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The structure and evolution of protoplanetary disks, especially the radialflows of gas through them, are sensitive to a number of factors. One that hasbeen considered only occasionally in the literature is externalphotoevaporation by far-ultraviolet (FUV) radiation from nearby, massive stars,despite the fact that nearly half of all disks will experiencephotoevaporation. Another effect apparently not considered in the literature isa spatially and temporally varying value of $\alpha$ in the disk [where theturbulent viscosity $\nu$ is $\alpha$ times the sound speed C times the diskscale height H]. Here we use the formulation of Bai \& Stone (2011) to relate$\alpha$ to the ionization fraction in the disk, assuming turbulent transportof angular momentum is due to the magnetorotational instability. We find thatdisk evolution is most sensitive to the surface area of dust. Typically $\alpha\lesssim 10^{-5}$ in the inner disk ($< 2$ AU), rising to $\sim 10^{-1}$ beyond20 AU. This drastically alters the structure of the disk and the flow of massthrough it: while the outer disk rapidly viscously spreads, the inner diskhardly evolves; this leads to a steep surface density profile with a slope < p> $\approx$ 2 - 5 in the 5-30 AU region) that is made steeper by externalphotoevaporation. We also find that the combination of variable $\alpha$ andexternal photoevaporation eventually causes gas as close as 3 AU, previouslyaccreting inward, to be drawn outward to the photoevaporated outer edge of thedisk. These effects have drastic consequences for planet formation and volatiletransport in protoplanetary disks.

astrophysical journal/the astrophysical journal

External Photoevaporation of the Solar Nebula II: Effects on Disk  Structure and Evolution with Non-Uniform Turbulent Viscosity due to the  Magnetorotational Instability