Circumplanetary Disk Dynamics in the Isothermal and Adiabatic Limits

TL;DR

This study uses 3D hydrodynamics simulations to explore the properties of circumplanetary disks in different thermal regimes, revealing their sizes, masses, and rotational support, which are crucial for understanding planet and satellite formation.

Contribution

It systematically compares isothermal and adiabatic CPDs across a range of planet masses, providing new insights into their structure, support mechanisms, and implications for planet formation.

Findings

Isothermal CPDs are disky and extend within ~10% of the Bondi radius.

Adiabatic CPDs are spherical and pressure-supported, with rotation rates scaling with $q_{thermal}$.

Even massive, cooled CPDs may have less than 1 Earth mass, impacting gas giant formation theories.

Abstract

Circumplanetary disks (CPDs) may be essential to the formation of planets, regulating their spin and accretion evolution. We perform a series of 3D hydrodynamics simulations in both the isothermal and adiabatic limits to systematically measure the rotation rates, sizes, and masses of CPDs as functions of $q_{\rm thermal}$, the ratio of the planet mass to the disk thermal mass. Our $q_{\rm thermal}$ ranges from 0.1 to 4; for our various disk temperatures, this corresponds to planet masses between 1 Earth mass and 4 Jupiter masses. Within this parameter space, we find that isothermal CPDs are disky and bound within $\sim$10\% of the planet's Bondi radius $r_{\rm B}$, with the innermost $\sim0.05\,r_{\rm B}$ in full rotational support. Adiabatic CPDs are spherical (therefore not actually "disks"), bound within $\sim0.2\,r_{\rm B}$, and mainly pressure-supported with rotation rates scaling linearly with $q_{\rm thermal}$; extrapolation suggests full rotational support of adiabatic envelopes at $\sim10\,q_{\rm thermal}$. Fast rotation and 3D supersonic flow render isothermal CPDs significantly different in structure from --- and orders of magnitude less massive than --- their 1D isothermal hydrostatic counterparts. Inside a minimum-mass solar nebula, even a maximally cooled, isothermal CPD around a 10 Earth-mass core may have less than 1 Earth mass, suggesting that gas giant formation may hinge on angular momentum transport processes in CPDs. Our CPD sizes and masses appear consistent with the regular satellites orbiting solar system giants.