Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2H029

Magnetic coupling in giant planet circumplanetary disks

Keith, Sarah L. (Macquarie University)
Wardle, Mark (Macquarie University)

During the final growth phase of giant planets, accretion is thought to be controlled by a surrounding circumplanetary disk. Current accretion disk models rely on hydromagnetic turbulence (through the magnetorotational instability) as the source of effective viscosity within the disk. However, whether the circumplanetary disk is able to interact sufficiently with the magnetic field to produce this turbulence remains a key uncertainty. Here, we examine the strength and nature of magnetic coupling in circumplanetary disks to identify accreting regions. We model the disk as a standard alpha-disk with a self-consistent opacity, and calculate the radial ionisation profile. We consider both turbulent and large-scale vertical fields for driving accretion. By calculating the ohmic, hall and ambipolar diffusivities, we find that the midplane is coupled out to only 30 Jupiter radii. Beyond this distance thermal ionisation drops rapidly prohibiting coupling and accretion across the bulk of the disk. We present an alternate model in which the entire midplane is thermally ionised, by allowing the viscosity parameter, alpha, to vary radially. Although the entire disk is coupled and accreting, a high column density is needed for the hot midplane (T ~ 1000K), such that the disk is self-gravitating at the outer edge. We also discuss the effectiveness for cosmic ray surface ionisation and electric discharge as additional ionising sources.

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