Dust dynamics in radially convective regions of protoplanetary disks (2501.09792v1)

Abstract: Hydrodynamic instabilities likely operate in protoplanetary disks. One candidate, Convective Overstability (COS), can be triggered in regions with a negative radial entropy gradient. The ensuing turbulence and flow structures are expected to affect dust dynamics directly. We revisit the interaction between dust and the COS with high-resolution spectral simulations in the unstratified, axisymmetric Boussinesq shearing box framework. We find zonal flows, or pressure bumps, formed by the COS trap dust, as expected, but dust densities increase at most by a factor of $O(10)$ over its background value due to the zonal flows' unsteady nature. Furthermore, dust feedback can impede the formation of zonal flows, even at small dust-to-gas ratios $\epsilon \sim O(0.1)$. We interpret this phenomenon as a competition between the negative gas angular momentum flux associated with zonal flow formation and the positive dust angular momentum flux associated with its drift towards pressure maxima. Dust concentration significantly weakens when a large-scale radial pressure gradient induces a background dust drift. Ultimately, we find that dust concentration by COS-induced zonal flows is limited to $\epsilon \lesssim 1$. Whether this can be improved under more realistic geometries must be addressed with stratified and full 3D simulations at equivalent resolutions.

• The paper demonstrates that convective overstability produces zonal flows that trap dust, increasing local dust densities by roughly 10 times.
• The paper reveals that even low dust-to-gas ratios (~0.1) can disrupt zonal flow formation through dust feedback, hindering dust concentration.
• The paper shows that a significant radial pressure gradient limits dust accumulation, indicating the need for more complex models to fully capture planetesimal formation.

An Investigation into Dust Dynamics in Radially Convective Regions of Protoplanetary Disks

The paper by Min-Kai Lin and Marius Lehmann focuses on the intricate dynamics of dust within protoplanetary disks, specifically in regions influenced by convective overstability (COS). Protoplanetary disks, composed predominantly of gas with embedded solid dust particles, are the birthplaces of planets, and understanding their dynamics is crucial for planet formation theories.

Key Findings and Methodology

The central aim of the paper is to explore the interaction between dust and the gas dynamics influenced by COS using high-resolution spectral simulations within the framework of an unstratified, axisymmetric Boussinesq shearing box. This approach allows the investigation of the hydrodynamic instabilities that can emerge in regions of the disk with a negative radial entropy gradient, which are capable of producing turbulence and flow structures impacting dust concentration and evolution.

• Zonal Flows and Dust Trapping: The simulations reveal that COS can generate zonal flows or pressure bumps that trap dust particles within the disk. However, the dust densities increase only by a factor of approximately 10 times the background value due to the unsteady nature of these zonal flows.
• Impact of Dust Feedback: It is demonstrated that dust feedback can obstruct zonal flow formation, even at relatively low dust-to-gas ratios of about 0.1. This interaction results in a competition influenced by the balance of angular momentum fluxes of gas and dust, impacting the dust's capability to concentrate.
• Constraints from Radial Pressure Gradients: A large-scale radial pressure gradient, inducing background dust drift, significantly weakens dust concentration ability. The paper suggests this is due to the background drift opposing the local pressures created by zonal flows.
• Limitations on Dust Concentration: The outcomes indicate that dust concentrations facilitated by COS-induced zonal flows are limited to values not exceeding dust-to-gas ratios of one, suggesting that further concentrations might necessitate more complex conditions or geometries that the unstratified model does not capture.

Implications and Future Directions

This investigation provides critical insights into the dust dynamics within protoplanetary disks, which holds substantial implications for planetesimal and ultimately planet formation.

• Theoretical Implications: The results underline the limitations of COS in achieving the high dust concentrations necessary for triggering phenomena like gravitational collapse or the streaming instability (SI). The necessity to consider more complex, possibly three-dimensional, and stratified disk models is clear, which could encompass vertical gravity effects and more realistic dust and gas coupling.
• Practical Implications: For practical advancements, the paper suggests the incorporation of vortex dynamics in future research to examine if COS-triggered vortices can concentrate dust to higher extents. This approach might reveal mechanisms enabling more effective dust collection and planetesimal formation, potentially involving the breakdown of zonal flows in three-dimensional scenarios.
• Advancements in Simulation Techniques: The application of spectral simulations using a Boussinesq framework paves the way for higher fidelity models that can incorporate multiple physical processes influencing disk stability, turbulence, and planet formation.

In conclusion, the research by Lin and Lehmann advances our understanding of dust dynamics in protoplanetary disks by illustrating the nuanced roles of convective overstability and feedback mechanisms in dust concentration. While the limitations of the unstratified, axisymmetric approach are acknowledged, the paper sets a foundation for more comprehensive models that could significantly enhance our grasp of early-stage planetary formation processes.