The effects of large scale magnetic fields around young protostars and their disks

Abstract

We study the evolution of magnetic fields in accretion flows around six different sink particles in a global star formation simulation. Using the adaptive mesh refinement capabilities of \ramses, the vicinity of the sinks is resolved with a resolution down to $2$ AU. The statistical properties of the magnetic field are tracked as a function of radius, height and time around each sink particle. All six systems are strongly magnetised with plasma beta being unity or below and we know that at least three of the sinks host an accretion disc. One of the discs is studied at a higher resolution of $0.06$ AU and we report its magnetic properties. We find that the angular momentum transport is dominated by large scale radial-azimuthal Maxwell stress. Furthermore, contrary to the weakly magnetised case studied in shearing box simulations, the large scale azimuthal field does not show smooth cycle periods but instead forms a banded structure. We conclude by speculating what this result might mean for observations and whether it will hold with highly resolved simulations including turbulence or non-ideal MHD effects.