The Three Dimensional Flow Field Around Planets on Eccentric Orbits

TL;DR

This study uses 3D hydrodynamic simulations to explore how eccentric orbits influence the flow patterns around protoplanets, revealing significant differences from circular orbits and implications for accretion processes.

Contribution

It provides the first detailed analysis of 3D flow fields around eccentric protoplanets, highlighting how eccentricity alters flow geometry and potential accretion rates.

Findings

Flow patterns depend on orbital phase in eccentric cases.

Increased inflow and outflow with higher eccentricity and lower mass.

Eccentricity may enhance pebble accretion rates.

Abstract

We investigate the properties of the hydrodynamic flow around eccentric protoplanets and compare them with the often assumed case of a circular orbit. To this end, we perform a set of 3D hydrodynamic simulations of protoplanets with small eccentricities ($e\leq 0.1$). We adopt an isothermal equation of state and concentrate resolution on the protoplanet to investigate flows down to the scale of the protoplanet's circumplanetary disk (CPD). We find enhanced prograde rotation exterior to the CPD for low planet masses undergoing subsonic eccentric motion. If the eccentricity is made large enough to develop a bow shock, this trend reverses and rotation becomes increasingly retrograde. The instantaneous eccentric flow field is dramatically altered compared to circular orbits. Whereas the latter exhibit a generic pattern of polar inflow and midplane outflow, the flow geometry depends on orbital phase in the eccentric case. For even the modest eccentricities tested here, the dominant source of inflow can come from the midplane instead of the poles. We find that the amount of inflow and outflow increases for higher $e$ and lower protoplanet masses, thereby recycling more gas through the planet's Bondi radius. These increased fluxes may increase the pebble accretion rate for eccentric planets up to several times that of the circular orbit rate. In response to eccentric motion, the structure and rotation of the planet's bound CPD remains unchanged. Because the CPD regulates the eventual accretion of gas onto the planet, we predict little change to the gas accretion rates between eccentric and circular planets.