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Abstract: Magnetic fields are usually considered dynamically important in starformation when the dimensionless mass-to-flux ratio is close to, or less than,unity lambda<~1. We show that, in disk formation, the requirement is far lessstringent. This conclusion is drawn from a set of 2D axisymmetric simulationsof the collapse of rotating, magnetized, singular isothermal cores. We findthat a weak field corresponding to 1ambda~100 can begin to disrupt therotationally supported disk through magnetic braking, by creating regions ofrapid, supersonic collapse in the disk. These regions are separated by one ormore centrifugal barriers, where the rapid infall is temporarily halted. Thenumber of centrifugal barriers increases with lambda. When lambda>~100, theymerge together to form a more or less contiguous, rotationally supported disk.Even though the magnetic field in such a case is extremely weak on the scale ofdense cores, it is amplified by collapse and differential rotation, to theextent that its pressure dominates the thermal pressure in both the disk andits surrounding region. For relatively strongly magnetized cores withlambda<~10, the disk formation is suppressed completely, as found previously. Anew feature is that the mass accretion is highly episodic, due to reconnectionof the accumulated magnetic field lines. For rotationally supported disks toappear during the protostellar mass accretion phase of star formation in densecores with realistic field strengths, the powerful magnetic brake must beweakened, perhaps through nonideal MHD effects and-or protostellar winds. Wediscuss the possibility of observing a generic product of the magnetic braking,an extended circumstellar region that is supported by a combination of toroidalmagnetic field and rotation - a -magnetogyrosphere-.

Author: Richard R. Mellon, Zhi-Yun Li


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