Magnetohydrodynamical simulations of the fragmentation of molecular cloud cores

Abstract

Multiple star systems are quite common throughout the universe, so it is of interest to know what physical process may promote and prevent the fragmentation of a collapsing gas cloud that leads to multiple star system formation. Protostars, condensed objects in which nuclear reactions have yet to begin, are predominantly formed in Molecular Cloud Cores, dense regions of Molecular clouds composed of mostly cold molecular hydrogen. The condition for fragmentation is described by the Jeans Length, the maximm radius a uniform spherical core can have, beyond which thermal pressure is insufficient to support the core against gravitational collapse. To simulate the formation of multiple star systems, a model was developed consisting of a sphere of uniform density with an azimuthal density perturbation to stimulate fragmentation, with some initial amount of rotation and a uniform magnetic field parallel to the rotation axis. The parameters tested were the ratios of the initial thermal, rotational, and magnetic energy to the gravitational potential energy of the sphere, and the ratio of specific heats, denoted by [alpha], [beta], [zeta subscript m], and [gamma] respectively. An [alpha]-[beta]-[zeta subscript m]-[gamma] parameter space survey was carried out using the magnetohydrodynamical computer code ZEUS-3D, augmented with a Fourier transform based gravity solver. The increase of the energy ratios [alpha], [beta], and [zeta subscript m] was found to generally provide more support against the collapse and result in the evolution of the cores to a lower density. Multiple dense fragments were produced primarily in the hydrodynamica simulations using adiabatic values of [gamma]. The addition of a magnetic field was found to homogenize the density distribution, suppess fragmentation, and prevent the movement of fluid perpendicular to field lines, resulting in a smaller collapsed core. Simulations using near-isothermal values of [gamma] were found to generally collapse to a higher density state than those with adiabatic values, and to produce a single central condensation rather than multiple fragments.