  Stars form in dense cores of magnetized molecular clouds. If the magnetic
flux threading the cores is dragged into the stars, the stellar field would be
orders of magnitude stronger than observed. This well-known "magnetic flux
problem" demands that most of the core magnetic flux be decoupled from the
matter that enters the star. We carry out the first exploration of what happens
to the decoupled magnetic flux in 3D, using an MHD version of the ENZO adaptive
mesh refinement code. The field-matter decoupling is achieved through a sink
particle treatment, which is needed to follow the protostellar accretion phase
of star formation. We find that the accumulation of the decoupled flux near the
accreting protostar leads to a magnetic pressure buildup. The high pressure is
released anisotropically, along the path of least resistance. It drives a
low-density expanding region in which the decoupled magnetic flux is expelled.
This decoupling-enabled magnetic structure has never been seen before in 3D MHD
simulations of star formation. It generates a strong asymmetry in the
protostellar accretion flow, potentially giving a kick to the star. In the
presence of an initial core rotation, the structure presents an obstacle to the
formation of a rotationally supported disk, in addition to magnetic braking, by
acting as a rigid magnetic wall that prevents the rotating gas from completing
a full orbit around the central object. We conclude that the decoupled magnetic
flux from the stellar matter can strongly affect the protostellar collapse
dynamics.
