Planet formation signposts: observability of circumplanetary disks via gas kinematics

Abstract: The identification of on-going 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 ALMA as features imprinted in the gas kinematics. We use 3D Smooth Particle Hydrodynamic (SPH) simulations of CPDs around 1 and 5 M_Jup 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) 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 3 signposts with ALMA Cycle 3 capabilities for both 1 and 5 M_Jup protoplanets, when embedded in an isothermal disk.

Observability of Circumplanetary Disks via Gas Kinematics

The paper by Perez et al. explores the potential of using gas kinematics to detect circumplanetary disks (CPDs) as part of ongoing planet formation processes. Utilizing observations made possible by the Atacama Large Millimeter Array (ALMA), the authors present a method to identify CPDs through their influence on the surrounding gas' velocity fields.

Scope and Methodology

The study begins by acknowledging the challenges in identifying CPDs, which are predicted to form around growing planetary cores but have largely eluded direct observation in spite of advancements in astronomical instrumentation. The authors employ three-dimensional (3D) Smooth Particle Hydrodynamic (SPH) simulations to model CPDs forming around planets of 1 and 5 Jupiter masses ($M_{\rm Jup}$) located at significant distances from their host stars. These simulations are crafted under locally isothermal and adiabatic disk conditions to capture variations that might occur under different thermal regimes.

Following the hydrodynamic simulations, 3D radiative transfer predictions are utilized to simulate CO isotopologue emissions. The observability of these emissions is compromised by adding realistic long baseline phase noise, similar to what has been documented in the recent HL Tau ALMA data.

Key Findings

The simulations indicate that CPDs imprint three notable features on the gas kinematics:

1. Velocity Separation: There is compact emission that is distinct in velocity from the larger circumstellar disk's Keplerian flow. This feature arises due to the CPD's individual keplerian dynamics affecting gas motion.

2. Distorted Velocity Patterns: As the Doppler-shifted line emission of the parent circumstellar disk intersects the CPD, a noticeable distortion or 'bending' occurs in the Keplerian pattern at the location of the CPD.

3. Increased Local Velocity Dispersion: There is an observable rise in velocity dispersion around the CPD, suggesting enhanced kinematic activity in its vicinity.

In alignment with these patterns, the study observes double-peaked profiles in the spectra reflective of distinct CPD kinematics, particularly prominent in the higher-mass planet scenarios (5 $M_{\rm Jup}$) under isothermal conditions.

Implications and Future Prospects

The paper's conclusions provide a framework for the direct detection of CPDs, which can be pivotal in confirming planetary formation theories and assessing the dynamics within protoplanetary disks. This work demonstrates that ALMA, with its high-resolution capabilities, can discern the subtle signs of CPDs if focused on the kinematic metrics proposed.

The paper lays a foundation for extending the scope of exoplanet discovery to the natal environments of protoplanets. Future work might integrate more complex radiative transfer physics into SPH simulations, transitioning from simplified isothermal models to those that encapsulate more comprehensive thermal feedback mechanisms.

Strategically, this paper suggests that the combined use of SPH and radiative models is effective in refining the observational signatures of CPDs, paving the way for more targeted observational campaigns with ALMA and other similar instruments worldwide. These campaigns could eventually substantiate CPD theories and aid in the understanding of planet formation processes at various scales and conditions.