[P-PS06] Planetary Sciences

[PPS06-22] Co-evolution of Giant Planets and Protoplanetary Disks : The Timing of Saturn's Formation

*Keiji Suzuki¹, Hidekazu Tanaka¹ (1.Tohoku university)

Keywords:Protoplanetary Disk, Giant Planet Formation, Solar System, Numerical Simulation

Giant planets are formed by the runaway accretion of gas from the protoplanetary disk. The gas accretion continues until the disk is dissipated by effects such as photoevaporation, at which point the final masses of the giant planets are fixed. Jupiter and Saturn are also thought to have formed in this way within the solar nebula. In this study, we investigated the co-evolution of multiple giant planets undergoing runaway gas accretion and the protoplanetary disk using numerical simulations that solve for the one-dimensional surface density evolution of the disk. Previous studies on the formation of Jupiter and Saturn have often assumed the minimum-mass solar nebula based on the Hayashi model to study their growth by runaway gas accretion. In contrast, we have investigated the conditions suitable for the formation of Jupiter and Saturn by varying parameters such as the disk mass and the onset time of runaway gas accretion to these planets. For the gas accretion rate to planets, we adopted the model of Tanigawa & Tanaka (2016), which is based on numerical hydrodynamical simulations. We adopted an alpha-viscous accretion disk and assumed that it dissipates by photoevaporation at a given rate. Since Saturn's mass is about one third of Jupiter's, its gas accretion must start later than Jupiter's. In our study, we conducted numerical simulations by varying three key parameters: the disk mass, the photoevaporation rate, and the time delay Delta t between the onset times of gas accretion to Jupiter and Saturn to find the appropriate parameters. If the disk mass at the onset of gas accretion to Jupiter is comparable to the Hayashi model (0.01 solar masses), a relatively high rate of photoevaporation is required to prevent Jupiter and Saturn from becoming too massive. As a result, the gas surface density near Jupiter at the onset of its accretion was reduced to 1/4 of the Hayashi model. In addition, the appropriate time delay Delta t for the onset of Saturn's accretion was found to be about 4×10⁵ years. The final masses of the planets are highly sensitive to both the photoevaporation rate and Delta t, requiring fine tuning. On the other hand, when the disk mass at the onset of Jupiter's accretion is 0.02 or 0.03 solar masses, the dependencies of the final planet masses on the photoevaporation rate and Delta t are relatively weak, making it more likely that Jupiter and Saturn reach their observed masses. In these cases, the surface density near Jupiter at the onset of its accretion was reduced to approximately 10% or less of the Hayashi model. As Jupiter and Saturn grew, the surface density in their vicinity decreased significantly, even outside their density gaps. Our results suggest that the gas surface density in the formation region of Jupiter and Saturn during their gas accretion phase was significantly lower than that expected from the minimum-mass solar nebula model.