Effects of disk irradiation on planet population synthesis

Abstract

Context. Planet formation is strongly dependent on disk properties. The surface density of gas and solids influences the formation of planetesimals and subsequently that of planetary cores. The temperature structure, on the other hand, affects planet migration and, as a result, the outcome of planet formation.Aims. We extended our previous disk structure calculations to include the effect of stellar irradiation in addition to viscous heating. We then used these new structures to compute new synthetic planet populations that we compared to those obtained without taking into account stellar irradiation.Methods. Our disk models are standard alpha-disk models calculated in the 1+1D approximation. The effect of the irradiation by the central star is treated by imposing an appropriate outer boundary conditions on temperature. The disks evolve through viscous transport and photo-evaporation on timescales that match observations.Results. When compared to the nominal non-irradiated case, the irradiated disk is, as expected, warmer beyond a few AUs. This translates into a greater scale-height, which implies, everything else being equal, a transition from type I to type II migration for larger core masses. Because type I migration is heavily damped in our nominal model and that type II starts at larger masses, the nominal model with an irradiated disk fails to account for the presence of the numerous hot Jupiters. In other words, the effect of accounting for irradiation is strong in conjunction with a heavily damped type I migration rate. Conclusions. We find that reducing the slowing down of the type I migration rate to a factor ten below the linear value (instead of a factor 103 as in the nominal model) is enough to retrieve a giant planet population consistent with observations. The same effect is observed if we change the transition from type I to type II by varying the nominal criterion without considering irradiation.