ON THE STABILITY AND EVOLUTION OF ISOLATED BOK GLOBULES

摘要

We present the results of three-dimensional hydrodynamic simulations of evolving isolated low-mass clouds and Bok globules, where the interstellar radiation field plays an important role in the chemical and thermal evolution. We consider two classes of cloud models. (1) clouds that are initially supported . against gravitational collapse by thermal pressure alone, and (2) clouds that are initially supported by a mildly supersonic, complex internal velocity field (``turbulence''). The models are based on our earlier work with a smoothed particle hydrodynamics code, but upgraded to include a larger chemical network, refined chemical and dust properties, and different boundary conditions. The chemical network predicts the abundances of several key tracers of cloud structure and evolution, including C+, C I, and CO. There are two main purposes of this work. The first is to calculate the effective Jeans masses of isolated and externally heated clouds under a range of initial conditions, in order to delineate the physical parameters necessary for gravitational collapse and star formation to occur. The second is to calculate density, tem- perature, and chemical species profiles for comparison with observations. We consider clouds with masses in the range 8 < = M < = 70 Mo, radii in the range 0.34 < = R < = l.8 pc, and initial number densities in the range 50 ≤ n ≤ 1000 cm~-3, corresponding to low-mass Bok globules. We examine the evolution of both uniform-density and centrally condensed clouds, and clouds with and without a turbulent velocity field. The main results of our calcu