On the stability and evolution of isolated Bok globules

We present the results of three-dimensional hydrodynamic simulations of evo lving isolated low-mass clouds and Bok globules, where the interstellar rad iation field plays an important role in the chemical and thermal evolution. We consider two classes of cloud models: (1) clouds that are initially sup ported against gravitational collapse by thermal pressure alone, and (2) cl ouds that are initially supported by a mildly supersonic, complex internal velocity held ("turbulence"). The models are based on our earlier work with a smoothed particle hydrodynamics code, but upgraded to include a larger c hemical network, refined chemical and dust properties, and different bounda ry 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 ini tial conditions, in order to delineate the physical parameters necessary fo r gravitational collapse and star formation to occur. The second is to calc ulate density, temperature, and chemical species profiles for comparison wi th observations. We consider clouds with masses in the range 8 less than or equal to M less than or equal to 70 M., radii in the range 0.34 less than or equal to R less than or equal to 1.8 pc, and initial number densities in the range 50 less than or equal to n less than or equal to 1000 cm(-3), co rresponding to low-mass Bok globules. We examine the evolution of both unif orm-density and centrally condensed clouds, and clouds with and without a t urbulent velocity held. The main results of our calculations are: 1. Clouds that proved to be gravitationally unstable collapsed to form cold , dense molecular cores, surrounded by warm, thermally supported, tenuous h alos in which the trace species were in ionic or atomic form. 2. The evolution of the thermally supported clouds is driven in the first i nstance by a pressure gradient through the cloud that arises because of the attenuation of the interstellar radiation field. Subsequent thermal evolut ion leads to cooling of the gas, which can induce gravitational instability . 3. Initially turbulent clouds evolve through the dissipation of their inter nal kinetic energy and then follow evolutionary paths similar to those of t he thermally supported clouds. The effect of the turbulence is to delay the collapse of the clouds until the turbulence decays, which occurs on a rapi d timescale through shock dissipation, and to increase the stability of the cloud models by a small amount. 4. The collapsing dense cores that arise in the simulations have masses in the range 3 less than or similar to M less than or similar to 20 M., radii in the range 0.1 less than or similar to R less than or similar to 0.2 pc, and temperatures in the range 8 less than or similar to T less than or simi lar to 12 K. These align closely with the observationally derived propertie s of Bok globule cores. 5. The characteristics of the collapsing dense cores are similar to those o f collapsing isothermal spheres, since the gas evolves toward a constant te mperature of 10 K before collapse ensues, because of gas-dust thermal coupl ing.