Formation of hot Jupiters through disk migration and evolving stellar tides

Abstract

Here we address the hot Jupiter (hJ) pile-up at 0.05 AU around young solar-type stars observed in stellar radial velocity surveys, the hJ longterm orbital stability in the presence of stellar tides, and the hJ occurrence rate of 1.2 (+-0.38)% in one framework. We calculate the combined torques on the planet from the stellar dynamical tide and from the protoplanetary disk in the type II migration regime. We model a 2D nonisothermal viscous disk parameterized to reproduce the minimum-mass solar nebula and simulate an inner disk cavity at various radial positions near the star. We choose stellar rotation periods according to observations of young star clusters. The planet is on a circular orbit in the disk midplane and in the star's equatorial plane. We show that the torques can add up to zero beyond the corotation radius around young stars and stop inward migration. Monte Carlo simulations predict hot Jupiter survival rates between ~3% (alpha disk viscosity of 1e-1) and 15% (alpha = 1e-3). Once the protoplanetary disk has been fully accreted, the surviving hJs are pushed outward from their tidal migration barrier and pile up near 0.05 AU, as we demonstrate using a numerical implementation of a stellar dynamical tide model. Orbital decay is negligible on a one-billion-year timescale due to the contraction of the highly dissipative convective envelopes in young Sun-like stars. We find that the higher pile-up efficiency around metal-rich stars can at least partly explain the observed positive correlation between stellar metallicity and hJ occurrence. Combined with the observed hJ occurrence rate, our results for the survival rate imply that <8 % (alpha = 1e-3) to <43 % (alpha = 1e-1) of sun-like stars initially encounter an inward migrating hJ. This reconciles models and observations of young spinning stars with the observed hJ pile up and hJ occurrence rates.