X-Ray Ionization of Planet-Opened Gaps in Protostellar Disks

Young planets with masses approaching Jupiter's have tides strong enough to clear gaps around their orbits in the protostellar disk. Gas ow through the gaps regulates the planets' further growth and governs the disks' evolution. Magnetic forces may drive that ow if the gas is suciently ionized to couple to the elds. We compute the ionizing eects of the X-rays from the central young star, using Monte Carlo radiative transfer calculations to nd the spectrum of Comptonscattered photons reaching the planet's vicinity. The scattered X-rays ionize the gas at rates similar to or greater than the interstellar cosmic ray rate near planets the mass of Saturn and of Jupiter, located at 5 au and at 10 au, in disks with the interstellar mass fraction of sub-micron dust and with the dust depleted a factor 100. Solving a gas-grain recombination reaction network yields charged particle populations whose ability to carry currents is sucient to partly couple the magnetic elds to the gas around the planet. Most cases can undergo Hall shear instability, and some can launch magnetocentrifugal winds. However the material on the planet's orbit has diusivities so large in all the cases we examine, that magneto-rotational turbulence is prevented and the non-ideal terms govern the magnetic eld's evolution. Thus the ow of gas in the gaps opened by the young giant planets depends crucially on the finite conductivity.