Influences of protoplanet-induced three-dimensional gas flow on pebble accretion $\rm\,I\hspace{-.1em}I\,$. Headwind regime

Abstract

Pebble accretion is one of the major theories in planet formation. Aerodynamically small particles, called pebbles, are highly affected by the gas flow. A growing planet embedded in a protoplanetary disk induces three-dimensional (3D) gas flow. In our previous study, Paper I, we focused on the shear regime of pebble accretion, and investigated the influence of planet-induced gas flow on pebble accretion. In Paper I, we found that pebble accretion is inefficient in the planet-induced gas flow compared to that in the unperturbed flow, in particular when ${\rm St}\lesssim10^{-3}$, where St is the Stokes number. Following Paper I, we investigate the influence of planet-induced gas flow on pebble accretion. We consider the headwind of the gas, which is not included in Paper I. We extend our study to the headwind regime of pebble accretion in this study. Assuming a nonisothermal, inviscid sub-Keplerian gas disk, we perform 3D hydrodynamical simulations on the spherical polar grid which has a planet with the dimensionless mass, $m=R_{\rm Bondi}/H$, located at its center, where $R_{\rm Bondi}$ and $H$ are the Bondi radius and the disk scale height. We then numerically integrate the equation of motion of pebbles in 3D using hydrodynamical simulation data. Combining our results with the spacial variety of turbulence strength and pebble size in a disk, we conclude that the planet-induced gas flow still allows for pebble accretion in the early stage of planet formation. Suppression of pebble accretion due to the planet-induced gas flow occurs only in the late stage of planet formation, in particular in the inner region of the disk. This may be helpful to explain the distribution of exoplanets and the architecture of the Solar System, both of which have small inner and large outer planets.