A Spherical Model for Starless Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains

In the standard picture of isolated star formation, dense ``starless'' coresare formed out of magnetic molecular clouds due to ambipolar diffusion. Underthe simplest spherical geometry, I demonstrate that ``starless'' cores formedthis way naturally exhibit a large scale inward motion, whose size and speedare comparable to those detected recently by Taffala et al. and Williams et al.in ``starless'' core L1544. My model clouds have a relatively low mass (oforder 10 $M_\odot$) and low field strength (of order 10 $\mu$G) to begin with.They evolve into a density profile with a central plateau surrounded by apower-law envelope, as found previously. The density in the envelope decreaseswith radius more steeply than those found by Mouschovias and collaborators forthe more strongly magnetized, disk-like clouds. At high enough densities, dust grains become dynamically important by greatlyenhancing the coupling between magnetic field and the neutral cloud matter. Thetrapping of magnetic flux associated with the enhanced coupling leads, in thespherical geometry, to a rapid assemblage of mass by the central protostar,which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap