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|Title:||Ambipolar Diffusion in Interstellar Clouds: One-Dimensional, Isothermal Collapse|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Astronomy and Astrophysics|
|Abstract:||The formulation of a theory of star formation faces two fundamental problems: (i) the angular momentum problem, and (ii) the magnetic flux problem. We have previously emonstrated by detailed calculations that magnetic braking by itself can resolve the angular momentum problem (at least for binary stars and, most likely, for single stars as well) during the early, relatively diffuse stages of cloud contraction. The magnetic flux problem lies in the observation that fluxes of typical stars are between 2 and 5 orders of magnitude smaller than fluxes of corresponding masses at interstellar densities. The detailed calculations of collapsing model clouds described in this paper show that the flux-to-mass ratio in cloud cores can decrease by more than 4 orders of magnitude at neutral densities n(,n) < 10('9) cm('-3) due to ambipolar diffusion alone. Ambipolar diffusion sets in at gas densities larger than those by which the bulk of the angular momentum problem is resolved by magnetic braking.
We follow numerically, sometimes for twenty eight initial central free-fall times, the evolution in time of model self-gravitating clouds which would have been in an exact equilibrium state with spatially nonuniform density and magnetic field if ambipolar diffusion had been ignored. Thus any evolution at all is entirely the result of ambipolar diffusion. A cloud is in pressure equilibrium with a magnetic, hot and tenuous external medium. In a typical case, appreciable drift velocities between ions and neutrals develop in the core where a combination of steep magnetic-field gradients and a small degree of ionization exist. Soon thereafter the core contracts dynamically under self-gravity, with ever-decreasing flux-to-mass ratio. Inward neutral velocities, and relative drift velocities, reach several km s('-1). A nonsteady, nearly nonmagnetic shock forms in this region, depending on the cloud temperature. Outward moving ions transfer momentum quite effectively to the neutrals in the envelope and, in most cases, a hydromagnetic shock forms in this region. The envelope expands to more than 2 - 7 times its original extent before it slows down and recollapses. (This behavior of the envelope is not, of course, found if the initial field is uniform everywhere in space.) Our two-fluid numerical code is implicit, conservative, with automated adaptive grid and uses artificial viscosity to treat shocks. It has been tested and run for more than 28 initial central free-fall times without numerical problems.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1983.
|Date Available in IDEALS:||2015-05-13|