MAGNETIC BRAKING, AMBIPOLAR DIFFUSION, AND THE FORMATION OF CLOUD CORES AND PROTOSTARS .3. EFFECT OF THE INITIAL MASS-TO-FLUX RATIO

Two previous papers have formulated the problem of the formation and c ontraction of protostellar cores in isothermal, rotating, self-gravita ting, magnetically supported model molecular clouds and have presented results, respectively, for a typical case and for the effects of vary ing five dimensionless free parameters of the problem. In this paper, we study the effect of varying the sixth parameter mu(d,e0) the initia l central mass-to-flux ratio in units of the critical value for collap se. Clouds with initial central mass-to-flux ratio ranging from highly subcritical (mu(d,e0) = 0.1) to initially critical (mu(d,e0) = 1.0) a re studied. Core formation is initially quasistatic (i.e., negligible acceleration) for the subcritical clouds but dynamic for the critical cloud. In the case of the critical cloud, magnetic-tension forces brin g an end to the magnetic-braking-induced, initial phase of (dynamic) c ollapse (caused by the rapid loss of rotational support); quasistatic contraction follows. After ambipolar diffusion increases (quasistatica lly) the central mass-to-flux ratio above the critical value, cores in all model clouds enter a dynamic phase of contraction. We find that, by the end of the isothermal phase of contraction, at a central densit y enhancement of about 10(6) (e.g., from 3 x 10(3) cm(-3) to 3 x 10(9) cm(-3)), the widest range of core masses and angular momenta is obtai ned from the variation of the free parameter mu(d,e0); specifically, w e find that M(core)proportional to mu(d,e0) and (J/M)(core) proportion al to mu(d,e0)(2). The observationally guided range of values of mu(d, e0) in our parameter study can explain naturally a range of core masse s 3-30 M. and specific angular momenta 10(19)-10(21) cm(2) s(-1).