Structure and evolution of magnetically supported molecular clouds: Evidence for ambipolar diffusion in the Barnard 1 cloud

Axisymmetric simulations have demonstrated that ambipolar diffusion initiates the formation and contraction of protostellar cores in predominantly magnetically supported, self-gravitating, isothermal molecular model clouds. New, fully implicit, multifluid, adaptive-grid codes have reliably followed both the early, quasistatic, ambipolar-diffusion-controlled phase of core formation as well as the later, dynamic contraction phase of thermally and magnetically supercritical cores. In this paper we apply these results and present the first evolutionary, dynamical model of any one specific molecular cloud. Using observational input on the structure of the B1 cloud, we first show that the "internal envelope" of B (mass ≤ 600 M_⊙ within r ≤ 2.9 pc, implying a mean density ≃2 × 10³ cm^-3; and mean magnetic field along the line of sight =16 ±3 μG) can be represented very well by a model in exact magnetohydrostatic equilibrium. An evolutionary calculation then follows the ambipolar-diffusion-induced formation and collapse of a supercritical protostellar core, whose predicted physical properties, including mass (13.4 M⊙), size (0.13 pc), mean density (1.3 ×10⁵ cm^-3), and mean magnetic field strength along the line of sight (29.1 μG) are in excellent agreement with observed values for the NH₃ core (M_core = 13 M⊙, R_core = 0.15 pc, n_n.core > 8 ×10⁴ cm-3, and B_los = 30 ±4 μG). Moreover, the calculated spatial profiles of the number density, column density, and magnetic field strength (hence, Alfvén speed) compare well with observations. The model makes further predictions concerning the structure of the protostellar core of B1 that can be tested by higher spatial resolution observations.