arXiv subjects

Dong Lai

Publications and source records attributed to Dong Lai.

At least 19 recordsLinked to original sources

arXiv2026

Harmonic brightness oscillations point to magnetar giant flares in rare short gamma-ray bursts

Kilohertz quasi-periodic oscillations (QPOs) have been observed in a few rare gamma-ray bursts (GRBs), but their physical origin remains uncertain. We reanalyze the two short GRBs with reported kilohertz QPOs, GRB 910711 and GRB 200415A, and find evidence for possible harmonic-like organization around a characteristic frequency scale near 360 hertz. The similar harmonic-like pattern in GRB 910711 and in the confirmed magnetar giant flare (MGF) event GRB 200415A, including features extending above 4 kilohertz, points to timing phenomenology on frequency scales relevant to magnetar oscillation modes. The temporal and spectral similarities between the two events and the difficulty of explaining these features in merger-driven scenarios favor an MGF-like interpretation of GRB 910711. This timing phenomenology may therefore provide an additional observational handle, complementary to spectral, temporal, and localization information, for distinguishing rare MGF-like transients from merger-driven GRBs.

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arXiv2026

Neutron Star-Main-Sequence Star Collision: Formation and Long-Term Evolution of Thorne-Żytkow Objects

Thorne-$\dot{\mathrm{Z}}$ytkow objects (T$\dot{\mathrm{Z}}$Os) are stars containing a neutron-star (NS) or black-hole (BH) core embedded in a stellar envelope and powered primarily by mass accretion onto the core. Classical evolution models of T$\dot{\mathrm{Z}}$Os begin with an assumed equilibrium configuration and predict chemical anomalies in the stellar envelope resulting from nuclear burning in the vicinity of the NS core. Here we connect dynamical formation to long-term evolution by combining a three-dimensional simulation of a post-supernova collision between a $1.4M_\odot$ NS and a $10M_\odot$ main-sequence star with a subsequent stellar evolution calculation using MESA. The collision successfully produces a bound, quasi-spherical T$\dot{\mathrm{Z}}$O with the NS settled at the center of the stellar envelope. In contrast to the classical picture, the collision heats a substantial fraction of the star and leads to widespread nuclear burning during the subsequent thermal-relaxation phase. As the envelope becomes convective, material processed by the carbon-nitrogen cycle is transported to the surface, leaving a potentially observable chemical imprint of formation that persists after the star attains the standard accretion-supported T$\dot{\mathrm{Z}}$O structure. At the end of our simulation, the stellar luminosity greatly exceeds Eddington in the outer envelope, suggesting that significant mass ejection will occur, leaving behind an isolated NS or BH.

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arXiv2026

Simulations of a Conducting Sphere Moving through Magnetized Plasma II: Effect of Internal Magnetic Fields and Enhancement of Alfvénic Drag

Magnetic interaction between astrophysical objects can play an important role and produce electromagnetic radiation in various systems, ranging from planet-moon and star-planet systems to binary compact objects. We extend our previous numerical study of an ideal magnetized plasma flowing past a conducting sphere to the case where the sphere is magnetized. Such a flow generates magnetic disturbances and produces a drag force on the sphere. We examine the drag coefficient as a function of the strength and orientation of the conductor's internal magnetic field relative to the background magnetic field. We find that a strong internal magnetic field that is aligned with the background field significantly enhances the drag coefficient compared to the unmagnetized case; this enhancement is related to the increased magnetic flux that connects the conductor and the open, external field region. We also explore how the drag coefficient changes as the flow transitions from the sub-Alfvénic to the super-Alfvénic regime. Our results suggest that magnetic interaction may play a larger role in orbital evolution of some binary systems than previously expected.

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arXiv2026

A Star's Death by a Thousand Cuts: The Runaway Periodic Eruptions of AT2023uqm

Stars on bound orbits around a supermassive black hole may undergo repeated partial tidal disruption events (rpTDEs), producing periodic flares. While several candidates have been suggested, definitive confirmation of these events remains elusive. We report the discovery of AT2023uqm, a nuclear transient that has exhibited at least five periodic optical flares, making it only the second confirmed case of periodicity after ASASSN-14ko. Uniquely, the flares from AT2023uqm show a nearly exponential increase in energy--a "runaway" phenomenon signaling the star's progressive destruction. This behavior is consistent with rpTDEs of low-mass, main-sequence stars or evolved giant stars. Multiwavelength observations and spectroscopic analysis of the two most recent flares reinforce its interpretation as an rpTDE. Intriguingly, each flare displays a similar double-peaked structure, potentially originating from a double-peaked mass fallback rate or two discrete collisions per orbit. The extreme ratio of peak separation to orbital period draws attention to the possibility of a giant star being disrupted, which could be distinguished from a low-mass main-sequence star by its future mass-loss evolution. Our analysis demonstrates the power of rpTDEs to probe the properties of disrupted stars and the physical processes of tidal disruption, though it is currently limited by our knowledge of these events. AT2023uqm emerges as the most compelling rpTDE thus far, serving as a crucial framework for modeling and understanding these phenomena.

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arXiv2026

Can isolated binaries form unequal-mass binary black-hole mergers with a high-spin primary black hole?

GWTC-5 has revealed a subpopulation of merging binary black holes (BBHs) with a high-spin primary black hole (BH) and possibly unequal BH masses. GW241110 additionally exhibits a large spin-orbit misalignment, which suggests a hierarchical-merger origin. However, other formation scenarios are possible or even likely, especially for events without constraints on spin-orbit misalignment. As an alternative to hierarchical mergers, we investigate whether binary evolution can produce unequal-mass BBH mergers with a high-spin primary BH. Rather than performing comprehensive population-synthesis calculations, we examine the evolutionary pathways of forming merging BBHs and assess their uncertainties. We identify two possible pathways for producing unequal-mass BBHs with a high-spin primary. In initially wide binaries, mass-ratio reversal can make the tidally spun-up second-born BH both more massive and more rapidly rotating than the first-born BH; alternatively, in an initially close, unequal-mass binary, the primary star may evolve chemically homogeneously, while the secondary star evolves normally, producing a high-spin first-born BH that is more massive than its companion. Generally, large spin-orbit misalignment can be produced by large natal kicks or tertiary-induced nodal precession and/or Zeipel-Lidov-Kozai oscillations. We conclude that hierarchical mergers are not uniquely required to produce unequal-mass BBHs with a high-spin primary BH, although each isolated-binary pathway faces important theoretical constraints. Future detections of more merger events with primary BH spins around 0.7 would discriminate between binary evolution and hierarchical merger origin.

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arXiv2026

Transiting Planetary Systems with Distant Giant Companions Remain Moderately Coplanar

The mutual inclination between inner planets and distant giant companions provides an important probe of planetary system formation and dynamical evolution, yet direct measurements of this quantity remain scarce. We combine radial velocity (RV) observations with Hipparcos--Gaia astrometry to constrain the orbital architecture of 19 planetary systems hosting at least one transiting inner planet and one outer giant companion. Using a hierarchical Bayesian framework, we infer the population-level distribution of the minimum mutual inclination, $ΔI$, between the inner and outer planetary orbits. We find that the $ΔI$ distribution is well described by a Rayleigh model with a scale parameter of $σ= 15.8^{+2.8}_{-2.6}°$, which is strongly preferred over an isotropic distribution ($Δ\log Z=5.45$). This result suggests that transiting systems hosting distant giant companions remain substantially more coplanar than expected for an isotropic population, consistent with the partial preservation of primordial coplanarity. A division by the mass ($0.3\,M_{\rm Jup}$) of the inner transiting planet suggests that giant-inner-planet systems may have lower $ΔI$ than small-inner-planet systems, with $P(σ_{\rm giant}<σ_{\rm small})=0.952$; however, the current data do not significantly favor a model allowing different $σ$ values for the two subsamples over one in which they share a common $σ$. Future Gaia DR4 astrometry will enable more robust population-level studies of the three-dimensional architectures of systems with distant giant companions.

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arXiv2026

Efficiency of Tidal Dissipation in Convective Flow Under Rapid Tidal Forcing

For close binaries and star-planet systems, tidal interactions mediate the energy transfer between the orbital motion and the internal flows of the bodies involved, thus playing a central role in their evolution. For equilibrium tides, the associated energy transfer is commonly modeled through an effective viscosity acting on the tidal flow. However, the scaling of viscous dissipation efficiency with tidal frequency $ω_\text{T}$ remains debated, particularly when $ω_\text{T}$ greatly exceeds the convective eddy turnover frequency $ω_\text{c}$. Previous numerical studies have addressed this issue by subjecting a turbulent convective flow to an oscillating background shear mimicking equilibrium tides. In this work, we adopt a novel three-layered convective box -- designed to represent a stellar convection zone sandwiched between two stable layers -- driven by an external periodic forcing. We quantify tidal dissipation efficiency by the forcing power on the flow in steady state. Our results yield a shallower scaling of tidal power per unit mass with $ω_\text{T}$ than reported in earlier shear-flow simulations. This scaling is consistent with the prediction by \cite{Terquem2021}, suggesting that the effective turbulent viscosity depends only weakly on $ω_\text{T}$, although our simulations are restricted to $ω_\text{T}\lesssim 10ω_\text{c}$. Moreover, we find no evidence of inverse energy transfer (or ``negative viscosity''), a phenomenon observed in some prior shear-flow simulations. We further investigate the influence of rotation within the same local framework. Slow rotation ($Ω\lesssim ω_\text{T}$) tends to enhance the tidal power, whereas fast rotation ($Ω\gtrsimω_\text{T}$) significantly suppresses it. We discuss the limitations of our approach and the broader implications of our findings.

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arXiv2026

Hierarchical Black Hole Mergers in Nuclear Star Clusters: A Combined Dynamical-Secular Channel for GW231123-like Events

The recent binary black hole (BH) merger GW231123, with both components likely in the high-mass gap and with high spins, challenges standard BH binary formation models. It is usually thought that the BHs are of second (or higher) generation (2G), resulting from the mergers of smaller BHs. But the physical processes that produce the merging 2G BH binaries are unclear and highly unconstrained. We show that such 2G mergers can be naturally produced in the nuclear star cluster of Milky Way-like galaxy. The dominant channel combines a sequence of binary-single interactions with secular evolution driven by the central supermassive BH. Our model produces a merger rate consistent with GW231123 and further predicts an abundant population of 2G BH-star (or low-mass BH) binaries; these binaries may observationally manifest as micro tidal disruption events or low-frequency gravitational-wave (GW) sources. Detecting these binaries would provide crucial insights into the dynamical pathways of hierarchical BH assembly.

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arXiv2026

TESS Photometry and Radial Velocity Analysis of the sub-Neptune Exoplanet π Mensae c and the Wider π Mensae Planetary System

Exoplanet characterization relies on precise measurements of planetary orbital and physical parameters. This is particularly important for planetary dynamics and atmospheric evolution, as orbital parameters help constrain system evolution, resolve ambiguities, and gauge atmospheric retention. The first exoplanet discovered by the Transiting Exoplanet Survey Satellite (TESS), $π$ Men c, is a warm sub-Neptune orbiting a bright Sun-like star in a system containing (at least) one other planet with a wildly different period and size. Lying near the 1.5-2.0 $R_{\oplus}$ radius gap, $π$ Men c is expected to have lost its primordial hydrogen and helium, but kept heavier compounds like H$_2$O and CO$_2$. The $π$ Men system is well observed with decades of radial velocity measurements, and TESS has continued to observe $π$ Men c, yielding six years and 21 sectors of photometry. We present a comprehensive analysis of these TESS data and 22 years of radial velocity measurements to provide updated orbital ephemerides for $π$ Men b, c, and the proposed third planet, $π$ Men d. Our newly derived $π$ Men c period error margins are an order of magnitude improved from previous estimates, and we estimate the mass range of $π$ Men d to be 13.4 $\leq$ M$_d$ $<$ 20 M$_{\oplus}$. We find that $π$ Men c is a uniquely interesting target for future transmission spectroscopy studies with JWST, and that existing radial velocity data are consistent with the existence of a third planet.

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arXiv2026

Impact of Resonant Compton Scattering on Magnetar X-Ray Polarization with QED Vacuum Resonance

Recent obeservations have revealed significant soft X-ray polarizations from several quiescent magnetars, including the intriguing $90^°$ polarization angle (PA) swing as a function of photon energy for some sources. We present a general semi-analytical framework for calculating energy-dependent soft X-ray polarization signatures from magnetars, consistently incorporating both QED vacuum resonance in the atmosphere and resonant Compton scattering (RCS) in the magnetosphere. Starting from the polarized radiative transfer equation for RCS and treating vacuum-resonance-induced mode conversion as an input, we employ a first-order approximation in RCS optical depth to evaluate the effect of different magnetospheric plasma density (which depends on magnetic twist), drift velocity and temperature, and viewing geometry on the observed radiation. Our analysis reveals that magnetic twist and plasma drift velocity are the critical parameters controlling the impact of RCS on both the absolute polarization degree and its variation across the soft X-ray spectrum. We find that sufficiently strong RCS can wash out the PA swing caused by vacuum resonance. Furthermore, in addition to the QED vacuum resonance effect, significant relativistic signatures arising from plasma drift velocity ($β_0 \gtrsim 0.5$) may introduce an extra $90^\circ$ PA swing in the spectrum. Our calculation framework, based on single-scattering approximation, bypasses the need for complex, multi-dimensional Monte Carlo simulations, providing an analytical pathway for modeling full-surface emission and rotational-phase-resolved radiation from magnetic neutron stars, in support of current and future X-ray polarization missions.

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arXiv2026

Bridging Roche Lobe Overflow and micro-TDEs: The Runaway Evolution of Eccentric Mass Transfer in Star-Black Hole Binaries

Binary systems may undergo mass transfer while maintaining significant orbital eccentricities. Stellar-mass black holes (sBHs) can strip stars on eccentric orbits and produce micro-tidal disruption events (micro-TDEs). While previous hydrodynamical studies have focused on compact systems on the verge of disruption, the transition between self-regulated eccentric mass transfer and runaway disruption remains poorly understood. We present SPH simulations of a Sun-like star interacting with a $10\,M_\odot$ sBH across a range of initial eccentricities ($e_0=0.30$--$0.70$) and pericenter distances ($b_0=3.33$--$3.57$ in units of the tidal radius), tracking the systems for tens to over 100 orbital periods. Our results reveal that these binaries can evolve along two distinct pathways, dictated by the competition between mass-transfer-driven stellar expansion and orbital widening: (i) Runaway disruption ($b_0\lesssim 3.45$), in which mass loss at pericenter drives adiabatic expansion of the stellar envelope, leading to unstable Roche-lobe overflow and runaway disruption of the star. The stripped debris forms a thick accretion flow with hyper-Eddington accretion rates onto the sBH, potentially powering fast X-ray/UV or blue/optical transients. (ii) Stable mass transfer ($b_0\gtrsim 3.57$), in which the binary settles into a long-lived, stable mass-transfer phase lasting up to 150 orbits (the limit of our simulation), regulated by orbital expansion from pericenter mass loss. These eccentric mass-transfer events could manifest observationally as repeating, quasi-periodic flares.

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arXiv2026

Planetary Desert around Compact Binaries: Dynamical Instability Triggered by Resonance-Induced Eccentricity Excitation

Compact binaries with orbital periods shorter than about 7 days show an absence of transiting planets, a feature known as the ``circumbinary planet desert". The physical mechanism behind this desert remains unclear. We investigate its origin by simulating the long-term dynamics of multi-planet circumbinary systems with evolving inner binaries. Our simulations are based on the single-averaged secular equations that average only over the binary orbital period and fully incorporate planet-planet interactions. When an eccentric binary decays via tides, an outer planet can be captured into resonance advection in eccentricity, a state in which its apsidal precession locks with that of the binary, driving extreme eccentricity growth. While such growth can occur in a binary-single planet system, the parameter space is limited and may not necessarily induce instability. In a multi-planet system, however, the excited orbit inevitably crosses those of its neighbors, which triggers violent planet-planet scatterings and produces collisions or ejections. Crucially, these mutual gravitational interactions amplify the ``localized" instability of a single planet into a system-wide chain reaction, drastically reshaping the orbital architecture and potentially clearing out the inner regions of planetary systems. Our results suggest that the resonance-induced instability provides a natural explanation for the observed circumbinary planet desert.

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arXiv2026

Impact of Cold Jupiter Scattering on the Mean-Motion Resonance of Inner Small Planets

A key feature of close-in, multiple super-Earth (SE) systems is the tendency for adjacent planet pairs to lie just wide of low-order mean-motion resonances (MMR). This period ratio distribution has motivated numerous theoretical studies, particularly those invoking post-disk processes that perturb initially resonant architectures. We investigate whether orbital instability among cold Jupiters (CJs) can perturb inner SE systems initially in MMR. We show that a single pericenter passage of a highly eccentric CJ can disrupt inner resonances once a critical perturbation strength is exceeded, increasing the libration amplitude of the resonant angles. However, N-body simulations show that deep penetration of CJs into the inner system is uncommon, with $\lesssim 10-20\%$ of cases reaching $\lesssim 10\%$ of the initial semi-major axis of the innermost CJ. Motivated by these results, we use secular perturbation theory to quantify the impact of time-dependent forcing from scattering CJs on the eccentricity and resonant-angle evolution of inner SEs. We find that for typical systems (e.g., with SEs at $\sim 0.1$ au and CJs at a few au), such forcing can efficiently disrupt resonances, driving resonance-angle circulation in most systems ($\gtrsim 60\%$ for 2:1 and $\sim 85\%$ for 3:2 configurations). Thus, even when the "final" CJ has little effect on the "current" SEs, its earlier scattering history can leave significant imprints on the system architecture. This mechanism, and similar ones involving more abundant cold Neptunes, provide a natural source of dynamical "kicks" and offer a pathway for producing the observed trough-peak structure in the period ratio distribution of Kepler multi-planet systems.

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arXiv2026

A Formation Crisis of Repeating Partial Tidal Disruption Events

A number of candidate repeating partial tidal disruption events (rpTDEs) have been reported in recent years. If these events are confirmed, the high fraction of observed rpTDEs among all tidal disruption events (TDEs) is in tension with prediction of the loss cone channel. We further point out an inequality $M_\bullet \lesssim 4\times 10^6 M_\odot (T_{\rm obt}/10\ {\rm yr})^{4/9}$ that must be satisfied for rpTDEs of solar type stars in the loss cone channel, where $M_\bullet$ is the central supermassive black hole (SMBH) mass and $T_{\rm obt}$ is the orbital period of the star. However the majority of reported rpTDE candidates potentially violate this inequality, indicating an alternative formation channel. In the commonly invoked Hills mechanism, the captured stars produced by tidal disruption of near-contact binaries can evade this inequality and may be the dominant source of rpTDEs. If the same process operates in the Galactic Center, there should exist a population of hypervelocity stars (HVSs) ejected with velocities as high as $3.6\times 10^3 (M_\bullet/10^6 M_\odot)^{1/6}\ {\rm km\ s}^{-1}$, which however have not been detected. A complete search for HVSs in the Milky Way will be critical for testing this prediction.

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arXiv2026

Understanding the UV/Optical Variability of AGNs through Quasi-Periodic Large-scale Magnetic Dynamos

The physical origin of the recently identified slow-moving temperature fluctuations in accretion disks around super-massive black holes (SMBHs) cannot be accounted for by reverberation models. In this work, we propose that large-scale dynamos (LSDs) operating in accretion disks could generate quasi-periodic perturbations in the turbulence viscosity, thereby producing outward-going temperature fluctuations with speeds comparable to those inferred from observations. Furthermore, we find that the UV/optical fluxes of our model are compatible with a damped-random-walk (DRW) process, with a damping time $τ_\text{d}$ consistent with observations. The scaling relation between $τ_\text{d}$ and the rest-frame wavelength $λ$ has a bended shape, $τ_\text{d}\proptoλ$ at short wavelengths and transitioning to a plateau at long wavelengths. At $λ=2500\textÅ$, the damping time roughly follows $\propto M_\text{BH}^{1/2}$ when $M_\text{BH}\gtrsim 10^6M_\odot$, consistent with observational constraints, though it tends to be underestimated for lower SMBH masses. Including additional refinements, such as the dependence of dynamo properties on $M_\text{BH}$ and AGN luminosity, and accounting for X-ray reprocessing, would further enhance the accuracy of the model. In addition, we show that generic disk models with spatially uncorrelated fluctuations cannot explain the observed DRW damping times; spatially correlated fluctuations, such as those discussed in this paper, may be an essential ingredient.

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arXiv2026

A universal brown dwarf desert formed between planets and stars

Giant planets and brown dwarfs play a crucial role in star and planet formation, as they are situated at the boundary between planets and stars with uncertain formation mechanisms. Previous observational searches for the formation boundary were hampered by the lack of large unified samples of wide-orbit giant planets and substellar companions. A combined analysis of radial velocity and astrometry mitigates this problem and has significantly enlarged the sample. Here we present a rigorous statistical analysis of the sample of 55 giant planets, brown dwarfs and low-mass stellar companions orbiting FGK stars. We quantitatively analyze the occurrence rates of brown dwarfs and identify a distinct brown dwarf desert at approximately $30\,M_\mathrm{J}$, with no evidence of disappearance up to 20 au. Unlike previous studies that predicted a declining planet occurrence rate beyond the water-ice line, we identify a new population of giant planets and low-mass brown dwarfs in this region. The metallicity and eccentricity trends in our sample suggest that these are the consequences of two different formation scenarios. Our combined population synthesis model successfully accounts for the observed brown dwarf desert, supporting the dual formation hypothesis.

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arXiv2026

Velocities of Free Floaters in a Sea of Stars

We investigate the velocity evolution of free-floating planets and interstellar objects (``free floaters'') through gravitational scatterings by field stars (with the stellar mass $m$ much larger than the mass of the floater, $m_p$). We show that the equilibrium velocity -- where dynamical friction balances stochastic acceleration -- is given by $σ\sqrt{2\ln(m/m_p)}$ (where $σ$ is the velocity disperson of the field stars), diverging from the standard energy equipartition scaling. While the timescale to reach this equilibrium is prohibitively long, we find that slow floaters ($v \lesssim σ$) undergo mass-independent acceleration, doubling their velocities within a few relaxation times. Consequently, free floaters initially following the Maxwellian distribution of their parent stars develop distinctly non-Maxwellian velocity distributions on a relaxation timescale. Since the relaxation time of the Galactic disk is longer than the age, our results suggest that the kinematics of low-mass free floaters in the disk may preserve signatures of their parent stars and ejection history.

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arXiv2026

Free-floating Planets Produced by Planet-Planet Scatterings: Ejection Velocity and Survival Rate of Their Moons

The discovery of numerous free-floating planets (FFPs) has intensified interest in their origins and dynamical histories. A leading formation mechanism is planet-planet scatterings in unstable multi-planetary systems, which can naturally lead to planetary ejections. If these planets originally host moons, it remains an open question whether such satellites can remain gravitationally bound to FFPs after ejection. In this work, we investigate both the ejection velocity of FFPs produced by planet-planet scatterings and the survival rate of their potential moons; we estimate the latter by determining the statistics of the minimum planet-planet distance prior to planet ejection, and comparing it to the initial orbital radius of the moon relative to its host planet. Using the circular restricted three-body framework, we derive an analytical boundary for the ejection velocity based on Jacobi energy conservation, which agrees with the results of integrations. We also identify a minimum planetary mass required for successful ejection. For two-planet systems with finite planetary masses, we use simulations and analytical arguments to determine how the ejection velocity scales with the planetary mass and initial semi-major axis. We contrast our results to the ejection of planets around binaries in unstable orbits. Extending our analysis to three-planet systems yields similar results, reinforcing the robustness of our conclusions. These findings offer insights into the property of FFPs and inform future efforts to search for exomoons around them.

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