arXiv subjects

Shigeru Ida

Publications and source records attributed to Shigeru Ida.

At least 19 recordsLinked to original sources

arXiv2026

Distribution of Chemically-Processed Dust in a Viscously Evolving Protoplanetary Disk: Application to Crystalline Silicates in Comets

Dust particles undergo chemical reactions in protoplanetary disks according to their environments, producing compositional diversity in planetary materials. Extraterrestrial records of irreversible reactions, such as crystallization of amorphous silicates, provide particularly strong constraints on the early evolution of the protosolar disk. In this study, we investigate such irreversible reactions and the spatiotemporal distribution of reacted dust in a viscously evolving disk using Monte Carlo particle-tracking simulations. We extend a predictive formula for the temperature at which irreversible reactions proceed efficiently ("reaction line"), originally developed for steady accretion disks, to viscously expanding disks. The spatiotemporal distribution of reacted dust is governed by the relative locations of the reaction line and the stagnation line, which separates inward and outward advection in the disk. The reaction line moves inward as the disk cools, while the stagnation line moves outward owing to the radial viscous spreading of the disk. When the reaction line lies far inside the stagnation line, the reacted dust remains inside the reaction line. On the other hand, when the reaction line lies near or beyond the stagnation line, the reacted dust located near the stagnation line or between the two lines is transported outward efficiently. It results in a radially broad distribution of reacted dust throughout the disk, including the outer regions where the temperatures remain too low for reactions. We assessed the disk conditions consistent with the crystalline silicates observed in Solar System comets and found that the protosolar disk was likely compact, moderately massive, and not strongly turbulent.

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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

Ring formation around giant planets by tidal disruption of a single passing large Kuiper belt object II: The dynamical fate of tidal fragments

Planetary rings are ubiquitous structure in our Solar System, but their formation mechanisms remain under debate. One of the proposed scenarios is the tidal disruption of a nearby passing body that enters within a planet's Roche limit, producing fragments that are gravitationally captured and finally form the rings. In this study, we investigate the detailed dynamical path and fate of such tidally captured fragments using direct Nbody simulations including collisional fragmentation with analytical arguments. Focusing on Saturn as a representative case, we explore how the inclination iTD and pericenter distance qTD of the orbit of the passing body control the subsequent orbital evolution, collisional grinding, and the survival of fragments mass. Our simulations show that initially highly eccentric and inclined fragments experience differential precession driven by the planet's J2 potential, followed by destructive high-velocity collisions that damp their eccentricities and inclinations. The timing and pathway of this evolution strongly depend on iTD, modifying the dynamical picture proposed in the previous work. For low to moderate iTD, a narrow, circular and equatorial rings finally form whose orbital radius is well predicted by an analytically derived equivalent circular radius based on the conservation of the vertical component of angular momentum. In contrast, for high iTD, collisional damping causes a substantial fraction of the material to fall onto the planet, preventing the formation of a massive ring. We compile our results of Nbody simulations with the analytical predictions on (qTD, iTD) parameter space and specify the parameter region where sufficient mass to form Saturn's present rings and inner satellites survives.

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arXiv2026

A Robust Launching Mechanism for Freely-Floating Planets from Host Stars with Close-in Planets

Secular perturbations from binary stars and distant massive planets can drive cold planets onto nearly parabolic orbits with pericenter passages extremely close to their host stars. Meanwhile, short-period super-Earths are frequently observed around nearby stars. Gravitational scattering between these two distinct populations can lead to substantial orbital energy exchange, liberating some intruders from the gravitational confinement of their host systems. This process offers a robust formation channel for a subset of the abundant freely floating planet population. It may also significantly perturb the original orbits of close-in planets, induce collisional trajectories between close-in planets and their host stars, and disrupt the dynamical evolution of cold planets toward close stellar encounters.

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arXiv2025

Outward Migration of a Gas Accreting Planet: A Semi-Analytical Formula

Type II orbital migration is a key process to regulate the mass and semimajor axis distribution of exoplanetary giant planets. The conventional formula of type II migration generally predicts too rapid inward migration to reconcile with the observed pile-up of gas giant beyond 1 au. Analyzing the recent high-resolution hydrodynamical simulations by Li et al. (2024) and Pan et al. (2025) that show robust outward migration of a gas accreting planet, we here clarify the condition for the outward migration to occur and derive a general semi-analytical formula that can be applied for broad range of planet mass and disk conditions. The striking outward migration is caused by azimuthal asymmetry in corotation torque exerted from cicumplanetary disk regions (connecting to horseshoe flow) that is produced by the planetary gas accretion, while the conventional inward migration model is based on radial asymmetry in the torques from the circumstellar protoplanetry disk. We found that the azimuthal asymmetry dominates and the migration is outward, when the gap depth defined by the surface density reduction factor of $1/(1+K')$ is in the range of $0.03 \lesssim K' \lesssim 50$. Using simple models with the new formula, we demonstrate that the outward migration plays an important role in shaping the mass and semimajor axis distribution of gas giants. The concurrent dependence of planets' accretion rate and migration direction on their masses and disk properties potentially reproduces the observed pile-up of exoplanetary gas giants beyond 1 au, although more detailed planet population synthesis calculations are needed in the future.

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arXiv2025

Concurrent Accretion and Migration of Giant Planets in their Natal Disks with Consistent Accretion Torque (II): Parameter Survey and Condition for Outward Migration

Migration typically occurs during the formation of planets and is closely linked to the planetary formation process. In classical theories of non-accreting planetary migration, both type I and type II migration typically result in inward migration, which is hard to align with the architecture of the planetary systems.In this work, we conduct systematic, high-resolution 3D/2D numerical hydrodynamic simulations to investigate the migration of an accreting planet. Under different disk conditions, we compared the dynamical evolution of planets with different planet-to-star mass ratios. We find that accretion of planets can significantly diminish the inward migration tendency of planets, or even change the migration direction. The migration of low-/high-mass planets is classified as Type I/II inward migration, respectively, while intermediate-mass planets, which have the strongest accretion, show an outward migration trend. We confirm that the outward migration is mainly attributed to the positive torque from the azimuthal asymmetric structures around the accreting planet, similar to Li et al. (2024). The termination of planetary mass growth is thus synonymous with the transition from outward to inward migration. For the high viscosity $\alpha=0.04$ and disk aspect ratio height $h_0=0.05$ cases, the mass ratio range for planetary outward migration is $1\times10^{-4}\lesssim q\lesssim4\times10^{-3}$. For the low viscosity case with $\alpha=0.001$, and/or the low disk aspect ratio cases $h_0=0.03$, the mass ratio range for the outward migration will shift toward the lower end. Our parameter survey reveals that a simple gap opening parameter determines the outward migration condition; details of the analytical interpretation are presented in Ida et al. (2025).

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arXiv2025

Formation of Free-Floating Planets via Ejection: Population Synthesis with a Realistic IMF and Comparison to Microlensing Observations

Microlensing observations suggest that the mass distribution of free-floating planets (FFPs) follows a declining power-law with increasing mass. The origin of such distribution is unclear. Using a population synthesis framework, we investigate the formation channel and properties of FFPs, and compare the predicted mass function with observations. Assuming FFPs originate from planet-planet scattering and ejection in single star systems, we model their mass function using a Monte Carlo based planet population synthesis model combined with N-body simulations. We adopt a realistic stellar initial mass function, which naturally results in a large fraction of planetary systems orbiting low-mass stars. The predicted FFP mass function is broadly consistent with observation: it follows the observed power-law at higher masses ($10 \lesssim m/M_\oplus < 10^4$), while at lower masses ($0.1 < m/M_\oplus \lesssim 10$) it flattens, remaining marginally consistent with the lower bound of the observational uncertainties. Low-mass, close-in planets tend to remain bound, while Neptune-like planets at wide orbits dominate the ejected population due to their large Hill radii and shallow gravitational binding. We also compare the mass distribution of bound planets with microlensing observations and find reasonably good agreement with both surveys. Our model predicts $\simeq 1.20$ ejected planets per star in the mass range of $0.33 < m/M_\oplus < 6660$, with a total FFP mass of $\simeq 17.98~M_\oplus$ per star. Upcoming surveys will be crucial in testing these predictions and constraining the true nature of FFP populations.

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arXiv2025

Ringed versus Ringless Worlds: How Poynting-Robertson Drag Shapes Rings across the Solar System

Planetary rings are not only ubiquitous around the giant planets in the outer Solar System, but have also been discovered around several small distant bodies. In contrast, no rings have been observed around any inner Solar System objects. To constrain the dynamical origin of this ringed-versus-ringless dichotomy, we employ a numerically cross-checked analytical model of gigayear-scale Poynting-Robertson (PR) drag due to the solar flux acting on an isolated particle, expressed as a function of the host body's heliocentric distance $\,a_{\mathrm{pla}}$ and the particle radius $\,r_{\mathrm{par}}$. Here we show that, in the absence of additional perturbations, PR drag alone can explain the observed ring architecture of the Solar System: outer planets and Centaurs/TNOs are able to retain rings for the age of the Solar System, whereas any rings around the inner planets are removed on much shorter timescales. Because the PR-drag lifetime scales steeply with heliocentric distance $\bigl(\tau_{\mathrm{decay}}\propto a_{\mathrm{pla}}^{2} \,r_{\mathrm{par}}\bigr)$, we predict that forthcoming surveys will reveal an ever-growing population of ring-bearing bodies in the distant Solar System.

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arXiv2025

Exploring Impact Vapor Plume Reactions from Asteroidal Impacts: Monte Carlo Simulations and Implications for Biomolecules Synthesis

During a hypervelocity impact, both the impactor and target materials evaporate, generating an impact vapor plume with temperatures reaching several thousand K. As the plume cools through adiabatic expansion, chemical reactions are predicted to quench, leading to a non-equilibrium composition. However, it is still unclear how chemical reactions proceed during the cooling impact vapor plume and lead to the synthesis of organic molecules. In this study, to investigate the evolution of chemical composition within impact vapor plumes, we conducted a Monte Carlo chemical reaction simulation for complex organic synthesis, developed in our previous work. Our model does not rely on a predefined reaction network; instead, it utilizes imposed conditions for chemical changes and an approximate method for calculating reaction rates suited to our objectives. Additionally, we developed a new approach to couple these chemical reaction calculations with the rapid temperature and pressure decay in the vapor plume. Results show diverse organic molecule production depending on the impactor materials assumed in this study. These products include important precursors to biomolecules such as amino acids, sugars, and nucleobases. On the other hand, for all impactor compositions, the abundance of biomolecules themselves remains extremely low throughout the reactions from an impact to quenching. Therefore, our results suggest that biomolecules are not directly produced in impact vapor plumes but rather synthesized through reactions of these precursor molecules in aqueous solutions, following H2O condensation as the vapor plume cools. Many of the detected organic compounds, including the precursor molecules such as imine compounds and formamide, are not included in the reaction networks of previous kinetic model simulations, and their formation has not been predicted.

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arXiv2025

A New Brown Dwarf Orbiting an M star and An Investigation on the Eccentricity Distribution of Transiting Long-Period Brown Dwarfs

The orbital eccentricities of brown dwarfs encode valuable information of their formation and evolution history, providing insights into whether they resemble giant planets or stellar binaries. Here, we report the discovery of TOI-5575b, a long-period, massive brown dwarf orbiting a low-mass M5V star ($\rm 0.21\pm0.02\,M_\odot$) delivered by the TESS mission. The companion has a mass and radius of $\rm 72.4\pm4.1\,M_J$ and $\rm 0.84\pm0.07\,R_J$ on a 32-day moderately eccentric orbit ($e=0.187\pm0.002$), making it the third highest-mass-ratio transiting brown dwarf system known to date. Building on this discovery, we investigate the eccentricity distributions of a sample of transiting long-period ($10\leq P\lesssim 1000$ days, $\sim$0.1-1.5 AU) giant planets, brown dwarfs and low-mass stars. We find that brown dwarfs exhibit an eccentricity behavior nearly identical to that of giant planets: a preference for circular orbits with a long tail toward high eccentricities. Such a trend contrasts sharply with direct imaging findings, where cold (5-100 AU) brown dwarfs and giant planets display distinct eccentricity distributions. Our results suggest that transiting long-period brown dwarfs and giant planets probably 1) form in different routes at exterior orbits but undergo analogous dynamical evolution processes and migrate inwards; or 2) both contain two sub-groups, one with widely spread eccentricities while the other has circular orbits, that jointly sculpt the eccentricity distributions. The low-mass-star systems appear to be a distinctive population, showing a peak eccentricity at about 0.3, akin to more massive stellar binaries.

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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.

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arXiv2025

A Population Synthesis Study on the Formation of Cold Jupiters from Truncated Planetesimal Disks

The occurrence rate of giant planets increases with orbital period and turns over at a location that roughly corresponds to the snow line of solar-type stars. Further, the density distribution of cold Jupiters (CJs) on the semi-major axis - mass diagram shows a relatively steep inner boundary, shaping the desert of warm Jupiters. The eccentricities of CJs show a broad distribution with a decreasing number density towards the larger end. Previous planet formation models fail to reproduce all these features at the same time. We use a planet population synthesis (PPS) model with truncated initial planetesimal distribution and compare the mass and orbital distribution of the simulated planets with the observation. We show that the occurrence of CJs with respect to the orbital period, the slope of the inner boundary of CJs on the semi-major axis - mass diagram, and the eccentricity distribution of CJs agree reasonably well with observation, if CJs form from truncated planetesimal disks of 10 au or wider with suppressed migration. While PPS simulations generally overestimate the fraction of giants with eccentricity below 0.2, $N$-body simulations produce a more consistent eccentricity distribution with observation. While the fraction of high-eccentricity planets can be increased by widening the planetesimal disk or reducing the migration speed, a deficit of giants with eccentricity between 0.2-0.4 exists regardless of the choices of parameters. Our results indicate that CJs are more likely born in truncated disks near the snow line than in classical uniform disks.

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arXiv2024

Effects from different grades of stickiness between icy and silicate particles on carbon depletion in protoplanetary disks

Earth and other rocky bodies in the inner Solar System are significantly depleted in carbon compared to the Sun and interstellar medium (ISM) dust. Observations indicate that over half of carbon in the ISM and comets is in refractory forms, like amorphous hydrocarbons and complex organics, which can be building blocks of rocky bodies. While amorphous hydrocarbons are destroyed by photolysis and oxidation, radial transport of solid particles can limit carbon depletion, except when complex organics, which are less refractory, are the main carbon source. We aim to identify conditions for severe carbon depletion in the inner Solar System by introducing more realistic factors: differences in stickiness between icy and silicate particles, and high-temperature regions in the disk's upper optically-thin layer, which were not considered in previous studies. We perform a 3D Monte Carlo simulation of radial drift and turbulent diffusion in a steady accretion disk, incorporating ice evaporation/re-condensation, photolysis/oxidation of hydrocarbons in the upper layer, and pyrolysis of complex organics. Our results show that the carbon fraction drops by two orders of magnitude inside the snow line under two conditions: i) silicate particles are much less sticky than icy particles, leading to a rapid decline in icy pebble flux while silicates accumulate inside the snow line, and ii) high-temperature regions in the disk's upper layer stir silicate particles into UV-exposed areas. These conditions reproduce carbon depletion patterns consistent with observations and allow for diverse carbon fractions in rocky bodies. This diversity may explain the wide variation of metals in white dwarf photospheres and suggest different surface environments for rocky planets in habitable zones.

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arXiv2024

A search for water vapor plumes on Europa by spatially resolved spectroscopic observation using Subaru/IRCS

We present near-infrared high-dispersion spectroscopic observations of Europa using the Infrared Camera and Spectrograph (IRCS) onboard the Subaru Telescope, seeking direct evidence of water plumes on Europa and exploring spatial variations in plume activity. Using high spectral/spatial resolution and sensitivity of Subaru/IRCS, our observations have enabled a spatially resolved search for water plumes on Europa. Within our detection limits and time of observation, we found no evidence for the presence of water emission. For a rotation temperature of 50 K, we derived an upper limit on the H$_{2}$O abundance of 9.46$\times$10$^{19}$ - 5.92$\times$10$^{20}$ m$^{-2}$ in each divided slit area and 4.61$\times$10$^{19}$ m$^{-2}$ in the entire area covered by the slit. This upper limit lies below the inferred water abundance from previous UV observations by the Hubble Space Telescope (HST), while being less sensitive by a factor of three compared to the Keck telescope and by one order of magnitude or more than the James Webb Space Telescope (JWST) observations. Our results align with previous studies and demonstrate that using Subaru/IRCS is an effective strategy for searching for water plumes on Europa with high spatial resolution. Continued observations across different surface areas and orbital phases are essential to fully characterize Europa's plume activity and complement upcoming space missions.

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arXiv2024

Forming planetary systems that contain only minor planets

Estimates of the frequency of planetary systems in the Milky Way are observationally limited by the low-mass planet regime. Nevertheless, substantial evidence for systems with undetectably low planetary masses now exist in the form of main-sequence stars which host debris discs, as well as metal-polluted white dwarfs. Further, low-mass sections of star formation regions impose upper bounds on protoplanetary disc masses, limiting the capacity for terrestrial or larger planets to form. Here, we use planetary population synthesis calculations to investigate the conditions that allow planetary systems to form only minor planets and smaller detritus. We simulate the accretional, collisional and migratory growth of $10^{17}$ kg embryonic seeds and then quantify which configurations with *entirely* sub-Earth-mass bodies ($\lesssim 10^{24}$ kg) survive. We find that substantial regions of the initial parameter space allow for sub-terrestrial configurations to form, with the success rate most closely tied to the initial dust mass. Total dust mass budgets of up to $10^2 M_{\oplus}$ within 10 au can be insufficiently high to form terrestrial or giant planets, resulting in systems with only minor planets. Consequently, the prevalence of planetary systems throughout the Milky Way might be higher than what is typically assumed, and minor planet-only systems may help inform the currently uncertain correspondence between planet-hosting white dwarfs and metal-polluted white dwarfs.

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arXiv2024

Monte Carlo simulation of UV-driven synthesis of complex organic molecules on icy grain surfaces

Complex organic molecules (COMs) have been widely observed in molecular clouds and protostellar environments. One of the formation mechanisms of COMs is radical reactions on the icy grain surface driven by UV irradiation. While many experiments have reported that various COMs can be synthesized under such ice conditions, the majority of the reaction processes are unclear. Complementary numerical simulations are necessary to unveil the synthetic process behind the formation of COMs. In this study, we develop a chemical reaction simulation using a Monte Carlo method. To explore the complex reaction network of COM synthesis, the model was designed to eliminate the need to prepare reaction pathways and to keep computational costs low. With this simulation, we investigate the chemical reactions occurring on icy dust surfaces during and after UV irradiation, assuming a protoplanetary disk environment. We aim to reveal the types of organic molecules produced in a disk and the formation mechanisms of COMs, in particular, amino acids and sugars. The results show that photodissociation and subsequent radical-radical reactions cause random rearrangement of the covalent bonds in the initial molecules composed of methanol, formaldehyde, ammonia, and water. Consequently, highly complex molecules such as amino acids and sugars were produced in a wide range of the initial conditions. We found that the final abundances of amino acids and sugars have extremely similar dependence on the atomic ratios of the initial molecules, which peak at C/H~0.1-0.3 and O/H~0.3-0.5, although the amino acids abundance is usually more than ten times higher than that of sugars. To understand this dependence, a semi-analytical formula was derived. Additionally, parameter surveys have suggested that the decomposition reactions of amino acids and sugars undergo a rapid transition within the threshold of a given parameter.

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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.

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arXiv2023

Large planets may not form fractionally large moons

One of the unique aspects of Earth is that it has a fractionally large Moon, which is thought to have formed from a Moon-forming disk generated by a giant impact. The Moon stabilizes the Earth's spin axis at least by several degrees and contributes to Earth's stable climate. Given that impacts are common during planet formation, exomoons, which are moons around planets in extrasolar systems, should be common as well, but no exomoon has been confirmed. Here we propose that an initially vapor-rich moon-forming disk is not capable of forming a large moon because growing moonlets, which are building blocks of a moon, experience strong gas drag and quickly fall toward the planet. Our impact simulations show that terrestrial and icy planets that are larger than $\sim 1.3-1.6 R_\oplus$ produce entirely vapor disks, which fail to form a large moon. This indicates that (1) our model supports the Moon-formation models that produce vapor-poor disks and (2) rocky and icy exoplanets whose radii are smaller than $\sim 1.6 R_\oplus$ are ideal candidates for hosting fractionally large exomoons.

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