ar X iv : a st ro - p h / 02 12 35 9 v 1 1 6 D ec 2 00 2 1 FRAGMENTATION AND COLLAPSE OF TURBULENT MOLECULAR CLOUDS

We performed simulations of self-gravitating hydro-dynamic turbulence to model the formation of filaments, clumps and cores in molecular clouds. We find that when the mass on the initial computational grid is comparable to the Jeans mass, turbulent pressure is able to prevent gravitational collapse. When the turbulence has damped away sufficiently, gravitational collapse can occur, and the resulting structure closely resembles the pre-singularity collapse of an isothermal sphere of Penston (1969). If several Jeans masses are initially placed on the grid, turbulence may not be sufficient to prevent collapse before turbulence can be significantly damped. In this case, the cores have density structures which are considerably shallower than expected for an isothermal gas, and resemble the solutions for a logatropic equation of state. There is abundant evidence that stars form in clumps within molecular clouds. In order to understand the mechanism by which stars form, we therefore must understand the environment within a molecular cloud. Observations show that molecular clouds are composed of a hierarchical network of filaments, clumps and cores (Mizuno et al. 1995; Johnstone & Bally 1999; Gahm et al. 2002). The most popular explanation for this substructure is that of turbulent fragmentation (Klessen & Burkert 2000; Os-triker et al. 2001). In this scenario, turbulent fluctuations within the molecular cloud can compress the gas through shocks to high enough densities that the self-gravity of such regions becomes strong enough to induce collapse, resulting in a cloud which has been broken up into several condensing cores. The cores, due to their origins as fragmenting shocks, are arranged along filamentary structures ; the intersections of these filaments have many of these smaller cores attracted to their higher average density , resulting in clusters which resemble the observed clumps. We performed simulations to test whether the structure of cores produced in this turbulent fragmentation scenario are consistent with observation. 2. Simulations Our simulations were performed using the NCSA ZEUS-MP hydrodynamics code. ZEUS-MP is a mesh-based Eu-lerian hydromagnetic code capable of performing 3D simulations. The self-gravity of the fluid is calculated using a FFT gravity solver. Our simulations are stopped when the numerical gravitational instability criterion of Tru-elove et al. (1997) is violated (L J < 4L pix). We use an isothermal equation of state and periodic boundary conditions. We start with a uniform initial density and no magnetic field, at a resolution of 128 3 grid cells. Into this box we …