arXiv subjects

Gongjie Li

Publications and source records attributed to Gongjie Li.

At least 37 records · Page 2Linked to original sources

arXiv2023

Enhanced Blackhole mergers in AGN discs due to Precession induced resonances

Recent studies have shown that AGN discs can host sources of gravitational waves. Compact binaries can form and merge in AGN discs through their interactions with the gas and other compact objects in the disc. It is also possible for the binaries to shorten the merging timescale due to eccentricity excitation caused by perturbations from the supermassive blackhole (SMBH). In this paper we focus on effects due to precession-induced (eviction-like) resonances, where nodal and apsidal precession rates of the binary is commensurable with the mean motion of the binary around the SMBH. We focus on intermediate mass black hole (IMBH)-stellar mass black hole (SBH) binaries, and consider binary orbit inclined from the circum-IMBH disk which leads to the orbital $J_2$ precession. We show that if a binary is captured in these resonances and is migrating towards the companion, it can undergo large eccentricity and inclination variations. We derive analytical expressions for the location of fixed points, libration timescale and width for these resonances, and identified two resonances in the near coplanar regime (the evection and eviction resonances) as well as two resonances in the near polar regime that can lead to mergers. We also derive analytical expressions for the maximum eccentricity that a migrating binary can achieve for given initial conditions. Specifically, the maximum eccentricity can reach 0.9 when captured in these resonances before orbital decay due to gravitational wave emission dominates, and the capture is only possible for slow migration ($\sim 10$ Myr) 2-3 order of magnitude longer than the resonance libration timescale. We also show that capture into multiple resonances is possible, and can further excite eccentricities.

astro-ph.HE↗

arXiv2023

Let's Sweep: The Effect of Evolving $J_2$ on the Resonant Structure of a Three-Planet System

Short and ultra-short planets are a peculiar type of exoplanets with periods as short as a few days or less. Although it is challenging to detect them, already several are observed with many additional candidates. If these planets have formation pathways to their longer period counterparts, they are predicted to reside in multi-planet systems. Thus, gravitational perturbation from potential planetary neighbors may affect their orbital configuration. However, due to their close proximity to their host star, they are also subjects to general relativity precession and torques from the stellar spin quadrupole moment ($J_2$). Here we show that an evolving $J_2$ due to magnetic braking, affects the magnitude and location of secular resonances of the short period planet in a multi-planet system. Thus, driving the short period planet into and out of a secular resonance, exciting the planet's eccentricity and inclination. The high inclination can hinder transit observation, and, in some cases, the high eccentricity may result in an unstable configuration. We propose that evolving $J_2$ in a multi-planet system can be critical in understanding the detectability and stability of short-period planets.

astro-ph.EP↗

arXiv2023

Secular Spin-orbit Resonances of Black Hole Binaries in AGN Disks

The spin-orbit misalignment of stellar-mass black hole (sBH) binaries provide important constraints on the formation channels of merging sBHs. Here, we study the role of secular spin-orbit resonance in the evolution of a sBH binary component around a supermassive BH (SMBH) in an AGN disk. We consider the sBH's spin-precession due to the $J_2$ moment introduced by a circum-sBH disk within the warping/breaking radius of the disk. We find that the sBH's spin-orbit misalignment (obliquity) can be excited via spin-orbit resonance between the sBH binary's orbital nodal precession and the sBH spin-precession driven by a massive circum-sBH disk. Using an $α$-disk model with Bondi-Hoyle-Lyttleton accretion, the resonances typically occur for sBH binaries with semi-major axis of $1$AU, and at a distance of $\sim 1000$AU around a $10^7$\msun SMBH. The spin-orbit resonances can lead to high sBH obliquities, and a broad distribution of sBH binary spin-spin misalignments. However, we note that the Bondi-Hoyle-Lyttleton accretion is much higher than that of Eddington accretion, which typically results in spin precession being too low to trigger spin-orbit resonances. Thus, the secular spin-orbit resonances can be quite rare for sBHs in AGN disks.

astro-ph.HE↗

arXiv2023

Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate

Climate modeling has shown that tidally influenced terrestrial exoplanets, particularly those orbiting M-dwarfs, have unique atmospheric dynamics and surface conditions that may enhance their likelihood to host viable habitats. However, sporadic libration and rotation induced by planetary interactions, such as that due to mean motion resonances (MMRs) in compact planetary systems may destabilize attendant exoplanets away from synchronized states (or 1:1 spin-orbit ratio). Here, we use a three-dimensional N-Rigid-Body integrator and an intermediately-complex general circulation model to simulate the evolving climates of TRAPPIST-1 e and f with different orbital and spin evolution pathways. Planet f perturbed by MMR effects with chaotic spin-variations are colder and dryer compared to their synchronized counterparts due to the zonal drift of the substellar point away from open ocean basins of their initial eyeball states. On the other hand, the differences between perturbed and synchronized planet e are minor due to higher instellation, warmer surfaces, and reduced climate hysteresis. This is the first study to incorporate the time-dependent outcomes of direct gravitational N-Rigid-Body simulations into 3D climate modeling of extrasolar planets and our results show that planets at the outer edge of the habitable zones in compact multiplanet systems are vulnerable to rapid global glaciations. In the absence of external mechanisms such as orbital forcing or tidal heating, these planets could be trapped in permanent snowball states.

astro-ph.EP↗

arXiv2022

Formation History of HD106906 and the Vertical Warping of Debris Disks by an External Inclined Companion

HD106906 is a planetary system that hosts a wide-orbit companion, as well as an eccentric and flat debris disk, which hold important constraints on its formation and subsequent evolution. The recent observations of the companion constrain its orbit to be eccentric and inclined relative to the plane of the debris disk. Here, we show that, in the presence of the inclined companion, the debris disk quickly ($\lesssim5$ Myr) becomes warped and puffy. This suggests that the current configuration of the system is relatively recent. We explore the possibility that a recent close encounter with a free floating planet could produce a companion with orbital parameters that agree with observations of HD106906b. We find that this scenario is able to recreate the structure of the debris disk while producing a companion in agreement with observation.

astro-ph.EP↗

arXiv2022

Blackhole Mergers Through Evection Resonances

Mechanisms have been proposed to enhance the merger rate of stellar mass black hole binaries, such as the Von Zeipel-Lidov-Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Here, we propose a novel pathway through which compact binaries could merge due to eccentricity increase in general, including in a near coplanar configuration. Specifically, a compact binary migrating in an AGN disk could be captured in an evection resonance, when the precession rate of the binary equals their orbital period around the supermassive black hole. In our study we include precession to due first-order post Newtonian precession as well as that due to disk around one or both components of the binary. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to $\sim 10^{3-5}$.

astro-ph.HE↗

arXiv2022

Data-driven discovery of non-Newtonian astronomy via learning non-Euclidean Hamiltonian

Incorporating the Hamiltonian structure of physical dynamics into deep learning models provides a powerful way to improve the interpretability and prediction accuracy. While previous works are mostly limited to the Euclidean spaces, their extension to the Lie group manifold is needed when rotations form a key component of the dynamics, such as the higher-order physics beyond simple point-mass dynamics for N-body celestial interactions. Moreover, the multiscale nature of these processes presents a challenge to existing methods as a long time horizon is required. By leveraging a symplectic Lie-group manifold preserving integrator, we present a method for data-driven discovery of non-Newtonian astronomy. Preliminary results show the importance of both these properties in training stability and prediction accuracy.

cs.LG↗

arXiv2022

Low Spin-Axis Variations of Circumbinary Planets

Having a massive moon has been considered as a primary mechanism for stabilized planetary obliquity, an example of which being our Earth. This is, however, not always consistent with the exoplanetary cases. This article details the discovery of an alternative mechanism, namely that planets orbiting around binary stars tend to have low spin-axis variations. This is because the large quadrupole potential of the stellar binary could speed up the planetary orbital precession, and detune the system out of secular spin-orbit resonances. Consequently, habitable zone planets around the stellar binaries in low inclination orbits hold higher potential for regular seasonal changes comparing to their single star analogues.

astro-ph.EP↗

arXiv2022

Raining Rocks: An analytical formulation for collision timescales in planetary systems

The dynamical interaction of minor bodies (such as comets or asteroids) with planets plays an essential role in the planetary system's architecture and evolution. As a result of these interactions, structures like the Kuiper belt and the Oort cloud can be created. In particular, the collision of minor bodies with planets can drastically change the planet's internal and orbital evolution. We present an analytic formulation to determine the collision timescale for a minor body to impact a planet for arbitrary geometry. By comparing with a suite of detailed N-body simulations and an analytical method for collision timescales in the solar system, we confirmed the accuracy of our analytic formulation. As a proof of concept, we focused on the collision rate of minor bodies randomly distributed around a Jupiter-like planet, emulating a Kuiper belt-like disk. We show that our analytical method yields in good agreement with the numerical simulations. The formalism presented here thus provides a succinct and accurate alternative to numerical calculations.

astro-ph.EP↗

arXiv2022

Mutual Inclination of Ultra-Short-Period Planets with Time Varying Stellar J2-moment

Systems with ultra-short-period planets (USPs) tend to possess larger mutual inclinations compared to those with planets located farther from their host stars. This could be explained due to precession caused by stellar oblateness at early times when the host star was rapidly spinning. However, stellar oblateness reduces over time due to the decrease in the stellar rotation rate, and this may further shape the planetary mutual inclinations. In this work, we investigate in detail how the final mutual inclination varies under the effect of a decreasing $J_2$. We find that different initial parameters (e.g., the magnitude of $J_2$ and planetary inclinations) will contribute to different final mutual inclinations, providing a constraint on the formation mechanisms of USPs. In general, if the inner planets start in the same plane as the stellar equator (or co-planar while misaligned with the stellar spin-axis), the mutual inclination decreases (or increases then decreases) over time due to the decay of the $J_2$ moment. This is because the inner orbit typically possesses less orbital angular momentum than the outer ones. However, if the outer planet is initially aligned with the stellar spin while the inner one is misaligned, the mutual inclination nearly stays the same. Overall, our results suggest that either the USP planets formed early and acquired significant inclinations (e.g., $\gtrsim30^\circ$ with its companion or $\gtrsim10^\circ$ with its host star spin-axis for Kepler-653c) or they formed late ($\gtrsim$Gyr) when their host stars rotate slower.

astro-ph.EP↗

arXiv2021

The Gliese 86 Binary System: A Warm Jupiter Formed in a Disk Truncated at $\approx$2 AU

Gliese 86 is a nearby K dwarf hosting a giant planet on a $\approx$16-day orbit and an outer white dwarf companion on a $\approx$century-long orbit. In this study we combine radial velocity data (including new measurements spanning more than a decade) with high angular resolution imaging and absolute astrometry from Hipparcos and Gaia to measure the current orbits and masses of both companions. We then simulate the evolution of the Gl 86 system to constrain its primordial orbit when both stars were on the main sequence; the closest approach between the two stars was then about $9\,$AU. Such a close separation limited the size of the protoplanetary disk of Gl 86 A and dynamically hindered the formation of the giant planet around it. Our measurements of Gl 86 B and Gl 86 Ab's orbits reveal Gl 86 as a system in which giant planet formation took place in a disk truncated at $\approx$2$\,$AU. Such a disk would be just big enough to harbor the dust mass and total mass needed to assemble Gl 86 Ab's core and envelope, assuming a high disk accretion rate and a low viscosity. Inefficient accretion of the disk onto Gl 86 Ab, however, would require a disk massive enough to approach the Toomre stability limit at its outer truncation radius. The orbital architecture of the Gl 86 system shows that giant planets can form even in severely truncated disks and provides an important benchmark for planet formation theory.

astro-ph.EP↗

arXiv2021

Milankovitch Cycles for a Circumstellar Earth-analog within $α$ Centauri-like Binaries

An Earth-analog orbiting within the habitable zone of $α$ Centauri B was shown to undergo large variations in its obliquity, or axial tilt, which affects the planetary climate by altering the radiative flux for a given latitude. We examine the potential implications of these obliquity variations for climate through Milankovitch cycles using an energy balance model with ice sheets. Similar to previous studies, the largest amplitude obliquity variations from spin-orbit resonances induce snowball states within the habitable zone, while moderate variations can allow for persistent ice caps or an ice belt. Particular outcomes for the global ice distribution can depend on the planetary orbit, obliquity, spin precession, binary orbit, and which star the Earth-analog orbits. An Earth-analog with an inclined orbits relative to the binary orbit can periodically transition through several global ice distribution states and risk runaway glaciation when periods of ice caps and an ice belt overlap. When determining the potential habitability for planets in stellar binaries, more care must be taken due to the orbital and spin dynamics.

astro-ph.EP↗

arXiv2021

TIC 172900988: A Transiting Circumbinary Planet Detected in One Sector of TESS Data

We report the first discovery of a transiting circumbinary planet detected from a single sector of TESS data. During Sector 21, the planet TIC 172900988b transited the primary star and then 5 days later it transited the secondary star. The binary is itself eclipsing, with a period of P = 19.7 days and an eccentricity of e = 0.45. Archival data from ASAS-SN, Evryscope, KELT, and SuperWASP reveal a prominent apsidal motion of the binary orbit, caused by the dynamical interactions between the binary and the planet. A comprehensive photodynamical analysis of the TESS, archival and follow-up data yields stellar masses and radii of M1 = 1.2384 +/- 0.0007 MSun and R1 = 1.3827 +/- 0.0016 RSun for the primary and M2 = 1.2019 +/- 0.0007 MSun and R2 = 1.3124 +/- 0.0012 RSun for the secondary. The radius of the planet is R3 = 11.25 +/- 0.44 REarth (1.004 +/- 0.039 RJup). The planet's mass and orbital properties are not uniquely determined - there are six solutions with nearly equal likelihood. Specifically, we find that the planet's mass is in the range of 824 < M3 < 981 MEarth (2.65 < M3 < 3.09 MJup), its orbital period could be 188.8, 190.4, 194.0, 199.0, 200.4, or 204.1 days, and the eccentricity is between 0.02 and 0.09. At a V = 10.141 mag, the system is accessible for high-resolution spectroscopic observations, e.g. Rossiter-McLaughlin effect and transit spectroscopy.

astro-ph.EP↗

arXiv2021

Field-mediated locomotor dynamics on highly deformable surfaces

In many systems motion occurs on deformed and deformable surfaces, setting up the possibility for dynamical interactions solely mediated by the coupling of the entities with their environment. Here we study the "two-body" dynamics of robot locomotion on a highly deformable spandex membrane in two scenarios: one in which a robot orbits a large central depression and the other where the two robots affect each other's motion solely through mutual environmental deformations. Inspired by the resemblance of the orbits of the single robot with those of general relativistic orbits around black holes, we recast the vehicle plus membrane dynamics in physical space into the geodesic motion of a "test particle" in a fiducial curved space-time and demonstrate how this framework facilitates understanding the observed dynamics. The two-robot problem also exhibits a resemblance with Einstein's general relativistic view of gravity, which in the words of Wheeler: "spacetime tells matter how to move; matter tells spacetime how to curve." We generalize this case the mapping to include a reciprocal coupling that translates into robotic curvature-based control schemes which modify interaction (promoting avoidance or aggregation) without long-range sensing. Our work provides a starting point for developing a mechanical analog gravity system as well as develops a framework that can provide insights into active matter in deformable environments and robot exploration in complex landscapes.

cs.RO↗

arXiv2021

Order in the chaos: eccentric black hole binary mergers in triples formed via strong binary-binary scatterings

Black hole (BH) triples represent one of the astrophysical pathways for BH mergers in the Universe detectable by LIGO and VIRGO. We study the formation of BH triples via binary-binary encounters in dense clusters, showing that one-third of triples formed through this channel are in a non-hierarchical, unstable configuration. We build a database of $32,000$ $N$-body simulations to investigate the evolution of these BH triples. Varying the mutual orbital inclination, the three BH masses and the inner and outer eccentricities, we show that retrograde, nearly planar configurations lead to a significant shrinkage of the inner binary. We find an universal trend of triple systems, namely that they tend to evolve toward prograde configurations and that the orbital flip, driven by the torque exerted on the inner BH binary (BHB) by the outer BH, leads in general to tighter inner orbits that, in some cases, merge within a Hubble time. We find that the mergers' mass distribution maps the original BH binary spectrum. A large fraction of merging BHBs with initial separation $1$ AU enter the $10^{-3}-10^{-1}$ Hz frequency band with large eccentricities, thus representing potential eccentric LISA sources. Mergers originating from initially tighter BHB ($a\sim 0.01$ AU), instead, often have eccentricities above 0.7 in the $1$ Hz band. This might imply that eccentric sources detected in high-frequency detectors are most likely connected with a high-velocity dispersion stellar environment, whereas eccentric sources detected in low-frequency detectors are likely to develop in low-density clusters. (Abridged)

astro-ph.HE↗

arXiv2021

GRIT: a package for structure-preserving simulations of gravitationally interacting rigid-bodies

Spin-orbit coupling of planetary systems plays an important role in the dynamics and habitability of planets. However, symplectic integrators that can accurately simulate not only how orbit affects spin but also how spin affects orbit have not been constructed for general systems. Thus, we develop symplectic Lie-group integrators to simulate systems consisting gravitationally interacting rigid bodies. A user friendly package (GRIT \url{https://github.com/GRIT-RBSim/GRIT}) is provided and external forcings such as tidal interactions are also included. As a demonstration, this package is applied to Trappist-I. It shows that the differences in transit timing variations due to spin-orbit coupling could reach a few min in ten year measurements, and strong planetary perturbations can push Trappist-I f, g and h out of the synchronized states.

astro-ph.EP↗

arXiv2021

Tilting Planets During Planet Scattering

Observational constraints on planet spin-axis has recently become possible, and revealed a system that favors a large spin-axis misalignment, a low stellar spin-orbit misalignment and a high eccentricity. To explain the origin of such systems, we propose a mechanism that could tilt the planet spin-axis during planet-planet scattering, which are natural outcomes of in-situ formation and disk migration. Specifically, we show that spin-orbit resonances could occur for a short time period during the scattering processes, and excite the misalignment of the planet spin-axis. This typically leads to planets with large spin-misalignment and a wide range of eccentricities and inclinations.

astro-ph.EP↗

arXiv2021

Exomoons in Systems with a Strong Perturber: Applications to $α$ Cen AB

The presence of a stellar companion can place constraints on occurrence and orbital evolution of satellites orbiting exoplanets, i.e., exomoons. In this work we revise earlier orbital stability limits for retrograde orbits in the case of a three body system consisting of star-planet-satellite. The latter reads $a_{\rm sat}^{\rm crit} \approx 0.668(1-1.236e_{\rm p})$ for $e_p \leq 0.8$ in units of the Hill Radius and represents the lower critical orbit as a function of the planetary eccentricity $e_{\rm p}$. A similar formula is determined for exomoons hosted by planets in binary star systems, where $e_{\rm p}$ is replaced with the components of free and forced eccentricity from secular orbit evolution theory. By exploring the dynamics of putative exomoons in $α$ Centauri AB we find that the outer stability limit can be much less than half the Hill Radius due to oscillations in the planetary orbital eccentricity caused by the gravitational interaction with the binary star. We show, furthermore, how the resulting truncation of the outer stability limit can affect the outward tidal migration and potential observability of exomoons through transit timing variations (TTVs). Typical TTV (RMS) amplitudes induced by exomoons in binary systems are $\lesssim$10 min and appear more likely for planets orbiting the less massive stellar component. A GitHub repository (saturnaxis/exomoon-in-binaries) is available to reproduce figures.

astro-ph.EP↗