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

Abstract

Young planets with masses approaching Jupiter’s have tides strong enough to clear gaps around their orbits in the protostellar disk. Gas flow through the gaps regulates the planets’ further growth and governs the disk’s evolution. Magnetic forces may drive that flow if the gas is sufficiently ionized to couple to the fields. We compute the ionizing effects of the X-rays from the central young star, using Monte Carlo radiative transfer calculations to find the spectrum of Compton-scattered 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 with the masses 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 by a factor 100. Solving a gas–grain recombination reaction network yields charged particle populations whose ability to carry currents is sufficient to partly couple the magnetic fields 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 diffusivities so large in all the cases we examine that magnetorotational turbulence is prevented and the non-ideal terms govern the magnetic field’s evolution. Thus the flow of gas in the gaps opened by young giant planets depends crucially on the finite conductivity.