Tuesday, June 11, 2013

1306.2276 (N. J. Turner et al.)

Magnetic Coupling in the Disks Around Young Gas Giant Planets    [PDF]

N. J. Turner, M. H. Lee, T. Sano
We examine the conditions under which the disks of gas and dust orbiting young gas giant planets are sufficiently conducting to experience turbulence driven by the magneto-rotational instability. By modeling the ionization and conductivity in the disk around proto-Jupiter, we find that turbulence is possible if the X-rays emitted near the Sun reach the planet's vicinity and either (1) the gas surface densities are in the range of the minimum-mass models constructed by augmenting Jupiter's satellites to Solar composition, while dust is depleted from the disk atmosphere, or (2) the surface densities are much less, and in the range of gas-starved models fed with material from the Solar nebula, but not so low that ambipolar diffusion decouples the neutral gas from the plasma. The results lend support to both minimum-mass and gas-starved models of the protojovian disk: (1) The dusty minimum-mass models have negligible internal angular momentum transfer by magnetic forces, as required for the material to remain in place while the satellites form. (2) The gas-starved models have magnetically-active surface layers and a decoupled interior "dead zone", analogous to the Solar nebula, with the active layers yielding accretion stresses in the range assumed in constructing the models. The results also point to areas where both classes of models can be further developed. Non-turbulent minimum-mass models will lose dust from their atmospheres by settling, enabling gas to accrete through a thin surface layer. In gas-starved models the stress-to-pressure ratio should increase with radius, likely leading to episodic accretion outbursts.
View original: http://arxiv.org/abs/1306.2276

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