Influences of three-dimensional gas flow induced by protoplanets on pebble accretion --$\rm\,I\,$. shear regime

Abstract

The pebble accretion model has the potential to explain the formation of various types of planets. A growing planet embedded in a disk induces three-dimensional (3D) gas flow, which may influence pebble accretion. In this study, we investigate the influence of the 3D planet-induced gas flow on pebble accretion. Assuming a non-isothermal, inviscid gas disk, we perform 3D hydrodynamical simulations on the spherical polar grid. Then we numerically integrate the equation of motion of pebbles in 3D using hydrodynamical simulations data. We find that the trajectories of pebbles in the planet-induced gas flow differ significantly from those in the unperturbed shear flow for a wide range of pebble sizes investigated (${\rm St}=10^{-3}$--$10^{0}$, where ${\rm St}$ is the Stokes number). The horseshoe flow and outflow of the gas alter the motion of the pebbles, which leads to the reduction of the width of the accretion window, $w_{\rm acc}$, and the accretion cross section, $A_{\rm acc}$. On the other hand, the changes in trajectories also cause an increase in relative velocity of pebbles to the planet, which offsets the reduction of $w_{\rm acc}$ and $A_{\rm acc}$. As a consequence, in the Stokes regime, the accretion probability of pebbles, $P_{\rm acc}$, in the planet-induced gas flow is comparable to that in the unperturbed shear flow except when the Stokes number is small, ${\rm St}\sim10^{-3}$, in 2D accretion, or when the thermal mass of the planet is small, $m=0.03$ in 3D accretion. In contrast, in the Epstein regime, $P_{\rm acc}$ in the planet-induced gas flow becomes smaller than that in the shear flow in the Stokes regime in both 2D and 3D accretion, regardless of assumed ${\rm St}$ and $m$. Our results suggest that the 3D planet-induced gas flow may be helpful to explain the distribution of exoplanets as well as the architecture of the solar system.