Gas Accretion Flows onto Giant Protoplanets: High‐Resolution Two‐dimensional Simulations

Abstract

We performed two-dimensional local hydrodynamic simulations of the accretion flows onto protoplanets (with different masses Mp and semimajor axis a) from a protoplanetary disk with extremely high resolution in order to clarify the fine structure of circumplanetary flows after the onset of the nucleated instability of the planetary atmosphere. We find that two types of shocks are formed: a pair of bow shocks extended outside the planetary gravitational sphere and a pair of spiral shocks inside the sphere winding toward the planet. The disk gas within narrow bands on both sides of the planetary orbit flows into the planetary gravitational sphere. The offset of the bands is determined by the energy dissipation across the bow shock on a streamline toward a stagnant point, and the width is determined by the energy dissipation across the spiral shock on another streamline toward the stagnant point of the opposite side of the planet. This means that the mass accretion rate onto the planet is also determined by the energy dissipation across the spiral shock. With the assumption that the gas is isothermal, we obtain the mass accretion rate as a function of normalized sound speed iso, which corresponds to the ratio of the disk scale height to the Hill radius, as = 8.0 × 10-3 M⊕(a/5.2 AU)-1.5 (Mp/10 M⊕)1.3 (Σ/Σmin)yr-1, where Σ is the surface density of the disk, Σmin is that for the minimum-mass solar nebula model. Note that the slower contraction of the planetary atmosphere (when Mp < 120 M⊕) as well as heating due to the gas accretion luminosity may make the accretion rate smaller. We also find that the circumplanetary spiral shocks could strongly affect the torques exerted on the planet by the disk gas.