arXiv subjects

Xue-Ning Bai

Publications and source records attributed to Xue-Ning Bai.

At least 19 recordsLinked to original sources

arXiv2026

Angular Momentum Transport in Protoplanetary Disks

We review our current understanding on the physical processes that govern angular momentum transport and evolution of protoplanetary disks. Extremely rich in physics, these processes are intimately connected to disk gas dynamics, with profound implications for planet formation. We organize them into a three-level hierarchical framework: (1) The coupling of gas with magnetic fields and radiation sets the microphysical foundation for understanding protoplanetary disk dynamics. Key ingredients include non-ideal magnetohydrodynamic effects (requiring ionization chemistry), along with heating and cooling processes. The disk can be divided into three radial sectors governed by distinct microphysics. (2) Protoplanetary disks host diverse gas dynamical processes, including hydrodynamic, magnetic and gravitational instabilities, along with thermally and magnetically-driven disk winds. Many of these {\it individual} processes are reasonably well understood, while others still require detailed investigation. (3) Protoplanetary disks are highly complex ecosystems where multiple processes interact. It is recognized that the bulk disk exhibits weak turbulence, with magnetically-driven wind likely serving as the primary transport mechanism. However, our knowledge remains highly limited regarding the disk's innermost region, early stages, long-term evolution, and environmental effects.

astro-ph.EP↗

arXiv2026

GRACE-DG: A Discontinuous Galerkin Method-Based Code for General Nonlinear Coagulation-Fragmentation Equations

Dust plays a crucial role in protoplanetary disks (PPDs) evolution and planet formation, influencing disk dynamics through gas-dust coupling, regulating disk temperature by dominating continuum opacity, and altering disk ionization fraction by capturing free electrons. In this work, we develop a high-order discontinuous Galerkin (DG) method-based open-source code GRACE-DG to solve the collision-induced coagulation-fragmentation equations. In particular, we have derived a new conservative formulation for the non-linear fragmentation term, which enables the DG method to capture the mass transfer process. The new solver exhibits good convergence in coupled aggregation and breakage simulations, making it highly suitable for future integration into hydrodynamic codes.

astro-ph.EP↗

arXiv2026

Modeling the Evolution of Protoplanetary Disks: Two Pathways from Gravitational Instability to MHD Wind-Driven Accretion

The global evolution of protoplanetary disks sets the initial conditions for planet formation. However, most models focus on individual evolutionary phases, with idealized initial conditions and oversimplified prescriptions for angular momentum transport and thermodynamics. We present a more realistic semi-two-dimensional ($1+1$D) model incorporating gravitational instability (GI), magnetohydrodynamic (MHD) winds, magneto-rotational instability (MRI), stellar irradiation, self-shadowing, and radiation transport. The radial distribution of large-scale magnetic flux drives two different pathways of disk evolution. When the vertical field is spatially uniform, a puffed-up, MRI-heated inner rim shadows the disk beyond it, sustaining a massive, gravitationally unstable region for $\sim 1$ Myr and, for several Myr, a compact ($\lesssim 10$ AU), cold ($\sim10$ K), low-turbulence ($α_\mathrm{SS}\sim10^{-4}$), high-density ($Σ\gtrsim300\,\mathrm{g\,cm^{-2}}$), optically thick reservoir, so that the disk mass inferred from mm-continuum emission can be greatly underestimated. When the field instead scales with midplane gas pressure, it drives stronger transport in the inner disk and eventually strips the shadow, leaving an extended, flared disk whose observable mass closely traces the true mass. Our results connect GI-dominated Class~0/I disks to MHD wind-driven Class~II disks, and point to three broader conclusions: (i) disk physics is strongly inhomogeneous in space and time, so constant-$α$ treatments miss essential physics; (ii) thermodynamics plays an active role, with self-shadowing simultaneously preserving GI and weakening MHD winds; and (iii) the distribution of large-scale magnetic flux is the key uncertainty, closely linked to whether the shadow is maintained. The two pathways align, respectively, with observations of compact, shadowed disks and extended, irradiated disks.

astro-ph.EP↗

arXiv2026

Complex gas flows in magnetized protoplanetary disks promote the formation of dust traps at low fragmentation velocities

Non-ideal magnetohydrodynamic simulations of protoplanetary disks show a plethora of complex gas structures, including winds, rings, and gaps. These affect dust transport and help form dust traps, which are essential for planetesimal formation. Although studies have explored the evolution of dust in such systems, they have done so either in 1D or without dust coagulation, and the effect of such systems on dust growth is still an active area of research. This work aims to investigate the effect of a complex gas flow architecture on global dust evolution, including dust growth and transport. We examine the timescales of different processes impacting dust evolution and discuss prospects of forming planetesimals. We post-process gas velocity output from a 2D non-ideal magnetohydrodynamic simulation using a 2D (r-z) Monte Carlo dust coagulation code to perform global simulations of dust growth and evolution. We perform three runs, one with a typical steady-state disk and two with the gas velocity from the MHD simulation, where we vary the fragmentation velocity. Our results show that the advection of small particles by the gas due to strong gas velocities can play an important role in setting the dust size distributions around protoplanetary disks. The gas flow structure has a transition region, and this region acts as a location of a dust pile-up, increasing the pebble-to-gas ratio by a factor of 2.5 when compared to the steady state disk. Lowering the fragmentation velocity improves the stability of the pile-up, but the pebble concentration is not as high. This scenario acts as a way to form a dust trap in a disk without a pressure bump. We discuss the possibilities for planetesimal formation in such a trap.

astro-ph.EP↗

arXiv2026

Guangqi: A two-dimensional radiation hydrodynamic code with realistic equation of states

We present Guangqi, a new two-dimensional, finite-volume radiation hydrodynamics code designed for high-performance astrophysical simulations. The code simultaneously resolves the hydrodynamic equations for complex equations of state (EoS) and implicit radiation transport under the flux-limited diffusion approximation. Written in Fortran and parallelized via the Message Passing Interface. Guangqi supports analytic hydrogen and helium EoS under the assumption of local thermal and chemical equilibrium. The framework is compatible with both Cartesian and spherical-polar geometries -- utilizing non-uniform grid spacing -- and incorporates static (SMR) and adaptive mesh refinement to optimize computational efficiency. To address the inherent challenges of angular momentum conservation in spherical-polar coordinates, we implement a robust and consistent "passive scalar angular momentum algorithm" (PSAMA). Domain decomposition is managed through both Z-order and Hilbert space-filling curves to ensure scalability. The code has been rigorously verified against a suite of standard benchmarks and newly designed test cases specifically intended to diagnose the non-linear coupling between gas dynamics, intricate EoS, radiation transport, and angular momentum conservation.

astro-ph.IM↗

arXiv2026

Cosmic Ray Magnetohydrodynamics: A New Two-Moment Framework with Numerical Implementation

Cosmic rays (CRs) play a pivotal role in various astrophysical systems, delivering feedback over a broad range of scales. However, modeling CR transport remains challenging due to its inherently multi-scale nature and complex microphysics. Recent advances in two-moment CR hydrodynamics have alleviated some of these challenges, improving understanding of CR feedback. Yet, current two-moment methods may not be able to directly incorporate all relevant CR transport processes, while the outcome of CR feedback sensitively depends on these underlying microphysics. Furthermore, numerical challenges persist, including instabilities from streaming terms and ambiguities in solver design for coupled CR-MHD systems. In this work, we develop a two-moment description for CR hydrodynamics from first principles. Beyond canonical CR streaming, our formulation accounts for CR pressure anisotropy and Alfvén waves propagating in both directions along the magnetic field, providing a general framework to incorporate more CR transport physics. We implement this framework as a new CR fluid module in the \textit{Athena}++ code, and validate it through a suite of benchmark tests. In particular, we derive the full dispersion relation of the two-moment CR-MHD system, identifying the CR-acoustic instability as well as other wave branches. These CR-MHD waves serve as rigorous benchmarks and also enable the use of realistic signal speeds in our Riemann solver. We propose a time step guideline to mitigate numerical instabilities arising from streaming source terms.

astro-ph.HE↗

arXiv2025

Radiation Hydrodynamics of Self-gravitating Protoplanetary Disks: I. Direct Formation of Gas Giants via Disk Fragmentation

Gravitational instability (GI) has long been considered a viable pathway for giant planet formation in protoplanetary disks (PPDs), especially at wide orbital separations or around low-mass stars where core accretion faces significant challenges. However, a primary drawback is that disk fragmentation from GI was generally found to produce over-massive clumps, typically in the mass range of brown dwarfs, although most numerical studies adopted simplified cooling prescriptions or with limited numerical resolution. We conduct a suite of global three-dimensional radiation hydrodynamics (RHD) simulations of self-gravitating PPDs using the meshless finite-mass (MFM) method. By implementing radiation transport via the M1 closure and systematically varying disk mass and opacity, we show that increasing disk mass and lowering opacity promote fragmentation by enhancing radiative cooling. Non-fragmenting disks settle into a gravito-turbulent state with low-order spiral structures and effective angular momentum transport characterized by $α\sim β_\mathrm{cool}^{-1}$. In fragmenting disks, a subset of gravitationally bound clumps survives as long-lived fragments. Their initial masses form a consistent distribution around $Σ\cdotλ_\mathrm{T} \cdot 2 (c_s/Ω_\mathrm{K})$ (with $λ_T$ the Toomre wavelength), corresponding to $\sim 0.3 - 10\,M_\mathrm{J}$ in our simulations, consistent with being gas giants. These results demonstrate that GI can produce planet-mass fragments under more realistic conditions, reinforcing it as a viable gas giant formation pathway and motivating further studies of fragment evolution and observational signatures.

astro-ph.EP↗

arXiv2025

Magnetohydrodynamic-guiding-center-particle-in-cell Method for Multiscale Plasma Kinetic Simulations

We present the formulation, algorithm and numerical tests of the magnetohydrodynamic-particle-in-cell (MHD-PIC) method with particles treated under the guiding center approximation, which we term the MHD-gPIC method, and it is implemented in the Athena++ MHD code. The new MHD-gPIC model consists of thermal (cold) fluid and high-energy particles whose dynamics are integrated through guiding center equations including drift motion, with carefully evaluated source terms as particle backreaction. The code is validated with a series of tests, and it is expected to be primarily applicable to study particle acceleration and transport in systems where gyro-resonance is considered insignificant. We also present preliminary studies of particle acceleration during non-relativistic magnetic reconnection.

astro-ph.HE↗

arXiv2025

Radiative Nonideal MHD Simulations of Inner Protoplanetary Disks: Temperature Structures, Asymmetric Winds, and Episodic Surface Accretion

We perform two-dimensional global magnetohydrodynamic (MHD) simulations including the full nonideal MHD effects (Ohmic diffusion, Hall effect, and ambipolar diffusion) and approximate radiation transport to understand the dynamics and thermal structure of the inner protoplanetary disks (PPDs). We have developed a simple radiative transfer model for PPDs that reasonably treats stellar non-thermal (XUV), stellar thermal (optical/infrared), and re-emitted radiations, reproducing the temperature structures from Monte Carlo radiative transfer. Our simulations show fast one-sided surface accretion ($\sim 10\%$ of Keplerian velocity) and asymmetric disk winds when the vertical magnetic field is aligned with the disk angular momentum. The asymmetry is due to the failure of the wind on the side with the accretion layer. On the accreting surface, clumps are repeatedly generated and accrete, driven by radiative feedback. For the anti-aligned fields, surface accretion becomes more moderate and time-variable, while the winds remain largely symmetric. For the thermal structure, accretion heating does not affect the disk temperature in any of our runs. This is because (1) the accretion energy dissipates via Joule heating at 2--3 gas scale heights, where low optical depth enables efficient radiative cooling, and (2) the winds remove $\gtrsim 10\%$ of the accretion energy. In contrast, the winds enhance radiative heating by elevating the irradiation front. These results highlight the importance of coupling between gas dynamics and radiation transport in PPDs, and provide observable magnetic activities such as fast episodic accretion, wind asymmetry, and molecular survival in XUV-irradiated winds.

astro-ph.EP↗

arXiv2025

Kinetic simulations of the cosmic ray pressure anisotropy instability: cosmic ray scattering rate in the saturated state

Cosmic ray (CR) feedback plays a vital role in shaping the formation and evolution of galaxies through their interaction with magnetohydrodynamic waves. In the CR self-confinement scenario, the waves are generated by the CR gyro-resonant instabilities via CR streaming or CR pressure anisotropy, and saturate by balancing wave damping. The resulting effective particle scattering rate by the waves, νeff, critically sets the coupling between the CRs and background gas, but the efficiency of CR feedback is yet poorly constrained. We employ 1D kinetic simulations under the Magnetohydrodynamic-Particle-In-Cell (MHD-PIC) framework with the adaptive δf method to quantify νeff for the saturated state of the CR pressure anisotropy instability (CRPAI) with ion-neutral friction. We drive CR pressure anisotropy by expanding/compressing box, mimicking background evolution of magnetic field strength, and the CR pressure anisotropy eventually reaches a quasi-steady state by balancing quasi-linear diffusion. At the saturated state, we measure νeff and the CR pressure anisotropy level, establishing a calibrated scaling relation with environmental parameters. The scaling relation is consistent with quasi-linear theory and can be incorporated to CR fluid models, in either the single-fluid or p-by-p treatments. Our results serve as a basis towards accurately calibrating the subgrid physics in macroscopic studies of CR feedback and transport.

astro-ph.HE↗

arXiv2025

Linear Analysis and Simulations of the Cosmic-Ray Streaming Instability: the Importance of Oblique Waves

Cosmic-ray (CR) streaming instability (CRSI) is believed to play an important role in CR transport and CR feedback to galaxies. It drives the growth of magnetohydrodynamic (MHD) waves that scatter CRs, and leads to energy/momentum exchange between CRs and interstellar medium. Despite extensive research on CRSI, its dependence on the thermodynamic state of the gas and its multidimensional effects have not been systematically studied. In this study, we derive the dispersion relation of the CRSI for three types of MHD waves including their dependence on propagation direction and plasma $β$ (the ratio of thermal pressure to magnetic pressure). We verify the analytical dispersion relation with one-dimensional and two-dimensional magnetohydrodynamic particle-in-cell simulations. Furthermore, we use 2D simulations to investigate the role of oblique MHD waves in scattering CRs, and find that these waves are important in helping low-energy particles overcome the 90-degree pitch angle barrier. While magnetosonic waves tend to be damped by transit time damping under typical conditions, oblique Alfvén waves likely play an important role in low-$β$ plasmas.

astro-ph.HE↗

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 $α$. 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.

astro-ph.EP↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2025

Solving for the 2D Water Snowline with Hydrodynamic Simulations. Emergence of gas outflow, water cycle and temperature plateau

In protoplanetary disks, the water snowline marks the location where ice-rich pebbles sublimate, releasing silicate grains and water vapor. These processes can trigger pile-ups of solids, making the water snowline a promising site for forming planetesimals. However, previous studies exploring the pile-up conditions typically employ 1D, vertically-averaged and isothermal assumptions. In this work, we investigate how a 2D flow pattern and realistic temperature structure affect the pile-up of pebbles at the snowline and how latent heat effects can leave observational imprints. We perform 2D (R-Z) multifluid hydrodynamic simulations, tracking chemically heterogeneous pebbles and the released vapor. With a recent-developed phase change module, the mass transfer and latent heat exchange during ice sublimation are calculated self-consistently. The temperature is calculated by a two-stream radiation transfer method under various opacities and stellar luminosity. We find that vapor injection at the snowline drives a previously unrecognized outflow, leading to a pile-up of ice outside the snowline. Vapor injection also decreases the headwind velocity in the pile-up, promoting planetesimal formation and pebble accretion. In active disks, we identify a water-cycle: after ice sublimates in the hotter midplane, vapor recondenses onto pebbles in the upper, cooler layers, which settle back to the midplane. This cycle promotes ice-trapping at snowline. Latent heat exchange flattens the temperature gradient across the snowline, broadening the width while reducing the peak solid-to-gas ratio of pile-ups. Due to the water cycle, active disks are more conducive to planetesimal formation than passive disks. The significant temperature dip (~ 40K) caused by latent heat cooling manifests as an intensity dip in the dust continuum, presenting a new channel to identify the water snowline in outbursting systems.

astro-ph.EP↗

arXiv2024

Dynamical Consequence of Shadows Cast to the Outer Protoplanetary Disks: I. Two-dimensional Simulations

There has been increasing evidence of shadows from scattered light observations of outer protoplanetary disks (PPDs) cast from the (unresolved) disk inner region, while in the meantime these disks present substructures of various kinds in the submillimeter. As stellar irradiation is the primary heating source for the outer PPDs, the presence of such shadows thus suggest inhomogeneous heating of the outer disk in azimuth, leading to a "thermal forcing" with dynamical consequences. We conduct a suite of idealized 2D disk simulations of the outer disk with azimuthally-varying cooling prescription to mimic the effect of shadows, generally assuming the shadow is static or slowly-rotating. The linear response to such shadows is two-armed spirals with the same pattern speed as the shadow. Towards the nonlinear regime, we find that shadows can potentially lead to the formation of a variety of types of substructures including rings, spirals and crescents, depending on viscosity, cooling time, etc. We have conducted systematic and statistical characterization of the simulation suite, and as thermal forcing from the shadow strengthens, the dominant form of shadow-induced disk substructures change from spirals to rings, and eventually to crescents/vortices. Our results highlight the importance of properly modeling the dynamical impact of inhomogeneous stellar irradiation, while call for more detailed modeling incorporating more realistic disk physics.

astro-ph.EP↗

arXiv2024

Evidence for Non-zero Turbulence in the Protoplanetary disc around IM Lup

The amount of turbulence in protoplanetary discs around young stars is critical for determining the efficiency, timeline, and outcomes of planet formation. It is also difficult to measure. Observations are still limited, but direct measurements of the non-thermal, turbulent gas motion are possible with the Atacama Large Millimeter/submillimeter Array (ALMA). Using CO(2-1)/$^{13}$CO(2-1)/C$^{18}$O(2-1) ALMA observations of the disc around IM Lup at ~0.4" (~60 au) resolution we find evidence of significant turbulence, at the level of $δv_{\rm turb}=(0.18-0.30)$c$_s$. This result is robust against systematic uncertainties (e.g., amplitude flux calibration, midplane gas temperature, disc self-gravity). We find that gravito-turbulence as the source of the gas motion is unlikely based on the lack of an imprint on the rotation curve from a massive disc, while magneto-rotational instabilities and hydrodynamic instabilities are still possible, depending on the unknown magnetic field strength and the cooling timescale in the outer disc.

astro-ph.EP↗

arXiv2024

Small and Large Dust Cavities in Disks around mid-M Stars in Taurus

High-angular resolution imaging by ALMA has revealed the near-universality and diversity of substructures in protoplanetary disks. However, disks around M-type pre-main-sequence stars are still poorly sampled, despite the prevalence of M-dwarfs in the galaxy. Here we present high-resolution (~50 mas, 8 au) ALMA Band 6 observations of six disks around mid-M stars in Taurus. We detect dust continuum emission in all six disks, 12CO in five disks, and 13CO line in two disks. The size ratios between gas and dust disks range from 1.6 to 5.1. The ratio of about 5 for 2M0436 and 2M0450 indicates efficient dust radial drift. Four disks show rings and cavities and two disks are smooth. The cavity sizes occupy a wide range: 60 au for 2M0412, and ~10 au for 2M0434, 2M0436 and 2M0508. Detailed visibility modeling indicates that small cavities of 1.7 and 5.7 au may hide in the two smooth disks 2M0450 and CIDA 12. We perform radiative transfer fitting of the infrared SEDs to constrain the cavity sizes, finding that micron-sized dust grains may have smaller cavities than millimeter grains. Planet-disk interactions are the preferred explanation to produce the large 60 au cavity, while other physics could be responsible for the three ~10 au cavities under current observations and theories. Currently, disks around mid-to-late M stars in Taurus show a higher detection frequency of cavities than earlier type stars, although a more complete sample is needed to evaluate any dependence of substructure on stellar mass.

astro-ph.EP↗