arXiv subjects

Dong Lai

Publications and source records attributed to Dong Lai.

At least 37 records · Page 2Linked to original sources

arXiv2025

Magnetically Driven Retrograde Precession in Misaligned Black Hole Accretion Flows

Observations of accreting black hole (BH) systems, such as microquasars and supermassive black holes, often reveal a precessing jet with changing directions, indicating a misaligned accretion flow relative to the BH spin. The precession is commonly attributed to the Lense-Thirring (LT) effect, which arises from the BH's rotation twisting the surrounding spacetime and accretion flow. In the strongly magnetized regime, which is preferred accretion flow conditions for M~87$^*$ and likely other jet-producing systems, the large-scale magnetic field can significantly influence the flow dynamics. Here, we perform large-scale three-dimensional general relativistic magnetohydrodynamic simulations of tilted accretion onto a rotating BH, and find a never-seen-before new retrograde precession. This precession arises from a magnetic torque on the disk generated by the poloidal magnetic field aligned with the BH's rotation, opposing the LT torque. This finding highlights the unique property of highly magnetized accretion flows around BHs and provides a new interpretation of jet precession observed in many systems.

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arXiv2025

Engulfment of Eccentric Planets by Giant Stars: Hydrodynamics and Light Curves

Recent observations suggest that planetary engulfment by a giant star may produce radiation that resembles subluminous red novae. We present three-dimensional hydrodynamical simulations of the interaction between an eccentric $5 \,M_J$ giant planet and its $1\,M_\odot$ red-giant host star. The planet's pericenter is initially $60\%$ of the stellar radius and is fully engulfed after tens of orbits. Once inside the stellar envelope, the planet generates pressure disturbances that steepen into shocks, ejecting material from the envelope. We use post-processing to calculate the light curves produced by planetary engulfment. We find that the hot stellar ejecta enhances the stellar luminosity by several orders of magnitude. A prolonged hydrogen recombination plateau appears when the ejecta cools to about $10^4\,\rm{K}$. The late-time rapid dimming of the light curve follows dust formation, which obscures the radiation. For planets with lower eccentricity, the orbital decay proceeds more slowly, although the observable properties remain similar.

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arXiv2025

Spin and Obliquity Distributions of Low-mass Planets Shaped by Dynamical Instability

Exoplanetary systems hosting multiple low-mass planets are thought to have experienced dynamical instability, during which planet-planet collisions and mergers occur; these collisions can impart substantial amount of angular momentum to the merger remnants, changing the obliquities of the resulting planets significantly. In this work, we carry out a series of $N$-body experiments to investigate the spin magnitude $(|\vec{S}|)$ and obliquity $(θ_{\rm SL})$ distributions of low-mass exoplanets that have gone through planetary collisions. In our fiducial super-Earth (with $m=3M_{\oplus}$, $R=1.3R_{\oplus}$) and mini-Neptune systems (with $m=9M_{\oplus}$, $R=2.5R_{\oplus}$), the collision products follow a nearly uniform distribution in $\cos{θ_{\rm SL}}$ and the spin-magnitude distribution is approximately linear in $|\vec{S}|$. Parameter studies and theoretical analysis show that increasing planetary radii or masses, or decreasing the initial planet-planet mutual inclinations, tend to polarize the obliquity distribution toward alignment or anti-alignment (i.e., excess probability near $\cos{θ_{\rm SL}}=\pm1$). Experiments with initially two-planet and three-planet systems produce qualitatively similar outcomes, suggesting that the trends in this study may generalize to systems with higher planetary multiplicities.

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arXiv2025

Stellar Obliquity Excitation via Disk Dispersal-Driven Resonances in Binaries

The stellar obliquity of a planetary system is often used to help constrain the system's formation and evolution. One of the mechanisms to reorient the stellar spin involves a secular resonance crossing due to the dissipation of the protoplanetary disk when the system also has an inclined, distant ($\sim 300\;\mathrm{AU}$) binary companion. This mechanism is likely to operate broadly due to the $\sim 50\%$ binary fraction of FGK dwarfs and can play an important role in setting the initial stellar obliquities prior to any dynamical evolution. In this work, we revisit this mechanism analytically for idealized, homologously evolving disk models and show that the resulting stellar obliquities are broadly distributed between $60^\circ$ and $180^\circ$ for most warm and cold planets. We further show that non-homologus disk dissipation, such as the development of a photoevaporatively-opened gap at $\sim 2\;\mathrm{AU}$, can help maintain orbital alignment of warm planets, in agreement with observations. Our results represent the proper primordial obliquities for planetary systems with distant binary companions. They also represent the obliquities of stars with no present-day binary companions if these companions are dynamically unbound during the birth cluster phase of evolution, a process that occurs on a comparable timescale as the disk-driven obliquity excitation.

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arXiv2025

Physics of Strong Magnetism with eXTP

In this paper we present the science potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission, in its new configuration, for studies of strongly magnetized compact objects. We discuss the scientific potential of eXTP for quantum electrodynamic (QED) studies, especially leveraging on the recent observations made with the NASA IXPE mission. Given eXTP's unique combination of timing, spectroscopy, and polarimetry, we focus on the perspectives for physics and astrophysics studies of strongly magnetized compact objects, such as magnetars and accreting X-ray pulsars. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to launch in early 2030.

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arXiv2025

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

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arXiv2025

Binary Stars Approaching Supermassive Black Holes: Hydrodynamics of Stellar Collisions, Mass Fallback and Partial TDEs

When binaries are injected into low-angular-momentum orbits around a central supermassive black hole (SMBH), various outcomes can occur, including binary tidal breakup, double stellar disruptions and stellar collision. We use hydrodynamical simulations to study stellar collisions triggered by binary-SMBH encounters, examining both head-on and grazing collisions in deep ($β_b=5$) and gentle ($β_b=0.6$) encounters, where $β_b$ is the ratio of the binary tidal disruption radius to the binary pericenter distance to the SMBH. Head-on collisions consistently result in appreciable mass loss ($\sim 5\%$) and a single merger remnant. Grazing collisions have varied outcomes. In gentle encounters, multiple collisions typically form a single remnant with minimal mass loss ($\lesssim 1 \%$). For deep encounters, the result depends on the specific collision parameters and stellar structure: $γ=5/3$ polytropic stars in our simulation produced two disturbed remnants, while solar-type stars (modeled with MESA) in our deep-grazing run formed a single merger remnant in a low-velocity collision. All merger remnants feature extended envelopes, making them susceptible to partial tidal disruptions when they return to the SMBH. The morphology and orbital energy distribution of collision-induced debris differ significantly from those of tidal disruption event (TDE) debris of single stars. Approximately half of the collision-generated debris falls back onto the SMBH, exhibiting a distinct time evolution of the fallback rate. We suggest that such mass loss and fallback can generate electromagnetic flares that mimic weak TDEs.

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arXiv2025

Equation of State of Decompressed Quark Matter, and Observational Signatures of Quark-Star Mergers

Quark stars are challenging to confirm or exclude observationally because they can have similar masses and radii as neutron stars. By performing the first calculation of the non-equilibrium equation of state of decompressed quark matter at finite temperature, we determine the properties of the ejecta from binary quark-star or quark star-black hole mergers. We account for all relevant physical processes during the ejecta evolution, including quark nugget evaporation and cooling, and weak interactions. We find that these merger ejecta can differ significantly from those in neutron star mergers, depending on the binding energy of quark matter. For relatively high binding energies, quark star mergers are unlikely to produce r-process elements and kilonova signals. We propose that future observations of binary mergers and kilonovae could impose stringent constraints on the binding energy of quark matter and the existence of quark stars.

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arXiv2025

A binary origin of ultra-long period radio pulsars

We propose a possible binary evolution model for the formation of ultra-long period pulsars (ULPPs). The model involves two key stages: first, a neutron star (NS) in wide binaries undergoes an effective spin-down phase through wind-fed accretion from its massive stellar companion; second, the supernova explosion of the companion leads to the disruption of the binary system, and produces two isolated compact stars. One of the them is the first-born, slowly rotating NSs, and our binary and spin evolution calculations show that the spin periods range from $\lesssim 0.1$ s to $\gtrsim 10^8$ s. This offers a possible formation channel for some of the long-period radio transients. We estimate that the formation rate of such systems in the Milky Way is approximately about $10^{-6}$ $\rm yr^{-1}$.

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arXiv2025

Resonance Capture of a Test Particle by an Eccentric Planet in the Presence of Externally-Driven Apsidal Precession

Planets undergoing convergent migration can be captured into mean-motion resonance (MMR), in which the planets' periods are related by integer ratios. The dynamics of MMR are typically considered in isolation, including only the forces between the planets and with the central star. However, the planets are often subjected to external forces that induce apsidal precessions, which may split the MMR into two sub-resonances and give rise to chaotic motion due to resonance overlap. In this study, we investigate how such externally induced differential apsidal precession affects capture into first-order $j:j+1$ MMRs. We study the restricted three-body problem for a test particle outside of an eccentric planet, with the planet undergoing outward migration. We find that capture can be sensitive to the differential apsidal precession, even when the precession rate is much smaller than the resonance overlap criterion. We identify two critical precession frequencies -- related to resonance overlap and secular apsidal resonance -- around which capture is disrupted even for very small planet eccentricity. Our results may help clarify the capture process of resonant trans-Neptunian objects by outward-migrating Neptune in early Solar System.

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arXiv2024

Binary Stars Approaching Supermassive Black Holes: Tidal Break-up, Double Stellar Disruptions and Stellar Collision

In galactic centers, stars and binaries can be injected into low-angular-momentum orbits, resulting in close encounters with the central supermassive black hole (SMBH). Previous works have shown that under different conditions, such close encounters can lead to the break-up of the binary, disruptions of both stars and collision between the stars. We use 3-body scattering experiments to characterize these different outcomes for a range of system parameters, such as $β_b$, the ratio of binary tidal radius to pericenter distance $r_p$ to the SMBH and the compactness of the binary. We focus on stellar collisions, which occur for a range of $β_b$'s, with a few to 10's percent probabilities (depending on the compactness of the binary). In gentle encounters ($β_b\lesssim 1$), stellar collisions occur after the pericenter passage, and the merger remnants are typically ejected from the SMBH at a small velocity. In deep encounters ($β_b\gtrsim 1$), collisions occur near the pericenter, with the impact velocity a few times the escape velocity of the star, and the merger remnants are typically bound to the SMBH. We suggest that stellar collisions induced by binary-SMBH encounters may produce exotic stars in galactic centers, trigger accretion flares onto the SMBH due to the mass loss, and result in bound merger remnants causing repeated partial TDEs.

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arXiv2024

Migration of Accreting Planets and Black Holes in Disks

Nascent planets are thought to lose angular momentum (AM) to the gaseous protoplanetary disk via gravitational interactions, leading to inward migration. A similar migration process also applies to stellar-mass black holes (BHs) embedded in the disks of active galactic nuclei. However, AM exchange via accretion onto the planet/BH may strongly influence the migration torque. In this study, we perform 2D global hydrodynamic simulations of an accreting planet/BH embedded in a disk, where AM exchange between the planet/BH and disk via gravity, accretion, pressure, and viscosity are considered. When accretion is turned off, we recover the linear estimate for Type I migration torque. However, for the two planet masses we investigated with our accreting simulations, we find outward migration due to the positive AM deposited onto the accreting body by the disk gas. Our simulations achieve the global steady state for the transport of mass and AM: The mass and AM fluxes are constant across the disk except for jumps ($Δ\dot M$ and $Δ\dot J$) at the planet's location, and the jumps match the accretion rate and torque on the planet. Our findings suggest that caution is needed when applying the standard results of disk migration to accreting planets and BHs.

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arXiv2024

Metal pollution in Sun-like stars from destruction of ultra-short-period planets

Chemical evidence indicates that an appreciable fraction of Sun-like stars have engulfed rocky planets during their main-sequence lifetimes. We investigate whether the tidal evolution and destruction of ultra-short-period planets (USPs) can explain this phenomenon. We develop a simple parameterized model for the formation and engulfment of USPs in a population of MS stars. With this model, it is possible to reproduce both the observed occurrence rate of USPs and the frequency of planet-engulfing Sun-like stars for a reasonable range of USP formation rates and tidal decay lifetimes. Our results support a theory of USP formation through gradual inward migration over many Gyr and suggest that engulfment occurs $\sim 0.1$-$1 \, {\rm Gyr}$ after formation. This lifetime is set by tidal dissipation in the USP itself instead of the host star, due to the perturbing influence of external companions. If USP engulfment is the main source of pollution among Sun-like stars, we predict a correlation between pollution and compact multi-planet systems; some $5$-$10\%$ of polluted stars should have a transiting planet of mass $\gtrsim 5 M_{\oplus}$ and period $\sim 4$-$12$ days.

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arXiv2024

Progress report on the BEBOP search for circumbinary planets with radial velocities

The BEBOP (Binaries Escorted By Orbiting Planets) survey is a search for circumbinary planets using the radial velocity spectrographs HARPS and SOPHIE, currently focusing on single-lined binaries with a mass ratio $<0.3$. Circumbinary systems are an important testing ground for planet formation theories as the dynamically complex influence of the binary makes planet formation and survival more difficult. Here we present the results of the survey so far including: confirmed planets such as BEBOP-1c the first circumbinary planet detected in radial velocity; the status of our observations; and preliminary occurrence rates. We compare the early results of the radial velocity survey to the population of circumbinary planets discovered in transit, and suggest that there may be a population of inflated planets close to the inner binary which are detectable in transit but more difficult in radial velocity. Using time-lag tidal theory, we show that this inflation is unlikely caused by tides.

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arXiv2024

Free-Floating Planets, Survivor Planets, Captured Planets and Binary Planets from Stellar Flybys

In star clusters, close stellar encounters can strongly impact the architecture of a planetary system or even destroy it. We present a systematic study on the effects of stellar flybys on two-planet systems. When such a system experiences flybys, one or both planets can be ejected, forming free-floating planets (FFPs), captured planets (CPs) around the flyby star, and free-floating binary planets (BPs); the remaining single-surviving-planets (SSPs) can have their orbital radii and eccentricities greatly changed. Through numerical experiments, we calculate the formation fractions (or branching ratios) of FFPs, SSPs, CPs and BPs as a function of the pericenter distance of the flyby, and use them to derive analytical expressions for the formation rates of FFPs, SSPs, CPs and BPs in general cluster environments. We find that the production rates of FFPs and SSPs are similar (for initial planet semi-major axis ratio $a_1/a_2=0.6-0.8$), while the rate for CPs is a few times smaller. The formation fraction of BPs depends strongly on $a_1/a_2$ and on the planet masses. For Jupiter-mass planets, the formation fraction of BPs is always less than $1\%$ (for $a_1/a_2=0.8$) and typically much smaller ($\lesssim 0.2\%$ for $a_1/a_2\lesssim 0.7$). The fraction remains less than $1\%$ when considering $4M_{\rm J}$ planets. Overall, when averaging over all flybys, the production rate of BPs is less than $0.1\%$ of that for FFPs. We also derive the velocity distribution of FFPs produced by stellar flybys, and the orbital parameter distributions of SSPs, CPs and BPs. These results can be used in future studies of exotic planets (including FFPs) and planetary systems.

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arXiv2024

Extreme resonant eccentricity excitation of stars around merging black-hole binary

We study the dynamics of a star orbiting a merging black-hole binary (BHB) in a coplanar triple configuration. During the BHB's orbital decay, the system can be driven across the apsidal precession resonance, where the apsidal precession rate of the stellar orbit matches that of the inner BHB. As a result, the system gets captured into a state of resonance advection until the merger of the BHB, leading to an extreme eccentricity growth of the stellar orbit. This resonance advection occurs when the inner binary has a non-zero eccentricity and unequal masses. The resonant driving of the stellar eccentricity can significantly alter the hardening rate of the inner BHB, and produce observational signatures to uncover the presence of nearby merging or merged BHBs.

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arXiv2024

Formation of Merging Stellar-Mass Black Hole Binaries by Gravitational Wave Emission in Active Galactic Nucleus Disks

Many stellar-mass Black Holes (sBHs) are expected to orbit supermassive black holes at galactic centers. For galaxies with Active Galactic Nuclei (AGN), it is likely that the sBHs reside in a disk. We study the formation of sBH binaries via gravitational wave emission in such disks. We examine analytically the dynamics of two sBHs orbiting a supermassive black hole, estimate the capture cross section, and derive the eccentricity distribution of bound binaries at different frequency bands. We find that the majority of the merging sBH binaries, assembled in this manner, can be measured as highly eccentric, detectable in the LIGO-Virgo-KAGRA (LVK) band from their formation, with $(1-e)\ll1$, through their circularization and up to their merger; the remaining binaries circularize to small eccentricities ($e\lesssim0.3$) before entering the LVK band. More eccentric mergers would be observed for sBHs with higher random velocities, closer to the supermassive black hole, or at lower observing frequency bands, as planned in future gravitational wave detectors such as the Einstein Telescope and LISA.

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arXiv2024

Mass Ejection Driven by Sudden Energy Deposition in Stellar Envelopes

A number of stellar astrophysical phenomena, such as tidal novae and planetary engulfment, involve sudden injection of sub-binding energy in a thin layer within the star, leading to mass ejection of the stellar envelope. We use a 1D hydrodynamical model to survey the stellar response and mass loss for various amounts ($E_{\mathrm{dep}}$) and locations of the energy deposition. We find that the total mass ejection has a nontrivial dependence on $E_{\mathrm{dep}}$ due to the varying strengths of mass ejection events, which are associated with density/pressure waves breaking out from the stellar surface. The rapid occurrence of multiple breakouts may present a unique observational signature for sudden envelope heating events in stars.

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