Numerical study of the effects of ambipolar diffusion on the collapse of magnetic gas clouds

We have calculated numerically the gravitational collapse of isothermal, nonrotating magnetic gas clouds including the effects of ambipolar diffusion. The fractional ionization in the clouds is approximated by a power-law function of the gas density f/sub i/ = Kn/sup -q/, where K and q are adjustable parameters. Eleven numerical experiments were run, and the results indicate that the asymptotic character of collapse is determined mainly by the value of q and is largely independent of the other parameters characterizing a cloud (e.g., K, cloud mass). In particular, there is nearly a one-to-one correspondence between q and the slope, x, of the central magnetic field strength-gas density relationship (i.e., B/sub c/ proportionalrho/sup x//sub c/). If q< or approx. =0.8, a cloud collapses asymptotically, as though the magnetic field were ''frozen'' to the neutral matter. The magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of qroughly-equal1, despite extremely low values of fractional ionization. A discussion of the theoretical basis for this unexpected behavior is given. Possible implications of our results for the problems of magnetic braking of rotating protostars and star formation in general are also presented