arXiv subjects

Beibei Liu

Publications and source records attributed to Beibei Liu.

At least 19 recordsLinked to original sources

arXiv2026

Disk dispersal freezes overstable resonant librations

Context. Convergent migration in a gaseous protoplanetary disk can capture a planet pair into mean-motion resonance. Eccentricity damping can subsequently make the resonant libration overstable and drive the pair out of resonance. Most studies of this process, however, assume a static disk. Aims. We examine how the decay of disk torques during dispersal changes this outcome and whether it can freeze an overstable libration before the pair escapes. Methods. We describe disk dispersal by allowing the migration and eccentricity-damping timescales to increase exponentially on a local timescale $\tau_{\rm d}$. Integrating the time-dependent growth rate predicts $\tau_{\rm d,crit}\propto\tau_{e,0}$. We test this scaling with direct $N$-body integrations and relate $\tau_{\rm d}$ to the time taken by a photoevaporative cavity edge to cross the local torque-producing region. Results. The simulations recover a linear boundary, $\tau_{\rm d,crit}\simeq S\tau_{e,0}$, with $S\simeq3$ over the explored parameter range. In a fiducial minimum-mass solar nebula, faster propagation of the cavity edge shortens the local dispersal time. The ratio $\tau_{\rm d}/\tau_{\rm d,crit}$ also decreases with orbital radius, so both effects favour resonant survival. Conclusions. When local disk dispersal is sufficiently rapid, the libration amplitude can freeze and the planet pair can remain in resonance instead of escaping through overstability. Late disk evolution can therefore alter the outcome of resonant overstability.

astro-ph.EP↗

arXiv2026

Actions on CAT(-1) spaces with critical exponent less than 1

We show that for a discrete isometry subgroup acting on a proper CAT(-1) space X, if the critical exponent is less than $1$, then the critical exponent equals the Hausdorff dimension of the entire limit set. Consequently, the limit set must be a Cantor set. As an application, we prove that any finitely generated, torsion-free discrete subgroup in Isom(X) with critical exponent less than one must be geometrically finite and free. This answers a question of Kapovich.

math.GT↗

arXiv2026

Cross Domain Few-Shot Class-Incremental Audio Classification Via Adversarial Contrastive Learning

Current Few-shot Class-incremental Audio Classification (FCAC) methods assume that samples of base and incremental classes are in the same domain (following the same distribution). However, there is generally a domain shift between the above two types of samples. In this paper, we explore the problem of Cross Domain FCAC where samples of base and incremental classes have domain shift. We propose a strategy of adversarial contrastive training which enables the model to effectively classify samples of different classes from unseen domains. The model consists of an encoder and a classifier. The encoder is trained in base session but frozen in incremental sessions, whereas the classifier is trained in all sessions. Experiments are done on six pairs of cross-domain datasets. Results show that our method exceeds state-of-the-art methods in average accuracy. The code is at https://github.com/YongjieSi/ACL.

eess.AS↗

arXiv2026

Hydrodynamical Simulations of Resonant Breaking in Multi-Planet Systems via Rebound Migration During Disk Dispersal

This study extends the investigation of rebound outward migration to multi-planet systems near an inner expanding disk cavity driven by stellar X-ray photoevaporation. Using 2D hydrodynamical simulations, we explore how systems of two and three planets that span masses from super-Earths to Jupiters evolve as the disk disperses from the inside out. Our results show that rebound migration can substantially reshape multi-planet architectures in the final stages of disk clearing. Owing to the strong, positive corotation torque exerted onto the planet near the cavity edge, divergent migration of the neighbouring planets can break resonant configurations and trigger dynamical instabilities, producing non-resonant orbits with widened period ratios. However, the outcome depends critically on planet mass and the disk dispersal timescale. In lower-mass disks where cavity expansion is too rapid, rebound migration is suppressed, and systems tend to preserve resonant chains. These findings suggest that the rebound mechanism can provide a compelling pathway to explain the prevalence of widely separated, non-resonant architecture observed in the exoplanet population.

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arXiv2026

Coupling dynamical accretion and chemical differentiation: A unified framework for the diversity of Earth and Mars

The physical and geochemical differences between Earth and Mars provide fundamental constraints on terrestrial planet formation, yet a self-consistent framework linking dynamical and chemical aspects remains elusive. Here we present an integrated modeling framework that couples high-resolution N-body simulations with impact-driven metal-silicate equilibration to track the dynamical accretion history and chemical differentiation for Earth and Mars. Using a narrow ring planetesimal accretion scenario, we show that Earth and Mars analogs naturally sample systematically different solid reservoirs within the protoplanetary disk. Earth analogs preferentially accrete reduced material around the planetesimal ring center, whereas Mars analogs acquire a larger fraction of oxidized material exterior to the ring. This leads to diverse bulk redox states, with composition further modified by impact-dependent pressure-temperature equilibration conditions during core formation. As a result, Earth analogs experience deeper equilibration and more efficient transfer of iron into the core, producing mantles with low iron oxide contents and larger core mass fractions. In contrast, Mars analogs equilibrate at shallower conditions, retain more iron in their mantles, and develop smaller cores. Our results demonstrate that the dynamical and geochemical differences between Earth and Mars emerge from the coupled effects of accretion pathways, the disk's radial redox structure, and impact-controlled differentiation rather than from any single process. Our unified framework physically explains the geochemical diversity of terrestrial planets and offers a potential pathway to interpret compositions of rocky planets in exoplanetary systems.

astro-ph.EP↗

arXiv2026

Chemical Abundances Shape History (CASH). I. A Link between Giant Planets Orbital Periods and Host Stellar C/O Ratios

The chemical abundance of host stars plays a pivotal role in shaping the formation history of planetary systems, yet the influence of elements beyond iron remains poorly understood. Here, we investigate the relationship between the carbon-to-oxygen (C/O) ratio of host stars and the orbital periods of giant planets. By analyzing high-resolution spectroscopic data from 598 planet-hosting stars (hosting 929 planets) across SDSS, Keck, and HARPS surveys, we identify a correlation: stars with higher C/O ratios are more likely to host longer-period giant planets. Theoretical models of pebble-driven planet formation and migration further support this observation, demonstrating that elevated C/O ratios enhance solid material availability at outer disk regions, promoting giant planet formation at larger distances and subsequent moderate inward migration. Our findings establish stellar C/O as a critical factor in shaping the orbital architecture of giant planets, bridging disk chemistry to planetary system evolution.

astro-ph.EP↗

arXiv2026

Trajectory Generation for Underactuated Soft Robot Manipulators using Discrete Elastic Rod Dynamics

Soft robots are well suited for contact-rich tasks due to their compliance, yet this property makes accurate and tractable modeling challenging. Planning motions with dynamically-feasible trajectories requires models that capture arbitrary deformations, remain computationally efficient, and are compatible with underactuation. However, existing approaches balance these properties unevenly: continuum rod models provide physical accuracy but are computationally demanding, while reduced-order approximations improve efficiency at the cost of modeling fidelity. To address this, our work introduces a control-oriented reformulation of Discrete Elastic Rod (DER) dynamics for soft robots, and a method to generate trajectories with these dynamics. The proposed formulation yields a control-affine representation while preserving certain first-principles force-deformation relationships. As a result, the generated trajectories are both dynamically feasible and consistent with the underlying actuation assumptions. We present our trajectory generation framework and validate it experimentally on a pneumatic soft robotic limb. Hardware results demonstrate consistently improved trajectory tracking performance over a constant-curvature-based baseline, particularly under complex actuation conditions.

cs.RO↗

arXiv2026

Reproducing the stellar-mass dependence of the giant planet occurrence rate with pebble accretion models

The stellar mass dependence of the unbiased giant planet occurrence rate may be the best statistical tool to constrain the formation of such planets. This rate rises and falls as a function of stellar mass, peaking around stars of $\sim 1.7{-}2 \Ms$. In this work, we carry out a population synthesis study, using pebble-driven core accretion model of planet formation, to investigate the planet formation conditions that may be responsible for this stellar-mass dependence. We use the inferred giant planet occurrence rated of three combined homogenised radial velocity surveys (EXPRESS, PPPS, and Lick giant star survey) to constrain the models. We find that we can produce a synthetic giant planet population with closely aligned occurrence and properties when we base our model on observationally-supported assumptions that accretion rates are higher and disk lifetimes are shorter around more massive stars, we can produce a synthetic giant planet population with closely aligned properties to the observed distribution. We also find that in this scenario, the runaway gas accretion occurs at a larger orbital distance and earlier times as the stellar mass increases.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2026

Capture and Stability of Resonant Planet Pairs in Turbulent Disk

We present a theoretical framework for the resonance capture and stability of two-planet systems in turbulent disks. By incorporating stochastic forcing (parameterized by $\kappa$) alongside laminar angular momentum and eccentricity damping timescales ($\tau_{\rm m}, \tau_{e}$), we derive an analytical criterion for the general $j:j-1$ mean motion resonances, and validate it through N-body simulations. The outcome is mapped in $\kappa$-$\tau_{\rm m}/\tau_{e}$ parameter space, revealing two distinct regimes: resonance trapping and turbulence-induced disruption -- which occurs either directly cross or via temporary capture followed by escape through turbulent diffusion. Crucially, our analysis identifies turbulence as a universal destabilizer. It amplifies the intrinsic overstability mechanism: In laminar disks, escape requires $\tau_{\rm m}/\tau_{e}$ to drop below a critical limit due to excessive eccentricity excitation. We demonstrate that turbulent diffusion lowers this limit, demanding stronger damping (larger $\tau_{\rm m}/\tau_{e}$) for stability. Thus, greater turbulence promotes escape, and sufficiently strong diffusion precludes resonance retention irrespective of eccentricity damping.

astro-ph.EP↗

arXiv2026

Effects of Stellar X-ray Photoevaporation on Planetesimal Formation via the Streaming Instability

The formation of planetesimals via the streaming instability (SI) is a crucial step in planet formation, yet its triggering conditions and efficiency are highly sensitive to both disk properties and specific evolutionary processes. We aim to study the planetesimal formation via the SI, driven by the stellar X-ray photoevaporation during the late stages of disk dispersal, and quantify its dependence on key disk and stellar parameters. We use the DustPy code to simulate the dust dynamics including coagulation, fragmentation, and radial drift in a viscously accreting disk undergoing stellar X-ray photoevaporation. Stellar X-rays drive the disk dispersal, opening a cavity at a few au orbital distance and inducing the formation of an associated local pressure maximum. This pressure maximum acts as a trap for radially drifting dust, therefore enhancing the dust density to the critical level required to initiate the streaming instability and the subsequent collapse into planetesimals. The fiducial model produces 31.4 M_\oplus of planetesimals with an initial dust to final planetesimal conversion efficiency of 20.4%. This pathway is most efficient in larger disks with higher metallicities, lower viscosities, higher dust fragmentation threshold velocities, and/or around stars with higher X-ray luminosities. This work demonstrates that stellar X-ray photoevaporation is a robust and feasible mechanism for triggering planetesimal formation via the SI during the final clearing phase of protoplanetary disk evolution.

astro-ph.EP↗

arXiv2025

Hydrodynamical Simulations of Planet Rebound Migration in Photo-evaporating Disks

This study investigates the orbital migration of a planet located near the truncated edge of protoplanetary disks, induced by X-ray photo-evaporation originating from the central star. The combined effects of turbulent viscous accretion and stellar X-ray photo-evaporation give rise to the formation of a cavity in the central few astronomical units in disks. Once the cavity is formed, the outer disk experiences rapid mass loss and the cavity expands inside out. We have conducted 2D hydrodynamical simulations of planet-disk interaction for various planet masses and disk properties. Our simulations demonstrate that planets up to about Neptune masses experience a strong positive corotation torque along the cavity edge that leads to sustained outward migration -- a phenomenon previously termed {\it rebound} migration. Rebound migration is more favorable in disks with moderate stellar photo-evaporation rates of ${\sim}10^{-8} ~ \rm M_{\odot}\,yr^{-1}$. Saturn-mass planets only experience inward migration due to significant gas depletion in their co-orbital regions. In contrast, Jupiter-mass planets are found to undergo modest outward migration as they cause the residual disk to become eccentric. This work presents the first 2D hydrodynamical simulations that confirm the existence and viability of rebound outward migration during the inside-out clearing in protoplanetary disks.

astro-ph.EP↗

arXiv2025

Colored knot Floer homology: structures and examples

Inspired by the $S^n$ colored version of Khovanov and Khovanov-Rozansky homology, we define a colored version of knot Floer homology by studying the colimit of a directed system of link Floer homology with infinite full twists. Specifically, our $n$-colored knot Floer homology of a knot $K$ is then defined as the colimit of the link Floer homology of $(n, mn)$-cables of $K$ by fixing $n$ and letting $m$ goes to infinity. We show that the colimit of the infinite full twists is a module over the colored knot Floer homology of the unknot. In addition, we give an explicit description of colored Heegaard Floer homology for L-space knots, and maps for colored knot Floer homology of crossing changes.

math.GT↗

arXiv2025

The impact of pre-main sequence stellar luminosity on giant planet formation

Luminosities of pre-main sequence stars evolve during the protoplanetary disc lifetime. This has a significant impact on the heating of their surrounding protoplanetary disks, the natal environments of planets. Moreover, stars of different masses evolve differently. However, this is rarely accounted for in planet formation models. We carry out pebble-driven core accretion planet formation modelling with focus on the impact of pre-main sequence stellar luminosity evolution on giant planet formation around host stars in the range of $1{-}2.4\ \rm M_{\odot}$. We find that giant planet formation is sensitive to the evolution of stellar luminosity, specifically the locations and times at which giant planet formation can occur depend on it. High stellar luminosity causes an increase in the scale height of the gas and pebbles, which may decrease the efficiency of pebble accretion, making it more challenging to form giant planets. This has important consequences for the composition of these giant planets, stressing the need to incorporate such aspects into planet formation models.

astro-ph.EP↗

arXiv2025

Brownian motion and orbit counting of Kleinian groups

In this paper, we investigate the relationship between the divergence of Kleinian groups $\Gamma$ and the recurrence of simple random walks on the Schreier graph associated with $\Gamma$. In particular, we show that if $\Gamma$ is a subgroup of a lattice and is of divergence type, then the Schreier graph is recurrent. Our approach builds connections among the growth rate of the $\Gamma$-orbit, the volume growth rate of the quotient manifolds, and the growth rate of the Schreier graph. Using the connections, we construct abundant Kleinian groups of divergence type.

math.GT↗

arXiv2025

JVLA Measurement of Grain Size in the Compact Dust Ring around Class I Protostar WL 17

The maximum grain size in protoplanetary disks is a critical parameter for planet formation, as the efficiency of mechanisms like streaming instability and pebble accretion depend on grain size. Even young class 0/I objects, such as HL Tau, show substructures in their disks, indicating the potential for early planet formation. In this study, we investigated the grain size in the dust surrounding the class I object WL 17 using the Karl G. Jansky Very Large Array. Observations were conducted across seven frequency bands (Q, Ka, K, Ku, X, C, and S bands) ranging from 2 to 48 GHz, corresponding to wavelengths of 15 cm to 6.3 mm, with a spatial resolution exceeding 0\farcs5. While the ring structure at 0\farcs1 of WL 17 remains unresolved in our data, its emission is clearly detected at all observed frequencies, except at 2 GHz. To estimate the maximum grain size ($a_{\rm max}$) within the ring, we compared the observed spectral energy distribution (SED) with theoretical SEDs calculated for various $a_{\rm max}$ values using radiative transfer models. Assuming the dust opacity follows the DSHARP model, our analysis suggests that certain structures internal to the ring achieved a maximum grain size of approximately 4.2 mm. Additionally, we discuss the gravitational stability of the ring and the potential planetary core mass that could form through pebble accretion within the structure.

astro-ph.EP↗

arXiv2025

Dependence of Planet populations on Stellar Mass and Metallicity: A Pebble Accretion-based Planet Population Synthesis

The formation and evolution of planetary systems are linked to their host stellar environment. In this study, we employ a pebble accretion-based planet population synthesis model to explore the correlation between planetary properties and stellar mass/metallicity. Our numerical results reproduce several main aspects of exoplanetary observations. First, we find that the occurrence rate of super-Earths $\eta_{\rm SE}$ follows an inverted V-shape in relation to stellar mass: it increases with stellar mass among lower-mass dwarfs, peaks at early-M dwarfs, and declines toward higher-mass GK stars. Second, super-Earths grow ubiquitously around stars with various metallicities, exhibiting a flat or weak $\eta_{\rm SE}$ dependence on $Z_{\star}$. Third, giant planets, in contrast, form more frequently around stars with higher-mass/metallicity. Lastly, we extend a subset of simulations to $1$ Gyr to investigate the long-term evolution of the systems' architecture. By converting our simulated systems into synthetic observations, we find that the eccentricities and inclinations of single-transit systems increase with stellar metallicity, while these dependencies in multi-planet systems remains relatively weak. The alignment between our results and observations provides key insights into the connection between planet populations and stellar properties.

astro-ph.EP↗

arXiv2025

Resonance Capture and Stability Analysis for Planet Pairs under Type I Disk Migration

We present a theoretical framework for investigating a two-planet system undergoing convergent type I migration in a protoplanetary disk. Our study identifies the conditions for resonant capture and subsequent dynamical stability. By deriving analytical criteria for general $j$:$j-1$ first-order mean-motion resonances (MMRs) applicable to planet pairs with arbitrary mass ratios, we validate these predictions through N-body simulations. The key results are demonstrated in $\tau_{\rm m}$-$\tau_{\rm m}/\tau_{e}$ plots, where $\tau_{\rm m}$ and $\tau_{e}$ are the timescales of the angular momentum and eccentricity damping, respectively. Specifically, we determine which combinations of orbital damping timescales allow for capture into resonance, showing that too fast migration or too strong eccentricity damping inhibit successful capture. After capture, the subsequent evolution can be classified into three regimes: stable trap, overstable trap and escape. Importantly, resonant capture always remains stable when the inner planet significantly outweighs the outer one. In contrast, when the mass of the inner planet is lower than or comparable to that of the outer planet, the system transitions from the stable to overstable trap, and eventually escapes the resonance, as the relative strength of eccentricity damping to migration ($\tau_{\rm m}/\tau_{e}$) decreases.

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