our ideal MHD simulation differs from theirs, presumably because our simulations have different initial conditions. This suggests that the mass spectrum of the clumps in the Padoan et al. turbulent fragmentation model for the IMF depends on the environment, which would conflict with evidence for a universal IMF. In addition, we give a general discussion of how the results of simulations of magnetized, turbulent, isothermal boxes can be scaled to physical systems. Each physical process that is introduced into the simulation, such as AD, introduces a dimensionless parameter, such as RAD, which must be fixed for the simulation, thereby reducing the number of scaling parameters by one. We show that the importance of selfgravity is fixed in any simulation of AD; it is not possible to carry out a simulation in which self-gravity and AD are varied independently unless the ionization is a free parameter. We show that our simulations apply to small regions in molecular clouds, generally with $$\ell_0$$ ≲ 0.4 pc and M ≲ 25 M⊙. A general discussion of the scaling relations for magnetized, isothermal, turbulent boxes, including self-gravitating systems, is given in the appendix
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF); National Center of Supercomputing Application
- Grant/Contract Number:
- AC52-07NA27344; AST-060683; AST-0908553; NNX09AK31G; TG-MCA00N020
- OSTI ID:
- 2301839
- Report Number(s):
- LLNL-JRNL-447531; 423360
- Journal Information:
- The Astrophysical Journal, Vol. 720, Issue 2; ISSN 0004-637X
- Publisher:
- IOP PublishingCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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