2010MNRAS.408..322K

In a previous paper we formulated the problem of the formation and evolution of fragments (or cores) in magnetically supported, self-gravitating molecular clouds in axisymmetric geometry, accounting for the effects of ambipolar diffusion and Ohmic dissipation, grain chemistry and dynamics, and radiative transfer. Here, we present results of star formation simulations that accurately track the evolution of a protostellar fragment over 11 orders of magnitude in density (from 300 to ~ 10¹⁴/cm3), i.e. from the early ambipolar-diffusion-initiated fragmentation phase, through the magnetically supercritical, dynamical-contraction phase and the subsequent magnetic decoupling stage, to the formation of a protostellar core in near hydrostatic equilibrium. As found by Fiedler & Mouschovias, gravitationally-driven ambipolar diffusion leads to the formation and subsequent dynamic contraction of a magnetically supercritical core. Moreover, we find that ambipolar diffusion, not Ohmic dissipation, is responsible for decoupling all the species except the electrons from the magnetic field, by a density of ~ 3x10¹²/cm3. Magnetic decoupling precedes the formation of a central stellar object and ultimately gives rise to a concentration of magnetic flux (a `magnetic wall') outside the hydrostatic core - as also found by Tassis & Mouschovias through a different approach. At approximately the same density at which Ohmic dissipation becomes more important than ambipolar diffusion (>rsim 7x10¹²/cm3), the grains carry most of the electric charge as well as the electric current. The prestellar core remains disc like down to radii ∼ 10 au, inside which thermal pressure becomes important. The magnetic flux problem of star formation is resolved for at least strongly magnetic newborn stars by this stage of the evolution, i.e. by a central density ~ 10¹⁴/cm3. The hydrostatic core has radius ~ 2 au, density ~ 10¹⁴/cm3, temperature ~ 300 K, magnetic field strength ~ 0.2 G, magnetic flux ~ 5x10²⁶ Mx, luminosity ∼ 10^–3 L_☉ and mass ∼ 10^–2 M_☉.