Gap opening beyond dead zones by photoevaporative winds: Implications for transition disks and planetary migration

Physics

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Scientific paper

In the present study, we infer connections between properties of protoplanetary disks in the process of their dispersal, so-called transition disks, and orbital configurations of exoplanetary systems. Our gas disk model takes into account dead zones and photoevaporative winds induced by X rays from the central stars. We find that a gap opens at a radius outside the dead zone, even when a large mass accretion onto the central star remains. This may explain large gap sizes and high mass accretion rates seen in transition disks. The timescale since a gap opens until gas accretion onto the central star stops increases with mass loss rate of photoevaporative winds unless it is too large. Using this disk model, we also investigate planetary migration. For Type I migration of rocky planets, we adopt the rates from recent theories which indicate that outward migration can occur in optically thick disks. For Type II migration, we simply assume that giant planets migrate together with gas, if the disk mass is larger than the planet mass. Our model shows that rocky planets migrate toward equilibrium radii with zero torque; the inner radius is the snow line and the outer radius locates slightly inside the outer edge of a dead zone. If giant planets form at the latter radius, they tend to migrate outward for a case with strong photoevaporative winds, whereas giant planets hardly stay at large radii with weak photoevaporation winds. Rocky planets can migrate close to their host stars only if optically thin disks can survive for a long time after dead zones disappear. Such a situation is realized with weak photoevaporative winds, but this may be not very common. This may explain why the number of Earth to Neptune size hot planets found by the Kepler mission, after corrections of observation biases, increases with size, as the planet migration rate increases with mass.

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