arXiv subjects

Pinghui Huang

Publications and source records attributed to Pinghui Huang.

15 recordsLinked to original sources

arXiv2026

Conditions for planetesimal formation via the streaming instability persist under turbulence driven by magnetorotational instability

Strong dust clumping by streaming instability (SI) is the leading proposed mechanism for forming planetesimals, the building blocks of terrestrial planets and giant-planet cores. The critical dust-to-gas density ratio above which the SI leads to dust concentration strong enough to result in gravitational collapse depends on local dust properties and disk conditions, such as particle Stokes number, pressure gradient, and turbulence. The role of turbulence has recently drawn attention because simulations have shown that even modest levels of istropically forced turbulence can significantly increase the critical dust-to-gas ratio. However, we show that this does not hold for turbulence self-consistently generated by the magnetorotational instability (MRI). We present the first parameter study of the SI in three-dimensional shearing-box simulations including non-ideal magnetohydrodynamics with ambipolar diffusion. Modest turbulence yields a clumping boundary as low as pure hydrodynamical cases, while stronger turbulence does increase the critical dust-to-gas density ratio, though appreciably less than in the models where turbulence is isotropically forced. Particle concentration occurs inside zonal flows, large-scale structures generated by the MRI. Our results suggest that self-consistent, MRI-driven turbulence does not necessarily inhibit planetesimal formation.

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arXiv2026

A Comparative Study of the Streaming Instability: Unstratified Models with Marginally Coupled Grains

The streaming instability is a leading mechanism for concentrating solids and initiating planetesimal formation in protoplanetary disks. Although numerous studies have explored its linear growth, nonlinear evolution, and implications for planet formation, the diversity of numerical methods and dust treatments used across the literature has made it difficult to assess which features of the instability are physically robust and which arise from code-dependent choices. We present the first systematic comparison of seven hydrodynamic codes--spanning finite-volume and finite-difference schemes and modeling dust either as Lagrangian particles or as a pressureless fluid--applied to the unstratified streaming instability with a dimensionless stopping time of unity. All codes reproduce the characteristic sequence of exponential growth, filament formation, and turbulent saturation, demonstrating broad agreement in the qualitative behavior of the instability. Quantitatively, however, the dust model remains the dominant source of variation at moderate resolution: particle-based simulations reach higher peak densities and exhibit broader high-density tails than fluid-based models at $512^2$ resolution, although increasing the number of particles brings their initial maximum density evolution into close agreement with that of dust-fluid models. At $1024^2$, these differences diminish substantially, indicating better agreement of the saturated-state statistics across dust treatments. In terms of computational performance, most particle implementations suffer from imbalanced parallelized loads, while execution on a GPU is at least two to three times more energy efficient and scales better at higher resolutions than on CPUs. Given the intrinsic stochasticity of this nonlinear system, only statistical diagnostics remain meaningful across codes.

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arXiv2025

Leaky Dust Traps in Planet-Embedded Protoplanetary Disks

From the survival of dust disks for a few Myr to the establishment of chemical dichotomy, dust traps are expected to play a pivotal role in sculpting protoplanetary disks and the early planet formation process. These traps however may not be perfect as evidenced by the detection of gas and dust inside the gaps and cavities of structured disks. Using two-fluid hydrodynamic global simulations in both two-dimensions (2D) and three-dimensions (3D), we directly compute the dynamics of dust grains as they aerodynamically interact with the disk gas that is being perturbed by an embedded planet of varying mass. In both 2D and 3D, we find the dust trap to be more leaky for lower mass planet and for higher turbulent $\alpha$. More crucially, we find the fraction of the dust mass that remain trapped within the pressure bump can be up to an order of magnitude more reduced in 3D vs. 2D with all else equal. Our simulations show a complex behavior of dust radial motion that is both azimuthally and poloidally non-uniform, with the overall dynamics dominated by the dust coupling to the gas flow even for relatively high St = 0.1. The leaky traps we find suggest pebble isolation mass is likely not truly isolating and that gap-opening planets do not establish as an unconditional impermeable barrier. Our findings have implications for recent JWST MINDS results, which show that volatiles, including water, are present in the inner regions of disks hosting outer dust rings.

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arXiv2025

Multi-wavelength Constraints on Dust Dynamics and Size Evolution in Protoplanetary Disk Rings. I. Method

Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (JVLA) have revealed many dust rings in protoplanetary disks, often interpreted as dust traps at gas pressure bumps. Previous studies have typically modeled these rings by assuming a single dust species in drift-diffusion equilibrium, neglecting dust size evolution resulting from coagulation and fragmentation. In this work, we perform numerical simulations that incorporate both dust-gas dynamics (drift and diffusion) and dust size evolution. Our results show that the radial distributions of different dust species (up to the fragmentation limit) are nearly identical in the dust ring, as dust growth dominates over drift and diffusion (e.g., with a typical dust-to-gas ratio of $\epsilon \sim 10^{-2}$). Building on this finding, we develop a comprehensive, self-consistent analytical theory that describes the dust ring structure while explicitly accounting for size evolution effects. Our model provides a unified framework for interpreting multi-wavelength observations by linking the physical dust distribution to the observed ring properties, thus laying the foundation for future observational modeling.

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arXiv2025

Dust Clumping in Outer Protoplanetary Disks: the Interplay Among Four Instabilities

Dust concentration in protoplanetary disks (PPDs) is the first step towards planetesimal formation, a crucial yet highly uncertain stage in planet formation. Although the streaming instability (SI) is widely recognized as a powerful mechanism for planetesimal formation, its properties can be sensitive to the gas dynamical environment. The outer region of PPDs is subject to the vertical shear instability (VSI), which could further induce the Rossby wave instability (RWI) to generate numerous vortices. In this work, we use the multifluid dust module in Athena++ to perform a 3D global simulation with mesh refinement to achieve adequate domain size and resolution to resolve and accommodate all these instabilities. The VSI mainly governs the overall gas dynamics, dominated by the breathing mode due to dust mass loading. The dust strongly settles to the midplane layer, which is much more densely populated with small vortices compared to the dust-free case. Strong dust clumping is observed, which is likely owing to the joint action of the SI and dusty RWI, and those sufficient for planetesimal formation reside only in a small fraction of such vortices. Dust clumping becomes stronger with increasing resolution, and has not yet achieved numerical convergence in our exploration. In addition, we find evidence of the Kelvin-Helmholtz instability (KHI) operating at certain parts of the dust-gas interface, which may contribute to the temporary destruction of dust clumps.

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arXiv2025

The Interplay between Dust Dynamics and Turbulence Induced by the Vertical Shear Instability

The interaction between gas and dust in protoplanetary disks (PPDs) plays a crucial role in setting the stage of planet formation. In particular, the streaming instability (SI) is well recognized as the mechanism for planetesimal formation out of this interaction. The outer region of PPDs is likely subject to the vertical shear instability (VSI), representing a major source of disk turbulence characterized by vertical corrugation that leads to strong dust stirring. In the meantime, the VSI turbulence in 3D generates vortices through the Rossby wave instability (RWI), which can trap dust and thereby promote dust concentration. In this study, we use the multifluid dust module in Athena++ to conduct 2D axisymmetric global simulations of PPDs with mesh refinement and 3D global simulations with modest resolution. In 2D, the VSI corrugation mode is weakened by dust back-reaction, while the SI can still survive regardless of initial conditions. Dust clumping occurs and is seeded by VSI-induced zonal flows. In 3D, dust can settle even more with increased dusty buoyancy, suppressing the VSI corrugation mode. Meanwhile, dust back-reaction enhances dust concentration in RWI vortices, though higher resolution is needed to assess dust clumping.

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arXiv2024

Vortex-Induced Rings and Gaps within Protoplanetary Disks

Observations of protoplanetary disks have revealed the presence of both crescent-shaped and ring-like structures in dust continuum emission. These crescents are thought to arise from dust-trapping vortices generated by the Rossby Wave Instability (RWI), which induces density waves akin to those caused by planets. These vortices have the potential to create gaps and rings within the disk, resulting from the dissipation of their density waves. We carry out 2D hydrodynamic simulations in the shearing box to investigate vortex-disk interaction. We find that long-lived vortices can produce dust rings and gaps in inviscid discs detectable by ALMA, and a more elongated vortex produces rings at larger separations. Vortex-induced density waves carry over two orders of magnitude higher angular momentum flux compared to planet-induced ones that shock at the same location, making the former much more effective at producing dust gaps and rings far away.

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arXiv2024

Observational characteristics of circum-planetary-mass-object disks in the era of James Webb Space Telescope

Recent observations have confirmed circumplanetary disks (CPDs) embedded in parental protoplanetary disks (PPDs). On the other hand, planetary-mass companions (PMCs) and planetary-mass objects (PMOs) are likely to harbor their own accretion disks. Unlike PPDs, CPDs and other disks around planet analogues are generally too compact to be spatially resolved by current instrumentation. In this study, we generate over 4,000 spectral energy distributions (SEDs) of circum-PMO-disks (CPMODs) with various host temperature and disk properties, which can be categorized into four prototypes, i.e., full, pre-transitional, transitional and evolved CPMODs. We propose a classification scheme based on their near-to-mid-infrared colors. Using those CPMOD models, we synthesize JWST (NIRCam and MIRI) photometry for F444W, F1000W and F2550W wide filters. We show F444W - F1000W and F444 - F2550W colors can be applied to distinguish different types of CPMODs, especially for those around hot hosts. Our results indicate that the ongoing and future JWST observations are promising to unveil structures and properties of CPMODs.

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arXiv2023

The dependence of the structure of planet-opened gaps in protoplanetary disks on radiative cooling

Planets can excite density waves and open annular gas gaps in protoplanetary disks. The depth of gaps is influenced by the evolving angular momentum carried by density waves. While the impact of radiative cooling on the evolution of density waves has been studied, a quantitative correlation to connect gap depth with the cooling timescale is lacking. To address this gap in knowledge, we employ the grid-based code Athena++ to simulate disk-planet interactions, treating cooling as a thermal relaxation process. We establish quantitative dependences of steady-state gap depth (Eq. 36) and width (Eq. 41) on planetary mass, Shakura-Sunyaev viscosity, disk scale height, and thermal relaxation timescale $(\beta)$. We confirm previous results that gap opening is the weakest when thermal relaxation timescale is comparable to local dynamical timescale. Significant variations in gap depth, up to an order of magnitude, are found with different $\beta$. In terms of width, a gap is at its narrowest around $\beta=1$, approximately $10\%$ to $20\%$ narrower compared to the isothermal case. When $\beta\sim100$, it can be $\sim20\%$ wider, and higher viscosity enhances this effect. We derive possible masses of the gas gap-opening planets in AS 209, HD 163296, MWC 480, and HL Tau, accounting for the uncertainties in local thermal relaxation timescale.

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arXiv2023

Atmospheric Recyling of Volatiles by Pebble-Accreting Planets

Planets, embedded in their natal discs, harbour hot envelopes. When pebbles are accreted by these planets, the contained volatile components may sublimate, enriching the envelope and potentially changing its thermodynamical properties. However, the envelopes of embedded planets actively exchange material with the disc, which would limit the buildup of a vapour-rich atmosphere. To properly investigate these processes, we have developed a new phase change module to treat the sublimation process with hydrodynamical simultions. Combined with the recently developed multi-dust fluid approach, we conduct 2D self-consistent hydrodynamic simulations to study how pebble sublimation influences the water content of super-Earths and sub-Neptunes. We find the extent and the amount of vapour that a planet is able to hold on to is determined by the relative size of the sublimation front and the atmosphere. When the sublimation front lies far inside the atmosphere, vapour tends to be locked deep in the atmosphere and keeps accumulating through a positive feedback mechanism. On the other hand, when the sublimation front exceeds the (bound) atmosphere, the ice component of incoming pebbles can be fully recycled and the vapour content reaches a low, steady value. Low disc temperature, small planet mass and high pebble flux (omitting accretion heating by pebbles) render the planet atmosphere vapour-rich while the reverse changes render it vapour-poor. The phase change module introduced here can in future studies also be employed to model the chemical composition of the gas in the vicinity of accreting planets and around snowlines.

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arXiv2022

A Multi-Fluid Dust Module in Athena++: Algorithms and Numerical Tests

We describe the algorithm, implementation and numerical tests of a multifluid dust module in the Athena++ magnetohydrodynamic (MHD) code. The module can accommodate an arbitrary number of dust species interacting with the gas via aerodynamic drag (characterized by the stopping time), with a number of numerical solvers. In particular, we describe two second-order accurate, two-stage, fully-implicit solvers that are stable in stiff regimes including short stopping time and high dust mass loading, and they are paired with the second-order explicit van-Leer and Runge-Kutta gas dynamics solvers in Athena++, respectively. Moreover, we formulate a consistent treatment of dust concentration diffusion with dust back-reaction, which incorporates momentum diffusion and ensures Galilean invariance. The new formulation and stiff drag solvers are implemented to be compatible with most existing features of Athena++, including different coordinate systems, mesh refinement, shearing-box and orbital advection. We present a large suite of test problems, including the streaming instability in linear and nonlinear regimes, as well as local and global setting, which demonstrate that the code achieves the desired performance. This module will be particularly useful for studies of dust dynamics and planet formation in protoplanetary disks.

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arXiv2020

Meso-scale Instability Triggered by Dust Feedback in Dusty Rings: Origin and Observational Implications

High spatial resolution observations of protoplanetary disks (PPDs) by ALMA have revealed many details that are providing interesting constraints on the disk physics as well as dust dynamics, both of which are essential for understanding planet formation. We carry out high-resolution, 2D global hydrodynamic simulations, including the effects of dust feedback, to study the stability of dusty rings. When the ring edges are relatively sharp and the dust surface density becomes comparable to the gas surface density, we find that dust feedback enhances the radial gradients of both the azimuthal velocity profile and the potential vorticity profile at the ring edges. This eventually leads to instabilities on meso-scales (spatial scales of several disk scale heights), causing dusty rings to be populated with many compact regions with highly concentrated dust densities on meso-scales. We also produce synthetic dust emission images using our simulation results and discuss the comparison between simulations and observations.

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arXiv2019

The Observability Of Vortex-Driven Spiral Arms In Protoplanetary Disk: Basic Spiral Properties

Some circumstellar disks are observed to show prominent spiral arms in infrared scattered light or (sub-)millimeter dust continuum. The spirals might be formed from self-gravity, shadows, or planet-disk interactions. Recently, it was hypothesized that massive vortices can drive spiral arms in protoplanetary disks in a way analogous to planets. In this paper, we study the basic properties of vortex-driven spirals by the Rossby Wave Instability in 2D hydrodynamics simulations. We study how the surface density contrast, the number, and the shape of vortex-driven spirals depend on the properties of the vortex. We also compare vortex-driven spirals with those induced by planets. The surface density contrast of vortex-driven spirals in our simulations are comparable to those driven by a sub-thermal mass planet, typically a few to a few tens of Earth masses. In addition, different from the latter, the former is not sensitive to the mass of the vortex. Vortex-driven spiral arms are not expected to be detectable in current scattered light observations, and the prominent spirals observed in scattered light in a few protoplanetary disks, such as SAO 206462 (HD 135344B), MWC 758, and LkHa 330, are unlikely to be induced by the candidate vortices in them.

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arXiv2019

New constraints on the dust and gas distribution in the LkCa 15 disk from ALMA

We search a large parameter space of the LkCa 15's disk density profile to fit its observed radial intensity profile of $^{12}$CO (J = 3-2) obtained from ALMA. The best-fit model within the parameter space has a disk mass of 0.1 $M_{\odot}$ (using an abundance ratio of $^{12}$CO/H$_2$ $=$ 1.4 $\times 10^{-4}$ in mass), an inner cavity of 45 AU in radius, an outer edge at $\sim$ 600 AU, and a disk surface density profile follows a power-law of the form $ρ_r \propto r^{-4}$. For the disk density profiles that can lead to a small reduced $χ^2$ of goodness-of-fit, we find that there is a clear linear correlation between the disk mass and the power-law index $γ$ in the equation of disk density profile. This suggests that the $^{12}$CO disk of LkCa 15 is optically thick and we can fit its $^{12}$CO radial intensity profile using either a lower disk mass with a smaller $γ$ or a higher disk mass with a bigger $γ$. By comparing the $^{12}$CO channel maps of the best-fit model with disk models with higher or lower masses, we find that a disk mass of $\sim$ 0.1 $M_{\odot}$ can best reproduce the observed morphology of the $^{12}$CO channel maps. The dust continuum map at 0.87 mm of the LkCa 15 disk shows an inner cavity of the similar size of the best-fit gas model, but its out edge is at $\sim$ 200 AU, much smaller than the fitted gas disk. Such a discrepancy between the outer edges of the gas and dust disks is consistent with dust drifting and trapping models.

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arXiv2018

Identifying Anticyclonic Vortex Features Produced by the Rossby Wave Instability in Protoplanetary Disks

Several nearby protoplanetary disks have been observed to display large scale crescents in the (sub)millimeter dust continuum emission. One interpretation is that these structures correspond to anticyclonic vortices generated by the Rossby wave instability within the gaseous disk. Such vortices have local gas over-densities and are expected to concentrate dust particles with Stokes number around unity. This process might catalyze the formation of planetesimals. Whereas recent observations showed that dust crescent are indeed regions where millimeter-size particles have abnormally high concentration relative to the gas and smaller grains, no observations have yet shown that the gas within the crescent region counter-rotates with respect to the protoplanetary disk. Here we investigate the detectability of anticyclonic features through measurement of the line-of-sight component of the gas velocity obtained with ALMA. We carry out 2D hydrodynamic simulations and 3D radiative transfer calculation of a protoplanetary disk characterized by a vortex created by the tidal interaction with a massive planet. As a case study, the disk parameters are chosen to mimic the IRS 48 system, which has the most prominent crescent observed to date. We generate synthetic ALMA observations of both the dust continuum and 12CO emission around the frequency of 345 GHz. We find that the anticyclonic features of vortex are weak but can be detected if both the source and the observational setup are properly chosen. We provide a recipe for maximizing the probability to detect such vortex features and present an analysis procedure to infer their kinematic properties.

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