Models of Planetary Envelopes for an Increasingly Hydrodynamic Theory of Planet Formation

Abstract

Core accretion, the process by which gas giants are generally thought to form, has been formulated as the quasi-static contraction of a gaseous envelope regulated by radiative cooling. In reality, the process can be far more dynamic with circumplanetary flows transporting mass and energy. The discovery of diverse exoplanet systems along with advances in computing power have prompted a re-examination of core accretion and the quasi-static assumption in particular. To this end, we investigate hydrodynamic extensions to core accretion with 3-dimensional fluid simulations of the envelopes around planetary cores. We first relax the assumption of a circular orbit and study changes to the flow field around embedded planets when subject to a small but non-zero eccentricity. We find a dramatic alteration of the flow geometry and an enhanced flux of gas being recycling through the planetary envelope. These increased fluxes may increase the pebble accretion rate for eccentric planets up to several times that of the circular orbit rate. While the rotational state of circumplanetary disks in these models remain unchanged, gas exterior to the disk can exhibit enhanced rotation or even retrograde motion in extreme cases. Because envelope growth is regulated by radiative cooling, we also present the results of a series of circular orbit hydrodynamic simulations utilizing an improved method of radiative transfer. These envelope models span a range of cooling times and opacities, making them applicable to both super-Earths and early stages of giant formation. We present calculated mass accretion rates along with details of envelope structure, finding them to be distinct from both adiabatic and isothermal models. We interpret the resultant scalings, judge the fidelity of other methods, and discuss the implications of our radiation-hydrodynamic models in the context of planet formation. We end with future applications and potential improvements to our 3D models that would enable further comparison with 1D models.