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M. Lecar, M.J. Kuchner (Harvard-Smithsonian CfA)
Planets in protoplanetary disks migrate inwards towards their host stars by tidal torquing. But some planets survive, always at semimajor axes a \ge 0.038 AU. Interior to the radius R0 where the disk reaches T=1500 K (R0 \approx 0.06 AU for solar-mass star), circumstellar dust sublimates and the gas can suddenly become ionized by the high temperature and penetrating UV photons. We assume that the ionization causes saturated Balbus-Hawley turbulence. Continuity dictates that the disk surface-density will plummet in the central zone by a factor of ~100 because of the turbulent high-viscosity accretion, so in the simplest circumstances there is not enough gas interior to R0 to affect the planet's orbit. Tidal torques become ineffective beyond the planet's 2:1 outer Linblad resonance, so planets find their migration halted at 0.63 R0 or 0.04 AU, where they can no longer interact with the dense disk beyond R0.
The author(s) of this abstract have provided an email address for comments about the abstract: mkuchner@cfa.harvard.edu