arXiv subjects

Eiichiro Kokubo

Publications and source records attributed to Eiichiro Kokubo.

At least 19 recordsLinked to original sources

arXiv2026

Planet-Planet Scattering Explains the Mass-Eccentricity Relation of Warm Jupiters

Warm giant planets with orbital periods of tens of days exhibit a positive correlation between mass and eccentricity. We interpret this trend as the outcome of planet-planet scattering, representing a transition from collision-dominated interactions among low-mass planets to ejection-dominated interactions among high-mass planets. This framework has important implications for warm Jupiter origins. It suggests that warm Jupiters originate from compact, multi-planet configurations. The dynamical interactions that shape their present-day architectures likely occur near their current semimajor axes, regardless of whether warm Jupiters formed through convergent disk-driven migration or in-situ formation. We argue that several observed properties of warm Jupiter systems, including the eccentricity bimodality, the mass-eccentricity relation, and generally low stellar obliquities, can be explained by this picture. We further predict that not only circular warm Jupiters, but also eccentric warm Jupiters, should frequently have additional planetary companions that are detectable through radial velocity observations. Finally, scattering can produce eccentricities high enough to trigger high-eccentricity tidal migration, potentially explaining the emerging population of proto-hot Jupiters on tidal migration tracks.

astro-ph.EP↗

arXiv2026

Dynamics of planetary rings under thermal forces

Planetary rings provide natural laboratories for studying the fundamental processes that govern the evolution of planetary systems. However, several key features, such as the sharp inner edges of Saturn's rings remain unresolved. In this work, we introduce and quantify the Eclipse-Yarkovsky (EY) effect, a thermal torque arising from asymmetric thermal emission of particles during planetary eclipses, which is effective for particles larger than millimeters in size. We formulate this effect within a continuum framework appropriate for collisionally coupled planetary rings and derive the continuum evolution equation that includes the EY torque and viscous diffusion (Eq.26), constraining its magnitude using ring particle spin distributions obtained from N-body simulations. We find that the EY effect systematically produces a positive angular momentum flux that could overcome the viscous torque, driving ring material outward and leading to long-term decretion. The total EY torque principally depends on the optical depth, in which we identify three dynamical regimes: dense, transitional, and tenuous regimes, each exhibiting distinct evolutionary pathways. In the dense or transition regimes, the EY torque can produce a sharp inner edge such as that of Saturn's A ring. In the tenuous regime, it can drive an entire ring outward while preserving shape. This outward transport may also facilitate satellite formation beyond the Roche limit. We also quantitatively show that planetary thermal radiation on rings exerts an opposing torque, namely planetary-Yarkovsky effect, whose importance depends on planetary emissivity and ring-particle albedo, and may lead to inward transport in Saturn's close-in rings.

astro-ph.EP↗

arXiv2026

Unraveling Year-Long Radial Velocity Variations in Red Clump Region -- I: Comprehensive analysis of a K0 Giant star, 2 Draconis

Slow-rotating evolved stars frequently exhibit radial velocity (RV) variations on annual timescales, complicated by instrumental systematics and aliasing in the one-year regime. Here we investigate the origin of the near-yearly periodicity in 2 Dra, a star located in the red-clump region, assessing possible causes between stellar activity, instrumental profile (IP) effects, sampling alias, and planetary companions. We applied two independent approaches: (1) constraining diagnostic signals and performing a correlation analysis ($r$) between period-confined signals, and (2) evaluating phase stability by partitioning Keplerian fits. These methods enabled us to examine the physical connections and phase coherence among stellar activity indicators, RV measurements, and IP diagnostics. Our analysis suggests a stellar rotation period of $\simeq270\text{--}320$\,d for 2~Dra. The 340-d RV signal does not appear to originate from stellar activity in this chromospherically quiet star ($|r| \lesssim 0.33$), nor from instrumental systematics near the annual period ($|r| \lesssim 0.1$). This conclusion is supported by contrasting phase behavior: the RV and stellar activity phases remain stable, whereas the IP phases do not. We therefore propose that the 340-d variation likely arises from either small-amplitude intrinsic variability or a tentative gas giant companion with potential weak activity-induced modulation. The case of 2~Dra provides a framework for distinguishing the origins of $\sim$1-yr RV variations in other evolved stars.

astro-ph.SR↗

arXiv2026

Analytical Solutions for Planet-Scattering Small Bodies

Gravitational scattering of small bodies (planetesimals) by a planet remains a fundamental problem in celestial mechanics. It is traditionally modeled within the circular restricted three-body problem (CR3BP), where individual particle trajectories are obtained via numerical integrations. Here, we use {Ö}pik's close-encounter framework to study the random walk of the orbital energy $x$ for an ensemble of test particles on planet-crossing orbits. We show that the evolution of each particle's orbital elements $(a, e, i)$ is fully encapsulated by the 3D rotation of the relative velocity vector $\bm{U}_\infty$, whose magnitude remains constant. Consequently, the system can be reduced to two degrees of freedom. By averaging over all possible flyby geometries, we derive explicit expressions for the drift and diffusion coefficients of $x$. We then solve the resulting Fokker--Planck equation to obtain a closed-form solution for the time evolution of the particle distribution. A characteristic scattering timescale naturally emerges, scaling as $(P_{p}/M_{p}^{2})/500$, where $P_{p}$ is the planet's orbital period and $M_{p}$ its mass ratio to the central star. The typical ejection speed of small bodies by a planet is estimated to be $3 v_p M_{p}^{1/3}$, where $v_p$ is the planet's orbital speed. Our analytical solution constitutes a universal law applicable to both the Solar System and exoplanetary systems, providing a computationally efficient alternative to costly $N$-body simulations for studying the orbital distributions and ejection of planetesimals and planets (e.g., Kuiper Belt, Oort Cloud, debris disks, interstellar objects, and free-floating planets).

astro-ph.EP↗

arXiv2025

Direct Imaging Explorations for Companions from the Subaru/IRD Strategic Program II; Discovery of a Brown-dwarf Companion around a Nearby mid-M~dwarf LSPM~J1446+4633

We report the discovery of a new directly-imaged brown dwarf companion with Keck/NIRC2+pyWFS around a nearby mid-type M~dwarf LSPM~J1446+4633 (hereafter J1446). The $L'$-band contrast ($4.5\times10^{-3}$) is consistent with a $\sim20-60\ M_{\rm Jup}$ object at 1--10~Gyr and our two-epoch NIRC2 data suggest a $\sim30\%$ ($\sim3.1σ)$ variability in its $L'$-band flux. We incorporated Gaia DR3 non-single-star catalog into the orbital fitting by combining the Subaru/IRD RV monitoring results, NIRC2 direct imaging results, and Gaia proper motion acceleration. As a result, we derive ${59.8}_{-1.4}^{+1.5}\ M_{\rm Jup}$ and $\approx4.3~{\rm au}$ for the dynamical mass and the semi-major axis of the companion J1446B, respectively. J1446B is one of the intriguing late-T~dwarfs showing variability at $L'$-band for future atmospheric studies with the constrained dynamical mass. Because the J1446 system is accessible with various observation techniques such as astrometry, direct imaging, and high-resolution spectroscopy including radial velocity measurement, it has a potential as a great benchmark system to improve our understanding for cool dwarfs.

astro-ph.SR↗

arXiv2025

A Scaling Law for the Orbital Architecture of Planetary Systems Formed by Gravitational Scattering and Collisions

In the standard formation models of terrestrial planets in the solar system and close-in super-Earths in non-resonant orbits recently discovered by exoplanet observations, planets are formed by giant impacts of protoplanets or planetary embryos after the dispersal of protoplanetary disk gas in the final stage. This study aims to theoretically clarify a fundamental scaling law for the orbital architecture of planetary systems formed by giant impacts. In the giant impact stage, protoplanets gravitationally scatter and collide with one another to form planets. Using {\em N}-body simulations, we investigate the orbital architecture of planetary systems formed from protoplanet systems by giant impacts. As the orbital architecture parameters, we focus on the mean orbital separation between two adjacent planets and the mean orbital eccentricity of planets in a planetary system. We find that the orbital architecture is determined by the ratio of the two-body surface escape velocity of planets $v_\mathrm{esc}$ to the Keplerian circular velocity $v_\mathrm{K}$, $k$ = The mean orbital separation and eccentricity are about $2 ka$ and $0.3 k$, respectively, where $a$ is the system semimajor axis. With this scaling, the orbital architecture parameters of planetary systems are nearly independent of their total mass and semimajor axis.

astro-ph.EP↗

arXiv2025

Semi-analytical model for the dynamical evolution of planetary system II: Application to systems formed by a planet formation model

The standard formation model of close-in low-mass planets involves efficient inward migration followed by growth through giant impacts after the protoplanetary gas disk disperses. While detailed N-body simulations have enhanced our understanding, their high computational cost limits statistical comparisons with observations. In our previous work, we introduced a semi-analytical model to track the dynamical evolution of multiple planets through gravitational scattering and giant impacts after the gas disk dispersal. Although this model successfully reproduced N -body simulation results under various initial conditions, our validation was still limited to cases with compact, equally-spaced planetary systems. In this paper, we improve our model to handle more diverse planetary systems characterized by broader variations in planetary masses, semi-major axes, and orbital separations and validate it against recent planet population synthesis results. Our enhanced model accurately reproduces the mass distribution and orbital architectures of the final planetary systems. Thus, we confirm that the model can predict the outcomes of post-gas disk dynamical evolution across a wide range of planetary system architectures, which is crucial for reducing the computational cost of planet formation simulations.

astro-ph.EP↗

arXiv2025

A Resonant Beginning for the Solar System Terrestrial Planets

In the past two decades, transit surveys have revealed a class of planets with thick atmospheres -- sub-Neptunes -- that must have completed their accretion in protoplanet disks. When planets form in the gaseous disk, the gravitational interaction with the disk gas drives their migration and results in the trapping of neighboring planets in mean motion resonances, though these resonances can later be broken when the damping effects of disk gas or planetesimals wane. It is widely accepted that the outer Solar System gas giant planets originally formed in a resonant chain, which was later disrupted by dynamical instabilities. Here, we explore whether the early formation of the terrestrial planets in a resonance chain (including Theia) can evolve to the present configuration. Using N-body simulations, we demonstrate that the giant planet instability would also have destabilized the terrestrial resonance chain, triggering moon-forming giant impacts in 20--50\% of our simulated systems, dependent on the initial resonance architecture. After the instability, the eccentricity and inclination of the simulated planets match their present-day values. Under the proposed scenario, the current period ratio of 3.05 between Mars and Venus -- devoid of any special significance in traditional late formation models -- naturally arises as a relic of the former resonance chain.

astro-ph.EP↗

arXiv2025

Semi-analytical model for the dynamical evolution of planetary systems via giant impacts

In the standard model of terrestrial planet formation, planets are formed through giant impacts of planetary embryos after the dispersal of the protoplanetary gas disc. Traditionally, $N$-body simulations have been used to investigate this process. However, they are computationally too expensive to generate sufficient planetary populations for statistical comparisons with observational data. A previous study introduced a semi-analytical model that incorporates the orbital and accretionary evolution of planets due to giant impacts and gravitational scattering. This model succeeded in reproducing the statistical features of planets in $N$-body simulations near 1 au around solar-mass stars. However, this model is not applicable to close-in regions (around 0.1 au) or low-mass stars because the dynamical evolution of planetary systems depends on the orbital radius and stellar mass. This study presents a new semi-analytical model applicable to close-in orbits around stars of various masses, validated through comparison with $N$-body simulations. The model accurately predicts the final distributions of planetary mass, semi-major axis, and eccentricity for the wide ranges of orbital radius, initial planetary mass, and stellar mass, with significantly reduced computation time compared to $N$-body simulations. By integrating this model with other planet-forming processes, a computationally low-cost planetary population synthesis model can be developed.

astro-ph.EP↗

arXiv2025

Stellar Obliquity of the Ultra-Short-Period Planet System HD 93963

We report an observation of the Rossiter-McLaughlin (RM) effect of the transiting planet HD 93963 Ac, a mini-Neptune planet orbiting a G0-type star with an orbital period of $P_{\rm{c}} = 3.65\,\mathrm{d}$, accompanied by an inner super-Earth planet with $P_{\rm{b}} = 1.04\,\mathrm{d}$. We observed a full transit of planet c on 2024 May 3rd UT with Keck/KPF. The observed RM effect has an amplitude of $\sim 1\,\mathrm{m\,s}^{-1}$ and implies a sky-projected obliquity of $λ= 14^{+17}_{-19}$ degrees for HD 93963 Ac. Our dynamical analysis suggests that the two inner planets are likely well aligned with the stellar spin, to within a few degrees, thus allowing both to transit. Along with WASP-47, 55 Cnc, and HD 3167, HD 93963 is the fourth planetary system with an ultra-short-period planet and obliquity measurement(s) of any planet(s) in the system. HD 93963, WASP-47, and 55 Cnc favor largely coplanar orbital architectures, whereas HD 3167 has been reported to have a large mutual inclination ($\sim$100$^\circ$) between its transiting planets b and c. In this configuration, the probability that both planets transit is low. Moreover, one planet would quickly evolve to be non-transiting due to nodal precession. Future missions such as ESO/PLATO should detect the resulting transit duration variations. We encourage additional obliquity measurements of the HD 3167 system to better constrain its orbital architecture.

astro-ph.EP↗

arXiv2025

Global N-body Simulation of Gap Edge Structures Created by Perturbations from a Small Satellite Embedded in Saturn's Rings II: The Effect of Satellite's Orbital Eccentricity and Inclination

Pan and Daphnis are embedded in Saturn's rings and opening a gap with satellite wakes at the gap edges. Furthermore, in the case of Daphnis, pronounced vertical wall structures casting shadows on the rings are also observed in the satellite wakes. In this paper, we perform a global 3D N-body simulation with non-zero $e_{\rm s}$ or non-zero $i_{\rm s}$ of the satellite orbit to investigate how they affect the gap edge structures. We found that the effect of satellite eccentricity is important both in the satellite wakes and the vertical walls at the gap edges. The non-sinusoidal sawtooth-like satellite wakes and azimuthally more localized vertical walls observed by Cassini are simultaneously reproduced in the detailed structures and spatial scales. Both of them periodically vary due to the satellite excursions between the apocenter and the pericenter. The ring particles in outer (inner) rings that undergo closest encounters with the satellite near the apocenter (pericenter) are excited the most highly. Because the excited eccentricities of the ring particles are converted to the inclinations through physical collisions, the conversion is the most active for the particles that acquire the highest eccentricities, resulting in the azimuthally more localized vertical wall structures. The predicted height of the tallest vertical walls is $\sim 0.2$ times the satellite Hill radius in the case of the satellite eccentricity comparable to Daphnis when adopting Hill scaling, which is twice as much as the height obtained in the case of the circular satellite orbit and is quantitatively more consistent with the Cassini observation. These results show that the observed vertical walls are actually formed by the satellite wakes followed by their conversion to the vertical motions through inter-particle collisions, rather than by the out-of-plane perturbation from the satellite in an inclined orbit.

astro-ph.EP↗

arXiv2025

Oligarchic growth of protoplanets in planetesimal rings

In the standard planet formation scenario, planetesimals are assumed to form throughout the protoplanetary disk and to be smoothly distributed in the radial direction except for the snowline. Planetesimal growth has been investigated using this assumption, and the oligarchic growth model is widely accepted. However, recent simulations of gas and dust evolution have shown that planetesimals form only in radially limited locations -- such as at gas pressure bumps and snowlines -- and are concentrated in ring-like regions. When planetesimals are distributed in a ring-like region, scattered ones leak from the ring edge, resulting in planetesimal diffusion. To investigate protoplanet growth in expanding planetesimal rings, we perform a series of $\textit{ N}$-body simulations. In all the simulations, protoplanet growth is well explained by oligarchic growth, while the ring width expands due to planetesimal scattering by the protoplanets. Massive protoplanets tend to form near the ring center, and protoplanets that form far from the ring center are less massive than those in the center. The scaled orbital separations depend on neither the initial ring width nor on the total mass, and they are consistent with estimates based on the oligarchic growth model and the diffused planetesimal distribution. The width of the expanded planetesimal ring does not depend on its initial width, but it does depend on its total mass. The maximum mass of protoplanets depends strongly on the total ring mass and weakly on its initial width.

astro-ph.EP↗

arXiv2024

Dynamics of Binary Planets within Star Clusters

We develop analytical tools and perform three-body simulations to investigate the orbital evolution and dynamical stability of binary planets within star clusters. Our analytical results show that the orbital stability of a planetary-mass binary against passing stars is mainly related to its orbital period. Critical flybys, defined as stellar encounters with energy kicks comparable to the binary binding energy, can efficiently produce a wide range of semimajor axes ($a$) and eccentricities ($e$) from a dominant population of primordially tight JuMBOs. The critical flyby criterion we derived offers an improvement over the commonly used tidal radius criterion, particularly in high-speed stellar encounters. Applying our results to the recently discovered Jupiter-Mass Binary Objects (JuMBOs) by the James Webb Space Telescope (JWST), our simulations suggest that to match the observed $\sim$9% wide binary fraction, an initial semimajor axis of $a_0 \sim$ 10-20 au and a density-weighted residence time of $χ\gtrsim 10^4$ Myr pc$^{-3}$ are favored. These results imply that the JWST JuMBOs probably formed as tight binaries near the cluster core.

astro-ph.EP↗

arXiv2024

On the elastoplastic behavior in collisional compression of spherical dust aggregates

Aggregates consisting of submicron-sized cohesive dust grains are ubiquitous, and understanding the collisional behavior of dust aggregates is essential. It is known that low-speed collisions of dust aggregates result in either sticking or bouncing, and local and permanent compaction occurs near the contact area upon collision. In this study, we perform numerical simulations of collisions between two aggregates and investigate their compressive behavior. We find that the maximum compression length is proportional to the radius of aggregates and increases with the collision velocity. We also reveal that a theoretical model of contact between two elastoplastic spheres successfully reproduces the size- and velocity-dependence of the maximum compression length observed in our numerical simulations. Our findings on the plastic deformation of aggregates during collisional compression provide a clue to understanding the collisional growth process of aggregates.

cond-mat.soft↗

arXiv2024

Molecular dynamics simulations of head-on low-velocity collisions between particles

The particle contact model is important for powder simulations. Although several contact models have been proposed, their validity has not yet been well established. Therefore, we perform molecular dynamics (MD) simulations to clarify the particle interaction. We simulate head-on collisions of two particles with impact velocities less than a few percent of the sound velocity to investigate the dependence of the interparticle force and the coefficient of restitution (COR) on the impact velocity and particle radius. In this study, we treat particles with a radius of 10-100 nm and perform simulations. We find that the interparticle force exhibits hysteresis between the loading and unloading phases. Larger impact velocities result in strong hysteresis and plastic deformation. For all impact velocities and particle radii, the coefficient of restitution is smaller than that given by the Johnson-Kendall-Robert theory. An inelastic contact model cannot reproduce our MD simulations. In particular, the COR is significantly reduced when the impact velocity exceeds a certain value. This significant energy dissipation cannot be explained even by the contact models including plastic deformation. We also find that the COR increases with increasing particle radius. We also find that the previous contact models including plastic deformation cannot explain the strong energy dissipation obtained in our MD simulations, although they agree with the MD results for very low impact velocities. Accordingly, we have constructed a new dissipative contact model in which the dissipative force increases with the stress generated by collisions. The new stress dependent model successfully reproduces our MD results over a wider range of impact velocities than the conventional models do. In addition, we proposed another, simpler, dissipative contact model that can also reproduce the MD results.

cond-mat.soft↗

arXiv2024

Global N-body Simulation of Gap Edge Structures Created by Perturbations from a Small Satellite Embedded in Saturn's Rings

Observations by the Voyager and Cassini spacecrafts have revealed various striking features of the gap structure in Saturn's ring, such as the density waves, sharp edge, and vertical wall structure. In order to explain these features in a single simulation, we perform a high-resolution (N~10^6-10^7) global full N-body simulation of gap formation by an embedded satellite considering gravitational interactions and inelastic collisions among all ring particles and the satellite, while these features have been mostly investigated separately with different theoretical approaches: the streamline models, 1D diffusion models, and local N-body simulation. As a first attempt of a series of papers, we here focus on the gap formation by separating satellite migration with fixing the satellite orbit in a Keplerian circular orbit. We reveal how the striking gap features - the density waves, sharp edge, and vertical wall structure - are simultaneously formed by an interplay of the satellite-ring and ring particle-particle interactions. In particular, we propose a new mechanism to quantitatively explain the creation of the vertical wall structure at the gap edge. Inelastic collisions between ring particles damp their eccentricity excited by the satellite's perturbations to enhance the surface density at the gap edge, making its sharp edges more pronounced. We find the eccentricity damping process inevitably raises the vertical wall structures the most effectively in the second epicycle waves. Particle-particle collisions generally convert their lateral epicyclic motion into vertical motion. Because the excited epicyclic motion is the greatest near the ring edge and the epicycle motions are aligned in the first waves, the conversion is the most efficient in the gap edge of the second waves and the wall height is scaled by the satellite Hill radius, which are consistent with the observations.

astro-ph.EP↗

arXiv2023

Two long-period giant planets around two giant stars: HD 112570 and HD 154391

We present the discoveries of two giant planets orbiting the red giant branch (RGB) star HD 112570 and the red clump (RC) star HD 154391, based on the radial velocity (RV) measurements from Xinglong station and Okayama Astrophysical Observatory (OAO). Spectroscopic and asteroseismic analyses suggest that HD 112570 has a mass of $1.15\pm0.12\,M_{\odot}$, a radius of $9.85\pm0.23\,R_{\odot}$, a metallicity [Fe/H] of $-0.46\pm0.1$ and a ${\rm log}\,g$ of $2.47\pm0.1$. With the joint analysis of RV and Hipparcos-Gaia astrometry, we obtain a dynamical mass of $M_{\rm p}={3.42}_{-0.84}^{+1.4}\ M_{\rm Jup}$, a period of $P={2615}_{-77}^{+85}$ days and a moderate eccentricity of $e={0.20}_{-0.14}^{+0.16}$ for the Jovian planet HD 112570 b. For HD 154391, it has a mass of $2.07\pm0.03\,M_{\odot}$, a radius of $8.56\pm0.05\,R_{\odot}$, a metallicity [Fe/H] of $0.07\pm0.1$ and a ${\rm log}\,g$ of $2.86\pm0.1$. The super-Jupiter HD 154391 b has a mass of $M_{\rm p}={9.1}_{-1.9}^{+2.8}\ M_{\rm Jup}$, a period of $P={5163}_{-57}^{+60}$ days and an eccentricity of $e={0.20}_{-0.04}^{+0.04}$. We found HD 154391 b has one of the longest orbital period among those ever discovered orbiting evolved stars, which may provide a valuable case in our understanding of planetary formation at wider orbits. Moreover, while a mass gap at $4\,M_{\rm Jup}$ seems to be present in the population of giant stars, there appears to be no significant differences in the distribution of metallicity among giant planets with masses above or below this threshold. Finally, The origin of the abnormal accumulation near 2 au for planets around large evolved stars ($R_{\star}>21\,R_{\odot}$), remains unclear.

astro-ph.EP↗

arXiv2023

Formation of inner planets in the presence of a Cold Jupiter: orbital evolution and relative velocities of planetesimals

We investigate the orbital evolution of planetesimals in the inner disk in the presence of nebula gas and a (proto-) cold Jupiter. By varying the mass, eccentricity, and semi-major axis of the planet, we study the dependence of the relative velocities of the planetesimals on these parameters. For classic small planetesimals ($10^{16}-10^{20} $g) whose mutual gravitational interaction is negligible, gas drag introduces a size-dependent alignment of orbits and keeps the relative velocity low for similar-size bodies, while preventing orbital alignment for different-size planetesimals. Regardless of the location and the mass ratio of the planetesimals, increasing the mass and eccentricity or decreasing the orbital distance of the planet always leads to higher relative velocities of planetesimals. However, for massive planetesimals, the interplay of viscous stirring, gas damping, and secular perturbation results in lower velocity dispersion of equal-size planetesimals when the planet is more massive or when it is located on a closer or more eccentric orbit. The random velocities of such planetesimals remain almost unperturbed when the planet is located beyond Jupiter's current orbit, or when it is less massive or less eccentric than Jupiter. Unlike small planetesimals, such large planetesimals can grow in a runaway fashion as in the unperturbed case. Our results imply that the presence of a cold Jupiter does not impede the formation of inner rocky planets through planetesimal accretion, provided that the planetesimals are initially large.

astro-ph.EP↗