The Effect of Protoplanetary Disk Cooling Times on the Formation of Gas Giant Planets by Gravitational Instability
Observational evidence exists for the formation of gas giant planets on wide orbits around young stars by disk gravitational instability, but the roles of disk instability and core accretion for forming gas giants on shorter period orbits are less clear. The controversy extends to population synthesis models of exoplanet demographics and to hydrodynamical models of the fragmentation process. The latter refers largely to the handling of radiative transfer in three dimensional (3D) hydrodynamical models, which controls heating and cooling processes in gravitationally unstable disks, and hence dense clump formation. A suite of models using the $β$ cooling approximation is presented here. The initial disks have masses of 0.091 $M_\odot$ and extend from 4 to 20 AU around a 1 $M_\odot$ protostar. The initial minimum Toomre $Q_i$ values range from 1.3 to 2.7, while $β$ ranges from 1 to 100. We show that the choice of $Q_i$ is equal in importance to the $β$ value assumed: high $Q_i$ disks can be stable for small $β$, when the initial disk temperature is taken as a lower bound, while low $Q_i$ disks can fragment for high $β$. These results imply that the evolution of disks toward low $Q_i$ must be taken into account in assessing disk fragmentation possibilities, at least in the inner disk, i.e., inside about 20 AU. The models suggest that if low $Q_i$ disks can form, there should be an as yet largely undetected population of gas giants orbiting G dwarfs between about 6 AU and 16 AU.