AMBIPOLAR DIFFUSION, INTERSTELLAR DUST, AND THE FORMATION OF CLOUD CORES AND PROTOSTARS .3. TYPICAL AXISYMMETRICAL SOLUTIONS

In a previous paper we formulated the problem of the formation of prot ostellar cores by ambipolar diffusion in axisymmetric, isothermal, sel f-gravitating, thermally supercritical but magnetically subcritical mo del molecular clouds, accounting for a cosmic abundance of interstella r grains (both charged and neutral). Using an implicit code with an ad aptive mesh, we follow the evolution to a central density enhancement of 10(6) (e.g., from 2.6 x 10(3) cm-3 to 2.6 x 10(9) cm-3). First, amb ipolar diffusion slowly increases the mass-to-flux ratio of a cloud's central flux tubes, leading to the formation and contraction of therma lly supercritical but magnetically subcritical cores. The timescale fo r this process is essentially the initial central flux-loss timescale, which exceeds the dynamical timescale (congruent-to free-fall timesca le) typically by a factor 10-20. Eventually, the mass-to-flux ratio ex ceeds the critical value for collapse. The subsequent contraction of t he thermally and magnetically supercritical cores becomes progressivel y more dynamic, while the envelopes remain relatively well supported b y magnetic forces, in agreement with early theoretical predictions by Mouschovias. A typical supercritical core consists of a uniform-densit y central region and a ''tail'' of infalling matter with a power-law d ensity profile n(n) is-proportional-to r(s), -1.5 approximately-greate r-than s approximately-greater-than -1.85. The mass infall (or accreti on) rate from the subcritical envelopes is controlled by ambipolar dif fusion, and differs both qualitatively and quantitatively from estimat es based on non-magnetic models and their extrapolations to magnetic c louds. Model clouds that include the macroscopic (collisional) effects of grains have their evolution retarded (typically by 50%) with respe ct to models accounting only for neutral-ion drag. Neutral-grain drag typically dominates the neutral-ion drag at core densities n(n, c) app roximately-greater-than 10(8) cm-3. Electrostatic attraction by electr on-shielded ions (''quasiparticles'') keeps charged grains partially a ttached to the magnetic field for densities n(n, c) approximately-grea ter-than 3 x 10(5) cm-3, at which detachment would otherwise occur bec ause of collisions with neutrals. Neutral grains also couple to the ma gnetic field by inelastic charge-capture processes. The grains lengthe n the timescale for the formation of a core, accentuate the core-envel ope separation, and, by any given central density enhancement, increas e a core's size, mass, and magnetic flux.