arXiv subjects

Yisheng Tu

Publications and source records attributed to Yisheng Tu.

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).

astro-ph.EP↗

arXiv2026

Modeling YSO Jets in 3D III: Dependence of Accretion and Jet Properties on Stellar Magnetospheric Field Strength and Rotation

Observations of Young Stellar Objects (YSOs) systems reveal a wide diversity of jet properties, from well-collimated bipolar jets to uni-polar jets and systems with no detectable jet. Both prograde and counter-rotating jets are reported, raising questions about how jets are launched and how their properties relate to the underlying star-disk system. Using 3D non-ideal MHD simulations, we present a suite of models in which jet properties depend sensitively on stellar rotation and magnetic field strength. In all models, jets are launched from ``two-legged'' magnetic field lines anchored to both the star and the turbulent, magnetically elevated disk surface, with interactions at the disk surface crucial for mediating the magnetosphere-disk coupling. The axial jet and its surrounding disk wind form a characteristic ``spine-tower'' structure: the spine is the kinematically-dominated jet along open field lines threading the star, and the tower is the surrounding toroidal-field--dominated disk wind. The stability of this structure depends on the balance between the spine's stabilizing power and the tower's destabilizing power; if the tower dominates, the disk wind can choke the jet, producing asymmetric or no jets. This relationship allows an upper limit estimate on the toroidal magnetic field strength in the disk wind-launching region using observed outflow properties. Counter-rotating jets naturally appear in models, particularly with non-rotating stars, showing that the classical rotation-poloidal velocity relation does not reliably indicate the jet-launching radius. Instead, it could be used to trace the stellar rotation rate, offering a potential observational diagnostic of stellar spin.

astro-ph.SR↗

arXiv2026

A New Method of Measuring Magnetic Field Strength in Highly Structured Protostellar Envelopes

Magnetic fields play a fundamental role in protostellar collapse and disk formation, yet direct measurements of magnetic field strength in deeply embedded protostellar envelopes remain difficult. We present a new method to estimate both the vertical and total magnetic field strength in collapsing, pseudodisk- or sheetlet-dominated protostellar envelopes, derived directly from the magnetohydrodynamic momentum equation. The method relates the magnetic field strength to two observationally accessible quantities: the projected gravitational acceleration toward the center of collapse and the face-on column density of the pseudodisk, and two dimensionless parameters, $a_{b, R}$ and $γ_{zR}$, which characterize magnetic contribution to the force balance and the field geometry, respectively, through $|B_z|=(2πa_{b,R}γ_{zR}g_RΣ)^{1/2}$. Using non-ideal magnetohydrodynamic simulations, we verify the assumptions underlying the method, justify the adopted approximations, and calibrate the two key dimensionless parameters. We provide canonical estimates of these two parameters, and show that they exhibit only weak spatial and temporal variations, allowing robust field strength estimates even when detailed gas kinematics or high-resolution polarization information is unavailable. We show that the method is applicable in both turbulent and non-turbulent envelopes and is insensitive to the ambipolar diffusion coefficient, making it robust against uncertainties in the local turbulence strength and ionization rate. We apply the method to the Class 0 source L1157, using column-density and gravitational-acceleration estimates from the literature to estimate the magnetic field strength for L1157. Our result is broadly consistent with previous estimates from independent methods, demonstrating the utility of this approach for constraining magnetic fields in embedded protostellar systems.

astro-ph.EP↗

arXiv2026

MHD Simulations of Strongly Magnetized HII Region Evolution: Evidence for Ionized Gas Filamentation

Recent JWST observations of HII regions in the Central Molecular Zone have shown a highly filamentary morphology distinct from HII regions in other parts of the galaxy. We present magnetohydrodynamic (MHD) simulations of strongly magnetized (plasma-$β$ << 1) HII region evolution that investigate and describe the formation of these ionized gas filaments. HII region evolution has been simulated in a 30 pc$^3$ box, in distinct models with pre-placed overdensities in the ambient medium and overdensities that have been generated with driven turbulence. We find that when these overdensities are seeded in the ambient medium before the birth of the ionizing source, the photoionized plasma stripped off of these dense blobs is funneled into long filaments along the magnetic field lines. The length and emission measure of these ionized gas filaments are similar to the filaments observed in the Central Molecular Zone. Given that these filaments are effectively magnetically-confined flows of photoionized gas, their density and curvature are influenced by the density of the blob and the geometry of the configuration.

astro-ph.GA↗

arXiv2026

Modeling YSO Jets in 3D II: Accretion-Fed, Star-Anchored Poynting Jets in the Low-Density Polar Cavity Powered by Disk-Magnetosphere Interaction

The origin of jets in young stellar objects (YSOs) remains a subject of active investigation. We present a 3D magnetohydrodynamic simulation of jet launching in YSOs, focusing on the interaction between the stellar magnetosphere and the accretion disk. In our model, a fast, low-density bipolar jet is powered by disk-magnetosphere interaction and launched through the polar cavity that is mass-loaded from the disk rather than the star. Specifically, outflows are driven by toroidal magnetic pressure generated along "two-legged" field lines, anchored at a magnetically dominated stellar footpoint and a mass-dominated point on the (magnetically elevated) disk surface via a cyclic "load-fire-reload" process: in the "load" stage, differential rotation between stellar and disk footpoints generates toroidal magnetic pressure; in the "fire" stage, vertical gradients in the toroidal field accelerate plasma and transport Poynting flux into the polar cavity; in the "reload" stage, magnetic reconnection allows the cycle to repeat, reforming "two-legged" field lines. These field lines are not required to be fully reset to a dipolar loop configuration; it is only required that the disk-end be shallowly embedded in the (elevated) disk surface. This rapid, asynchronous process produces a continuous, large-scale outflow. The resulting magnetically dominated (Poynting) jet, accelerated by magnetic pressure within the low-density polar cavity, is distinct from the denser, slower disk wind launched through the classic magnetic-tower mechanism. Comparison with a disk-only model shows that the rotating stellar magnetosphere promotes bipolar jet launching by shaping a magnetic geometry favorable to symmetric outflows.

astro-ph.SR↗

arXiv2025

Protostellar Disks Fed By Dense Collapsing Gravo-Magneto-Sheetlets

Stars form from the gravitational collapse of turbulent, magnetized molecular cloud cores. Our non-ideal MHD simulations reveal that the intrinsically anisotropic magnetic resistance to gravity during the core collapse naturally generates dense gravo-magneto-sheetlets within inner protostellar envelopes -- disrupted versions of classical sheet-like pseudodisks. They are embedded in a magnetically dominant background, where less dense materials flow along the local magnetic field lines and accumulate in the dense sheetlets. The sheetlets, which feed the disk predominantly through its upper and lower surfaces, are the primary channels for mass and angular momentum transfer from the envelope to the disk. The protostellar disk inherits a small fraction (up to 10\%) of the magnetic flux from the envelope, resulting in a disk-averaged net vertical field strength of 1-10 mG and a somewhat stronger toroidal field, potentially detectable through ALMA Zeeman observations. The inherited magnetic field from the envelope plays a dominant role in disk angular momentum evolution, enabling the formation of gravitationally stable disks in cases where the disk field is relatively well-coupled to the gas. Its influence remains significant even in marginally gravitationally unstable disks formed in the more magnetically diffusive cases, removing angular momentum at a rate comparable to or greater than that caused by spiral arms. The magnetically driven disk evolution is consistent with the apparent scarcity of prominent spirals capable of driving rapid accretion in deeply embedded protostellar disks. The dense gravo-magneto-sheetlets observed in our simulations may correspond to the ``accretion streamers" increasingly detected around protostars.

astro-ph.EP↗

arXiv2025

Fragmentation of Dense Rotation-Dominated Structures Fed by Collapsing Gravomagneto-Sheetlets and Origin of Misaligned 100 au-Scale Binaries and Multiple Systems

The majority of stars are in binary/multiple systems. How such systems form in turbulent, magnetized cores of molecular clouds in the presence of non-ideal MHD effects remains relatively under-explored. Through ATHENA++-based non-ideal MHD AMR simulations with ambipolar diffusion, we show that the collapsing protostellar envelope is dominated by dense gravo-magneto-sheetlets, a turbulence-warped version of the classic pseudodisk produced by anisotropic magnetic resistance to the gravitational collapse, in agreement with previous simulations of turbulent, magnetized single-star formation. The sheetlets feed mass, magnetic fields, and angular momentum to a Dense ROtation-Dominated (DROD) structure, which fragments into binary/multiple systems. This DROD fragmentation scenario is a more dynamic variant of the traditional disk fragmentation scenario for binary/multiple formation, with dense spiral filaments created by inhomogeneous feeding from the highly structured larger-scale sheetlets rather than the need for angular momentum transport, which is dominated by magnetic braking. Provided that the local material is sufficiently demagnetized, with a plasma-$β$ of 10 or more, collisions between the dense spiraling filaments play a key role in facilitating gravitational collapse and stellar companion formation by pushing the local magnetic Toomre parameter $Q_\mathrm{m}$ below unity. This mechanism can naturally produce {\it in situ} misaligned systems on the 100-au scale, often detected with high-resolution Atacama Large Millimeter Array (ALMA) observations. Our simulations also highlight the importance of non-ideal MHD effects, which affect whether fragmentation occurs and, if so, the masses and orbital parameters of the stellar companions formed.

astro-ph.SR↗

arXiv2025

Modeling YSO Jets in 3D I: Highly Variable Asymmetric Magnetic Pressure-Driven Jets in the Polar Cavity from Toroidal Fields Generated by Inner Disk Accretion

Jets and outflows are commonly observed in young stellar objects (YSOs), yet their origins remain debated. Using 3D non-ideal magnetohydrodynamic (MHD) simulations of a circumstellar disk threaded by a large-scale open poloidal magnetic field, we identify three components in the disk-driven outflow: (1) a fast, collimated jet, (2) a less collimated, slower laminar disk wind, and (3) a magneto-rotational instability (MRI)-active turbulent disk wind that separates the former two. At high altitudes, the MRI-active wind merges with the laminar disk wind, leaving only the jet and disk wind as distinct components. The jet is powered by a novel mechanism in the star formation context: a lightly mass-loaded outflow driven by toroidal magnetic pressure in the low-density polar funnel near the system's rotation axis. A geometric analysis of the magnetic field structure confirms that magnetic tension does not contribute to the outflow acceleration, with magnetic pressure acting as the dominant driver. While the outflow in our model shares similarities with the magneto-centrifugal model-such as angular momentum extraction from the accreting disk-centrifugal forces play a negligible role in jet acceleration. In particular, the flow near the jet base does not satisfy the conditions for magneto-centrifugal wind launching. Additionally, the jet in our simulation exhibits strong spatial and temporal variability. These differences challenge the applicability of rotation-outflow velocity relations derived from steady-state, axisymmetric magneto-centrifugal jet models for estimating the jet's launching radius. For the slower disk wind, vertical motion is driven by toroidal magnetic pressure, while centrifugal forces widen the wind's opening angle.

astro-ph.SR↗

arXiv2025

YSO Jets Magnetocentrifugally Driven by Reconnecting Atmospheric Avalanche Accretion Streams Above Inner Circumstellar Disks

Fast, collimated jets are ubiquitous features of young stellar objects (YSOs). They are generally thought to be powered by disk accretion, but the details are debated. Through 2D (axisymmetric) MHD simulations, we find that a fast ($>100$~km/s) collimated bipolar jet is continuously driven along the north and south poles of the circumstellar disk that is initially magnetized by a large-scale open poloidal field and contains a thermally ionized inner magnetically active zone surrounded by a dead zone. The fast jet is primarily driven magneto-centrifugally by the release of the gravitational binding energy of the so-called ``avalanche accretion streams" near the boundary of an evacuated poloidal field-dominated polar region and a thick disk atmosphere raised by a toroidal magnetic field. Specifically, the fast outflow is driven along the upper (open) branch of the highly pinched poloidal field lines threading the (strongly magnetically braked) accretion streams where the density is relatively low so that the lightly loaded material can be accelerated magneto-centrifugally along the open field line to a high speed. The highly pinched poloidal magnetic fields threading the avalanche accretion streams tend to reconnect, enabling mass to accrete to the center without dragging along the poloidal magnetic flux with it. The reconnection provides a potential heating source for producing chondrules and calcium- and aluminum-rich inclusions (CAIs).

astro-ph.SR↗

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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2024

How negative feedback and the ambient environment limit the influence of recombination in common envelope evolution

We perform 3D hydrodynamical simulations to study recombination and ionization during the common envelope (CE) phase of binary evolution, and develop techniques to track the ionic transitions in time and space. We simulate the interaction of a $2\,M_\odot$ red giant branch primary and a $1\,M_\odot$ companion modeled as a particle. We compare a run employing a tabulated equation of state (EOS) that accounts for ionization and recombination, with a run employing an ideal gas EOS. During the first half of the simulations, $\sim15$ per cent more mass is unbound in the tabulated EOS run due to the release of recombination energy, but by simulation end the difference has become negligible. We explain this as being a consequence of (i) the tabulated EOS run experiences a shallower inspiral and hence smaller orbital energy release at late times because recombination energy release expands the envelope and reduces drag, and (ii) collision and mixing between expanding envelope gas, ejecta and circumstellar ambient gas assists in unbinding the envelope, but does so less efficiently in the tabulated EOS run where some of the energy transferred to bound envelope gas is used for ionization. The rate of mass unbinding is approximately constant in the last half of the simulations and the orbital separation steadily decreases at late times. A simple linear extrapolation predicts a CE phase duration of $\sim2\,\mathrm{yr}$, after which the envelope would be unbound.

astro-ph.SR↗

arXiv2023

Modeling CN Zeeman Effect Observations of the Envelopes of a Low-Mass Protostellar Disk and a Massive Protostar

We use the POLARIS radiative transfer code to produce simulated circular polarization Zeeman emission maps of the CN $J = 1 - 0$ molecular line transition for two types of protostellar envelope magnetohydrodynamic simulations. Our first model is a low mass disk envelope system (box length $L = 200\text{ au}$), and our second model is the envelope of a massive protostar ($L = 10^4\text{ au}$) with a protostellar wind and a CN enhanced outflow shell. We compute the velocity-integrated Stokes $I$ and $V$, as well as the implied $V/I$ polarization percentage, for each detector pixel location in our simulated emission maps. Our results show that both types of protostellar environment are in principle accessible with current circular polarization instruments, with each containing swaths of envelope area that yield percentage polarizations that exceed the 1.8\% nominal sensitivity limit for circular polarization experiments with the Atacama Large Millimeter/submillimeter Array (ALMA). In both systems, high polarization ($\gtrsim$1.8\%) pixels tend to lie at an intermediate distance away from the central star and where the line-center opacity of the CN emission is moderately optically thin ($τ_{LC} \sim 0.1-1$). Furthermore, our computed $V/I$ values scale roughly with the density weighted mean line-of-sight magnetic field strength, indicating that Zeeman observations can effectively diagnose the strength of envelope-scale magnetic fields. We also find that pixels with large $V/I$ are preferentially co-located where the absolute value of the velocity-integrated $V$ is also large, suggesting that locations with favorable percentage polarization are also favorable in terms of raw signal.

astro-ph.SR↗

arXiv2022

Grain Growth During Protostellar Disk Formation

Recent observations indicate that mm/cm-sized grains may exist in the embedded protostellar disks. How such large grains grow from the micron size (or less) in the earliest phase of star formation remains relatively unexplored. In this study we take a first step to model the grain growth in the protostellar environment, using two-dimensional (2D axisymmetric) radiation hydrodynamic and grain growth simulations. We show that the grain growth calculations can be greatly simplified by the "terminal velocity approximation", where the dust drift velocity relative to the gas is proportional to its stopping time, which is proportional to the grain size. We find that the grain-grain collision from size-dependent terminal velocity alone is too slow to convert a significant fraction of the initially micron-sized grains into mm/cm sizes during the deeply embedded Class 0 phase. Substantial grain growth is achieved when the grain-grain collision speed is enhanced by a factor of 4. The dust growth above and below the disk midplane enables the grains to settle faster towards the midplane, which increases the local dust-to-gas ratio, which, in turn, speeds up further growth there. How this needed enhancement can be achieved is unclear, although turbulence is a strong possibility that deserves further exploration.

astro-ph.EP↗

arXiv2022

Centrifugal Barrier and Super-Keplerian Rotation in Protostellar Disk Formation

With the advent of ALMA, it is now possible to observationally constrain how disks form around deeply embedded protostars. In particular, the recent ALMA C3H2 line observations of the nearby protostar L1527 have been interpreted as evidence for the so-called "centrifugal barrier," where the protostellar envelope infall is gradually decelerated to a stop by the centrifugal force in a region of super-Keplerian rotation. To test the concept of centrifugal barrier, which was originally based on angular momentum conserving-collapse of a rotating test particle around a fixed point mass, we carry out simple axisymmetric hydrodynamic simulations of protostellar disk formation including a minimum set of ingredients: self-gravity, rotation, and a prescribed viscosity that enables the disk to accrete. We find that a super-Keplerian region can indeed exist when the viscosity is relatively large but, unlike the classic picture of centrifugal barrier, the infalling envelope material is not decelerated solely by the centrifugal force. The region has more specific angular momentum than its surrounding envelope material, which points to an origin in outward angular momentum transport in the disk (subject to the constraint of disk expansion by the infalling envelope), rather than the spin-up of the envelope material envisioned in the classic picture as it falls closer to the center in order to conserve angular momentum. For smaller viscosities, the super-Keplerian rotation is weaker or non-existing. We conclude that, despite the existence of super-Keplerian rotation in some parameter regime, the classic picture of centrifugal barrier is not supported by our simulations.

astro-ph.SR↗

arXiv2020

Common Envelope Evolution on the Asymptotic Giant Branch: Unbinding within a Decade?

Common envelope (CE) evolution is a critical but still poorly understood progenitor phase of many high-energy astrophysical phenomena. Although 3D global hydrodynamic CE simulations have become more common in recent years, those involving an asymptotic giant branch (AGB) primary are scarce, due to the high computational cost from the larger dynamical range compared to red giant branch (RGB) primaries. But CE evolution with AGB progenitors is desirable to simulate because such events are the likely progenitors of most bi-polar planetary nebulae (PNe), and prominent observational testing grounds for CE physics. Here we present a high resolution global simulation of CE evolution involving an AGB primary and $1\,\mathrm{M}_\odot$ secondary, evolved for $20$ orbital revolutions. During the last $16$ of these orbits, the envelope unbinds at an almost constant rate of about $0.1$-$0.2\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$. If this rate were maintained, the envelope would be unbound in less than $10\,\mathrm{yr}$. The dominant source of this unbinding is consistent with inspiral; we assess the influence of the ambient medium to be subdominant. We compare this run with a previous run that used an RGB phase primary evolved from the same $2\,\mathrm{M}_\odot$ main sequence star to assess the influence of the evolutionary state of the primary. When scaled appropriately, the two runs are quite similar, but with some important differences.

astro-ph.SR↗

arXiv2020

Fields and Characteristic Impedances of Dipole and Quadrupole Cylindrical Stripline Kickers

We present semi-analytical methods for calculating the electromagnetic field in dipole and quadrupole stripline kickers with curved plates of infinitesimal thickness. Two different methods are used to solve Laplace's equation by reducing it either to a single or to two coupled matrix equations; they are shown to yield equivalent results. Approximate analytic solutions for the lowest order fields (dipole or quadrupole) are presented and their useful range of validity are shown. The kickers plates define a set of coupled transmission lines and the characteristic impedances of modes relevant to each configuration are calculated. The solutions are compared with those obtained from a finite element solver and found to be in good agreement. Mode matching to an external impedance determines the kicker geometry and this is discussed for both kicker types. We show that a heuristic scaling law can be used to determine the dependence of the characteristic impedance on plate thickness. The solutions found by semi-analytical methods can be used as a starting point for a more detailed kicker design.

physics.acc-ph↗

arXiv2019

How Drag Force Evolves in Global Common Envelope Simulations

We compute the forces, torque and rate of work on the companion-core binary due to drag in global simulations of common envelope (CE) evolution for three different companion masses. Our simulations help to delineate regimes when conventional analytic drag force approximations are applicable. During and just prior to the first periastron passage of the in-spiral phase, the drag force is reasonably approximated by conventional analytic theory and peaks at values proportional to the companion mass. Good agreement between global and local 3D "wind tunnel" simulations, including similar net drag force and flow pattern, is obtained for comparable regions of parameter space. However, subsequent to the first periastron passage, the drag force is up to an order of magnitude smaller than theoretical predictions, quasi-steady, and depends only weakly on companion mass. The discrepancy is exacerbated for larger companion mass and when the inter-particle separation reduces to the Bondi-Hoyle-Lyttleton accretion radius, creating a turbulent thermalized region. Greater flow symmetry during this phase leads to near balance of opposing gravitational forces in front of and behind the companion, hence a small net drag. The reduced drag force at late times helps explain why companion-core separations necessary for envelope ejection are not reached by the end of limited duration CE simulations.

astro-ph.SR↗