AMBIPOLAR DIFFUSION, INTERSTELLAR DUST, AND THE FORMATION OF CLOUD CORES AND PROTOSTARS .4. EFFECT OF ULTRAVIOLET IONIZATION AND MAGNETICALLY CONTROLLED INFALL RATE

We extend our previous studies of the self-initiated formation and con traction of protostellar cores (due to ambipolar diffusion) in axisymm etric, isothermal, self-gravitating, disklike, thermally supercritical but magnetically subcritical model molecular clouds, to include the e ffect of the external (interstellar) ultraviolet radiation field. UV i onization dominates cosmic-ray ionization up to optical depths of abou t 10 and increases the degree of ionization in the envelopes of model clouds by more than 2 orders of magnitude. It thereby decreases by a s imilar factor the rate at which ambipolar diffusion progresses in the envelopes. We follow the evolution of four model clouds to a central d ensity enhancement of 10(6) (e.g., from 2.6 x 10(3) to 2.6 x 10(9) cm( -3)). Magnetically supercritical cores form on the initial central flu x-loss timescale, which exceeds the dynamical timescale (similar or eq ual to free-fall time) by a factor 10-20. As in the case of no UV radi ation, a typical magnetically supercritical core consists of a uniform -density central region and a ''tail'' of infalling matter with a powe r-law density profile n(n) proportional to r(s), -1.5 greater than or similar to s greater than or similar to 2 -1.85. Models that include t he macroscopic (collisional) effects of grains have the evolution of t heir cores retarded (typically by 50%) with respect to models that acc ount only for neutral-ion drag, independently of the effects of W radi ation. Model clouds that account for the effect of UV ionization have envelopes that are even better supported by magnetic forces than envel opes of models ionized only by cosmic rays. The effect that a well-sup ported envelope has on an oblate cloud's central gravitational field i s to increase the field strength, which speeds up the evolution of a c ore in a typical model cloud by 30%. In all cases, the mass infall (or accretion) rate in (or from) the magnetically supported envelope is c ontrolled by slow ambipolar diffusion. Ambipolar diffusion is so ineff ective in the envelopes of model clouds with W ionization that mass in fall decreases precipitously outside the supercritical protostellar co res.