arXiv subjects

Min-Kai Lin

Publications and source records attributed to Min-Kai Lin.

At least 19 recordsLinked to original sources

arXiv2026

Dust and Gas Transport in Substructured Nonideal MHD Wind-Launching Disks with Embedded Planets

Radial dust transport in protoplanetary disks is a key process shaping planet formation and disk chemistry. We investigate how this transport, along with gas transport, is regulated in wind-launching disks with embedded planets using three-dimensional nonideal MHD simulations. We find that disk substructures do not act as absolute barriers to transport. Low-mass planets leave the disk structure dominated by the magnetic wind, while a Jupiter-mass planet opens a deep gap and drives spiral shocks. However, even in this regime, wind-driven accretion persists; the planet reshapes rather than replaces the magnetically driven flow, leaving the gap intrinsically time-dependent and partially permeable. Early-phase suppression of inward transport is followed by the development of localized, azimuthally intermittent inflow channels that enable continued cross-gap transport. This transport is strongly size-dependent: small grains remain coupled to the gas and readily penetrate the gap, whereas larger grains are efficiently trapped outside the planet. Consequently, a giant planet acts as an efficient but incomplete filter rather than a perfect barrier. These results support a "leaky gap" scenario, where radial transport is regulated rather than halted by substructures. Volatile-rich material can be delivered to the inner disk both before gap opening and via continued leakage, providing a natural explanation for the diverse inner disk compositions inferred from JWST. Similarly, pebble isolation during core growth should be viewed as a gradual filtering process rather than a binary transition. More generally, disk substructures are dynamically evolving features whose transport efficiency depends on their physical origin (magnetic versus planet-driven).

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arXiv2026

Puffed-up Edges of Planet-opened Gaps in Protoplanetary Disks. II. The Role of the Planet's Orbital Eccentricity

Eccentric planets constitute a large population of known exoplanets and may drive significant substructures in protoplanetary disks through planet-disk interactions if their eccentricities are excited early in the planet formation process. In this paper, we investigate the impact of a planet's orbital eccentricity on gas and dust structures in protoplanetary disks using three-dimensional multifluid hydrodynamic simulations. We find that an eccentric planet can drive stronger meridional gas circulation around the planet-opened gap, which significantly enhances the dust puff-up feature at the gap edge relative to the circular-orbit case. The planet-induced gap can also become highly leaky to dust grains when the planet is eccentric, allowing dust grains to be transported radially and thereby fill the gap. Furthermore, dust rings composed of pebble-sized grains are expected to become both larger and radially wider when the planet is eccentric, with this trend becoming more pronounced at higher planet eccentricities. Overall, our results suggest that a planet's orbital eccentricity can play a significant role in shaping gas and dust structures in protoplanetary disks, with important implications for planet formation theory and disk observations of the WISPIT 2 system.

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arXiv2026

Streaming instabilities in weakly ionized protoplanetary discs: the Ambipolar Streaming Instability (AmSI)

The regions of protoplanetary discs where planets can form are believed to be weakly ionised, suggesting thereby that non-ideal magneto-hydrodynamics (MHD) effects play an important role in the disc dynamics and in the planet formation process. In particular, the combined effect of ohmic resistivity and ambipolar diffusion can be responsible for launching MHD-driven disc winds. In this context, we focus on the effect of ambipolar diffusion (AD) and examine the stability of a dusty, magnetized disc by employing both linear stability analyses and numerical simulations. We show that dust feedback tends to stabilize the MRI oblique modes involved in the ambipolar-shear instability. We also find that ambipolar diffusion leads to the onset of a strong resonant drag instability (RDI), in which an Alfv\'en wave is destabilized by the relative drift between the gas and dust components. The main impact of AD is to modify the Alfv\'en wave frequency, resulting in a large resonance width. The instability is found to have significant growth rates even in dust-poor discs and for tightly coupled particles, which may help to bridge the gap between growth of dust grains through coagulation and planetesimal formation.

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arXiv2026

Diffusive Instabilities in Dusty Disks: Linear Growth and Nonlinear Breakdown

We revisit the diffusive instability in dusty disks that arises when the dust mass diffusivity and/or viscosity decreases sufficiently steeply with increasing dust density. Our updated model includes an incompressible, viscous gas that responds azimuthally and couples to the dust through drag. We show that the basic criterion for diffusion-slope-driven instability remains approximately $\beta_\mathrm{diff}\lesssim -2$ for small dust stopping times, with gas feedback providing only modest quantitative changes for parameters motivated by streaming-instability turbulence. We perform nonlinear numerical calculations and confirm linear growth and mode selection toward the fastest-growing wavenumber. However, for power-law closures $D\propto\Sigma^{\beta_\mathrm{diff}}$ with $\beta_\mathrm{diff}<0$, the nonlinear evolution does not saturate. Instead, steepening gradients amplify the nonlinear dust-pressure term and drive finite-time collapse into increasingly sharp spikes. Motivated by the absence of multidimensional saturation channels in our 1D framework, we test a simple piecewise closure in which the negative diffusion slope operates only over a finite density interval. This modification eliminates blowup and produces peak densities controlled by the imposed saturation scale. Our results support diffusive instabilities as a linear organizing mechanism in dusty turbulence, while highlighting that realistic nonlinear saturation requires additional physics beyond the present closure.

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arXiv2026

Evolution of dust in a protoplanetary disc driven by stellar flybys: implications for the streaming instability

Stellar flybys are a common dynamical process in young stellar clusters and can significantly reshape protoplanetary discs. However, their impact on dust dynamics remains poorly understood, particularly in the weakly coupled regime (St$\gg$1). We present three-dimensional hydrodynamical simulations of parabolic stellar flybys-both coplanar and inclined-interacting with a gaseous and dusty protoplanetary disc. Dust species with Stokes numbers ranging from 15 to 100, corresponding to four grain sizes under a uniform initial gas surface density, are included. Perturber masses of 0.1 and 1$\mathrm{M}_{\odot}$ are considered. The induced spiral structures exhibit distinct dynamical behaviours in gas and dust: dust spirals retain a nearly constant pattern speed, while gas spirals gradually decelerate. The pitch angles of both components decrease over time, with dust evolving more rapidly. In the weakly coupled regime, gas and dust spirals are spatially offset, facilitating dust accumulation around both structures. Equal-mass flybys truncate the disc at approximately $\sim$0.55$r_{\mathrm{Hill}}$, producing tightly wound, ring-like spirals that promote dust concentration. By mapping the streaming instability growth rates in the solid abundance-Stokes number space across three evolutionary phases, we find that a low-mass flyby suppresses dust concentration below the critical clumping threshold after periastron and maintains this suppression over time, indicating long-lasting inhibition of dust clumping. An equal-mass flyby raises local solid abundance well above the threshold, suggesting that such encounters may foster conditions favourable for dust clumping. Flyby-induced spirals play a central role in shaping dust evolution, leading to distinct spatial and temporal behaviours in weakly coupled discs.

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arXiv2025

Coexistence of coagulation and streaming instabilities in protoplanetary discs

The streaming instability is considered one of the leading candidates for the formation of planetesimals, due to its ability to overcome the bouncing and fragmentation barriers. The formation of dense dust clumps through this process, however, is possible provided it involves solids with dimensionless stopping times $\sim 0.1$ in standard discs, which typically corresponds to 1-10 cm-sized particles. This implies that dust coagulation is required for the SI to be an efficient process. Here, we employ unstratified, shearing-box simulations combined with a moment equation for solving the coagulation equation to examine the effect of dust growth on the SI. In dust-rich discs with a dust-to-gas ratio $\epsilon\gtrsim 1$, coagulation is found to have little impact on the SI; while in dust-poor discs with $\epsilon\sim 0.01$, we observe the formation of vertically extended filaments through the action of the coagulation instability (CI), which is triggered due to the dependence of coagulation efficiency on dust density. For moderate dust-to-gas ratios $\epsilon\sim 0.1$ and Stokes numbers $St \lesssim 0.1$, we find onset of the SI within these filaments, with a linear growth rate significantly higher compared to standard SI. We refer to this regime as coagulation-assisted SI. The synergy between both instabilities in that case leads to isotropic turbulence and dust concentrations that are increased by a factor of $30-40$. As dust continues to grow, SI tends to overcome the effect of the CI such that the nonlinear saturation phase is similar to pure SI. Our results suggest that coagulation, by simply increasing dust size, may facilitate the formation of dense clumps through the SI; even though it has only little effect on its nonlinear evolution.

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arXiv2025

JCMT detection of HCN emission from 3I/ATLAS at 2.1 AU

We report the detection of HCN ($J=3-2$) rotational emission from comet 3I/ATLAS at a heliocentric distance of 2.13 AU with the James Clerk Maxwell Telescope (JCMT). Observations were conducted from 07 August 2025 and 14 September 2025 (UT) using the $^{\prime}\overline U^{\prime}\overline u$ heterodyne receiver and ACSIS spectroscopic backend. The HCN line was detected at $>6\sigma$ on 14 Sep 2025 (UT) and a production rate of $Q({\rm HCN})=(4.0\pm1.7)\times10^{25}\ {\rm s}^{-1}$ was derived by non-LTE radiative transfer modelling. Preliminary estimates of the HCN/H$_2$O and CN/HCN abundance ratios suggest values similar to Solar System comets.

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arXiv2025

An MHD-based model for wind-driven disc-planet interactions

Hydrodynamic simulations of protoplanetary discs with planets typically assume that the disc is viscously driven, even though magnetic disc winds are now considered the primary driver of angular momentum transport through the disc. Magnetic disc winds are typically left out of hydrodynamic simulations because they require a magneto-hydrodynamic (MHD) treatment and an entire 3D domain, both of which are computationally expensive. Some studies have attempted to incorporate disc winds into disc-planet simulations without full MHD by adding a torque to mimic the effects of a disc wind. However, these studies predate any explicit 3D MHD simulations of planets in the presence of a disc wind. In light of recent MHD studies of disc winds beginning to include a planet, we develop a new disc wind prescription based on these studies and test its efficacy. With three main components, namely (i) excess torque in the planetary gap region, (ii) an MHD-based radial profile for the background torque, and (iii) a moderate level of viscosity, we find that we can essentially reproduce planetary gap profiles for planets above the thermal mass. With lower-mass planets, however, we find it more difficult to reproduce their gap structure. Lastly, we explore the planet's migration path and find that the planet rapidly migrates inwards due to the excess torque in the gap.

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arXiv2025

Dust Growth in ALMA Rings: II. Dusty Rossby Wave Instability

Annular substructures serve as ideal venues for planetesimal formation. In this series, we investigate the linear stage of dust growth within rings. The first paper examines the global streaming instability, while this study focuses on the dusty Rossby wave instability (DRWI). We perform a linear analysis of the two-fluid equations on a background pressure bump, representing annular substructures. The spectral code \textsc{Dedalus} is used to solve the linear eigenvalue problem. We identify two distinct DRWI modes: Type I, which originates from dust-modified gas RWI, and Type II, which results from dust-gas coupling. These modes never coexist for a given azimuthal wavenumber $\ky$, but transition between each other as $\ky$ varies. Type I modes are driven by the advection of background vorticity, whereas Type II modes involve two primary waves: Rossby waves, driven by advection, and thin waves, driven by dust-gas drag. Finally, we assess the relevance of DRWI in ALMA rings using DSHARP sources. Our findings suggest that Type I modes could explain the absence of azimuthal asymmetries in many ALMA disks, whereas Type II modes are entirely absent in all eight observed rings, implying that unresolved narrow rings or alternative mechanisms may play a role in dust growth within annular substructures.

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arXiv2025

Dust Concentration Via Coupled Vertical Settling and Radial Migration in Substructured Non-Ideal MHD Discs and Early Planet Formation

We investigate the dynamics of dust concentration in actively accreting, substructured, non-ideal MHD wind-launching disks using 2D and 3D simulations incorporating pressureless dust fluids of various grain sizes and their aerodynamic feedback on gas dynamics. Our results reveal that mm/cm-sized grains are preferentially concentrated within the inner 5-10 au of the disk, where the dust-to-gas surface density ratio (local metalicity Z) significantly exceeds the canonical 0.01, reaching values up to 0.25. This enhancement arises from the interplay of dust settling and complex gas flows in the meridional plane, including midplane accretion streams at early times, midplane expansion driven by magnetically braked surface accretion at later times, and vigorous meridional circulation in spontaneously formed gas rings. The resulting size-dependent dust distribution has a strong spatial variation, with large grains preferentially accumulating in dense rings, particularly in the inner disk, while being depleted in low-density gas gaps. In 3D, these rings and gaps are unstable to Rossby wave instability (RWI), generating arc-shaped vortices that stand out more prominently than their gas counterparts in the inner disk because of preferential dust concentration at small radii. The substantial local enhancement of the dust relative to the gas could promote planetesimal formation via streaming instability, potentially aided by the "azimuthal drift" streaming instability (AdSI) that operates efficiently in accreting disks and a lower Toomre Q expected in younger disks. Our findings suggest that actively accreting young disks may provide favorable conditions for early planetesimal formation, which warrants further investigation.

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arXiv2025

Convective Overstability in Radially Global Protoplanetary Disks. II. Impact on planetesimal formation

The Convective Overstability (COS) is a hydrodynamic instability occurring in protoplanetary disk (PPD) regions with an adverse radial entropy gradient. It is a potential driver of turbulence and may influence planetesimal formation. In this second paper of our series, we study the effects of the COS on dust dynamics in radially global PPD simulations, focusing on the mid-plane region, where vertical gravity on the dust is included. Axisymmetric 2D simulations show susceptibility to both the COS and the Vertically Shearing Streaming Instability. For a Stokes number tau = 0.1, strong dust clumping occurs only for highly super-solar initial metallicities Z greater than 0.05. In 3D non-axisymmetric simulations, the COS generates large-scale, long-lived vortices that have the potential to efficiently concentrate dust, with dust accumulation strengthening as tau increases. For tau = 0.01, no strong clumping occurs even at metallicities as high as Z = 0.1, and vortices remain robust and long-lived. At tau = 0.04, strong dust clumping is observed for solar metallicity (Z = 0.01) and higher. For tau = 0.1, clumping occurs even at strongly sub-solar metallicities (Z greater or equal to 0.004), peaking at Z approximately 0.01 to 0.03, including solar values. Under these conditions, vortices weaken significantly and become more spatially extended. At higher metallicities (Z greater or equal to 0.04) with tau = 0.1, large-scale vortex formation is suppressed, leading to nearly axisymmetric dust rings, which can still undergo clumping via the classical Streaming Instability.

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arXiv2025

Dust dynamics in radially convective regions of protoplanetary disks

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.

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arXiv2024

Observational Signatures of Dust Traffic Jams in Polar-Aligning Circumbinary Disks

Misaligned circumbinary disks will produce dust traffic jams during alignment or anti-alignment to the binary orbital plane. We conduct a hydrodynamical simulation of an initially misaligned circumbinary disk undergoing polar alignment with multiple dust species. Due to differential precession between the gas and dust components, multiple dust traffic jams are produced within the disk during polar alignment. The radial locations of the dust traffic jams depend on the Stokes number of the grains, which depends on grain size. We compute the dust temperature structure using post-processing radiative transfer to produce continuum images at cm-wavelengths. Multiple distinct rings emerge in the continuum images, corresponding to the dust traffic jams. The angular resolution of upcoming observations from SKA and ngVLA will be sufficient to detect centimeter-sized grains in protoplanetary disks and resolve the widths of dust traffic jams. Therefore, dust traffic jams resulting from the differential precession of gas and dust in misaligned circumbinary disks will be a prime target for more extended wavelength observations.

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arXiv2024

Streaming instabilities in accreting protoplanetary disks: A parameter study

The streaming instability (SI) is currently the leading candidate for triggering planetesimal formation in protoplanetary disks. Recently, a novel variation, the `azimuthal-drift' streaming instability (AdSI), was discovered in disks exhibiting laminar gas accretion. Unlike the classical SI, the AdSI does not require pressure gradients and can concentrate dust even at low abundances. We extend previous simulations of the AdSI to explore the impact of dust abundance, accretion flow strength, pressure gradients, and grain size. For a dimensionless accretion flow strength $\alpha_{\mathrm{M}}=0.1$ and particle Stokes number $\operatorname{St}=0.1$, we find the AdSI produces dust filaments for initial dust-to-gas ratios as low as $\epsilon=0.01$. For $\epsilon\gtrsim 1$, maximum dust-to-gas ratios of order 100 are attained, which can be expected to undergo gravitational collapse. Furthermore, even in systems dominated by the classical SI, an accretion flow drives filament formation, without which the disk remains in a state of small-scale turbulence. Our results suggest that an underlying accretion flow facilitates dust concentration and may thus promote planetesimal formation.

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arXiv2024

Polar alignment of a dusty circumbinary disc -- II. Application to 99 Herculis

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. In our first paper, we found as the circumbinary disc evolves towards a polar configuration perpendicular to the binary orbital plane, the differential precession between the gas and dust components leads to multiple dust traffic jams. These dust traffic jams evolve to form a coherent dust ring. In part two, we use 3D smoothed particle hydrodynamical simulations of gas and dust to model an initially highly misaligned circumbinary disc around the 99 Herculis (99 Her) binary system. Our results reveal that the formation of these dust rings is observed across various disc parameters, including the disc aspect ratio, viscosity, surface density power law index, and temperature power law index. The dust traffic jams are long-lived and persist even when the disc is fully aligned polar. The midplane dust-to-gas ratio within the rings can surpass unity, which may be a favourable environment for planetesimal formation. Using 2D inviscid shearing box calculations with parameters from our 3D simulations, we find streaming instability modes with significant growth rates. The streaming instability growth timescale is less than the tilt oscillation timescale during the alignment process. Therefore, the dust ring will survive once the gas disc aligns polar, suggesting that the streaming instability may aid in forming polar planets around 99 Her.

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arXiv2024

Rossby Wave Instability and Substructure Formation in 3D Non-Ideal MHD Wind-Launching Disks

Rings and gaps are routinely observed in the dust continuum emission of protoplanetary discs (PPDs). How they form and evolve remains debated. Previous studies have demonstrated the possibility of spontaneous gas rings and gaps formation in wind-launching disks. Here, we show that such gas substructures are unstable to the Rossby Wave Instability (RWI) through numerical simulations. Specifically, shorter wavelength azimuthal modes develop earlier, and longer wavelength ones dominate later, forming elongated (arc-like) anti-cyclonic vortices in the rings and (strongly magnetized) cyclonic vortices in the gaps that persist until the end of the simulation. Highly elongated vortices with aspect ratios of 10 or more are found to decay with time in our non-ideal MHD simulation, in contrast with the hydro case. This difference could be caused by magnetically induced motions, particularly strong meridional circulations with large values of the azimuthal component of the vorticity, which may be incompatible with the columnar structure preferred by vortices. The cyclonic and anti-cyclonic RWI vortices saturate at moderate levels, modifying but not destroying the rings and gaps in the radial gas distribution of the disk. In particular, they do not shut off the poloidal magnetic flux accumulation in low-density regions and the characteristic meridional flow patterns that are crucial to the ring and gap formation in wind-launching disks. Nevertheless, the RWI and their associated vortices open up the possibility of producing non-axisymmetric dust features observed in a small fraction of protoplanetary disks through non-ideal MHD, although detailed dust treatment is needed to explore this possibility.

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arXiv2024

Rossby wave instability in weakly ionized protoplanetary disks. I. azimuthal or vertical B-fields

Rossby wave instability (RWI) is considered the underlying mechanism to crescent-shaped azimuthal asymmetries, discovered in (sub-)millimeter dust continuum of many protoplanetary disks. Previous works on linear theory were conducted in the hydrodynamic limit. Nevertheless, protoplanetary disks are likely magnetized and weakly ionized. We examine the influence of magnetic fields and non-ideal magnetohydrodynamic (MHD) effects - namely, Ohmic resistivity, Hall drift, and ambipolar diffusion - on the RWI unstable modes. We perform radially global linear analyses, employing constant azimuthal ($B_\phi$) or vertical ($B_z$) background magnetic fields. It is found that, in the ideal MHD regime, magnetism can either enhance or diminish RWI growth. Strong non-ideal MHD effects cause RWI growth rates to recover hydrodynamic results. The sign of Hall Els\"{a}sser number subtly complicates the results, and vertical wavenumbers generically diminish growth rates.

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arXiv2024

Shoulder of Dust Rings Formed by Planet-disk Interactions

Recent analyses of mm-wavelength protoplanetary disk observations have revealed several emission excesses on the previously identified dust rings, referred to as dust shoulders. The prevalence of dust shoulders suggests that they trace a common but unclear mechanism. In this work, we combine 3D, multifluid hydrodynamic simulations with radiative transfer calculations to explain the formation of dust shoulders. We find that the ring-shoulder pairs can result from the 3D planet-disk interactions with massive, gap-opening planets. The key driver is the dust filtration effect at the local pressure maximum due to planet-driven outward gas flows. Our work provides a possible explanation for the outer dust shoulders in recent super-resolution analyses of ALMA observations. It also provides insights into the formation of the inner dust shoulder in the PDS 70 disk and highlights the role of 3D effects in planet-disk interaction studies.

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