Protostellar Fragmentation Enhanced by Magnetic Fields

Abstract

Fragmentation, the breakup of molecular cloud cores during their self-gravitational collapse to form stars, is the leading explanation for the formation of binary and multiple protostars. Molecular cloud cores appear to be supported against collapse in large part by magnetic fields. However, most protostellar fragmentation calculations have either ignored the effects of magnetic fields or found that in the presence of frozen-in magnetic fields, fragmentation is prohibited. Allowing for magnetic field loss by ambipolar diffusion prior to collapse leads again to fragmentation, but these calculations did not take into account magnetic field tension, which effectively dilutes the self-gravitational forces once a thin disk forms. Because self-gravity drives fragmentation, magnetic tension might then prevent fragmentation. Here we report on the first three-dimensional calculations that show that because magnetic tension also helps in avoiding a central density singularity during protostellar collapse, the net effect is to enhance fragmentation of collapsing magnetic cloud cores. Magnetic clouds can thereby fragment into binary and multiple protostar systems, the latter of which an likely to be unstable to subsequent orbital decay.