AAS 205th Meeting, 9-13 January 2005
Session 117 Star Formation, Embedded Young Stars and Their Disks
Oral, Wednesday, January 12, 2005, 10:00-11:30am, Golden Ballroom

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[117.01] Collapse and Fragmentation of Magnetically-Supported Infinite Sheets

A. P. Boss (DTM -- Carnegie Institution)

The collapse and fragmentation of initially sheet-like, magnetic molecular clouds is calculated in three dimensions with a gravitational, radiative hydrodynamics code. The code includes a crude representation of magnetic field effects and ambipolar diffusion, through the magnetic pressure and magnetic tension approximations, and a simple parameterization based on previous magnetohydrodynamical calculations, respectively. The computational volume is a spherical portion of an initially isothermal, infinite sheet of self-gravitating gas, symmetric about its midplane, with the portion of the cloud exterior to the spherical volume being represented through its effect on the gravitational potential inside the spherical volume. The gas layer is initially in hydrostatic equilibrium, but with a mass equal to or greater than the critical mass (~1 M\odot) for the growth of gravitational instability. The magnetic field pressure acts to further stabilize the initial cloud. Over 106 active grid points are employed in the models, sufficient to resolve the Jeans length and so avoid artificial fragmentation. The parameters varied are the ratio of the ambipolar diffusion time to the midplane free fall time (10 or 20), the cloud's reference magnetic field strength (100 or 200 microgauss), the ratio of rotational to gravitational energy of the sheet (0.0 or 0.01), and the form of the initial density perturbation applied to the infinite sheet. Three types of outcomes are observed: formation of one or two protostars near the edge of the spherical volume, formation of a protostar near (but not at) the center of the cloud, or formation of a rotating ring near the center of the cloud, which appears likely to fragment into two or more protostars. Flow speeds of ~ 0.1 km s-1 are generated as the sheet begins to break-up into collapsing protostars. The forming protostars are separated by distances approximately equal to the cloud diameter, consistent with the spacing predicted by the linear theory of the gravitational stability of an infinite sheet. This research has been supported in part by NSF grants AST-0305913 and MRI-9976645.

The author(s) of this abstract have provided an email address for comments about the abstract: boss@dtm.ciw.edu

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