Planet Formation Signposts: Observability of Circumplanetary Disks via Gas Kinematics

Abstract

The identification of ongoing planet formation requires the finest angular resolutions and deepest sensitivities in observations inspired by state-of-the-art numerical simulations. Hydrodynamic simulations of planet–disk interactions predict the formation of circumplanetary disks (CPDs) around accreting planetary cores. These CPDs have eluded unequivocal detection—their identification requires predictions in CPD tracers. In this work, we aim to assess the observability of embedded CPDs with the Atacama Large Millimeter/submillimeter Array (ALMA) as features imprinted in the gas kinematics. We use 3D smooth particle hydrodynamic simulations of CPDs around 1 and 5 MJup planets at large stellocentric radii in locally isothermal and adiabatic disks. The simulations are then connected with 3D radiative transfer for predictions in CO isotopologues. Observability is assessed by corrupting with realistic long baseline phase noise extracted from the recent HL Tau ALMA data. We find that the presence of a CPD produces distinct signposts: (1) a compact emission separated in velocity from the overall circumstellar disk’s Keplerian pattern, (2) a strong impact on the velocity pattern when the Doppler-shifted line emission sweeps across the CPD location, and (3) a local increase in the velocity dispersion. We test our predictions with a simulation tailored for HD 100546—which has a reported protoplanet candidate. We find that the CPDs are detectable in all three signposts with ALMA Cycle 3 capabilities for both 1 and 5 MJup protoplanets, when embedded in an isothermal disk.