arXiv subjects

Takanori Sasaki

Publications and source records attributed to Takanori Sasaki.

17 recordsLinked to original sources

arXiv2026

Dependence on the Equation of State in SPH Simulations of Proto-Uranian Disk Formation from a Giant Impact

The $98^\circ$ obliquity of Uranus is widely attributed to a giant impact that ejected material and formed a debris disk, which subsequently coalesced into its regular satellites. Previous Smoothed Particle Hydrodynamics (SPH) studies have yielded inconsistent disk compositions, a discrepancy often linked to the variety of numerical and physical modeling assumptions. We address this by presenting SPH simulations that systematically test three distinct EOS models alongside two SPH schemes (standard SPH, and the enhanced density-independent SPH). We utilized a $3M_{\oplus}$ impactor and explored a range of impact parameters which are capable of reproducing Uranus's current spin state. Our primary finding is that for impacts capable of reproducing Uranus's current rotation, the choice of EOS or SPH scheme barely affects macroscopic features such as the post-impact rotation period, disk mass, or disk size; these properties are primarily controlled by the impact's angular momentum. In contrast, the disk's rock fraction is highly EOS-dependent. Our results clarify that while disk mass and size are robust outcomes, the final disk composition is highly model-dependent. Therefore, accurate EOS modeling, integrated with detailed disk evolution studies, is essential to definitively validate the giant impact scenario for Uranus.

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arXiv2025

Multiple Giant Impacts and Chemical Equilibria: An Integrated Approach to Rocky Planet Formation

During the formation of rocky planets, the surface environments of growing protoplanets were dramatically different from those of present-day planets. The release of gravitational energy during accretion would have maintained a molten surface layer, forming a magma ocean. Simultaneously, sufficiently massive protoplanets could acquire hydrogen-rich proto-atmospheres by capturing gas from the protoplanetary disk. Chemical equilibration among the atmosphere, magma ocean, and iron core plays a key role in determining the planet's interior composition. In this study, we investigate terrestrial planet formation under such primitive surface conditions. We conduct N-body simulations to model the collisional growth from protoplanets to planets, coupled with chemical equilibrium calculations at each giant impact event, where surface melting occurs. Our results show that planetary growth proceeds through a series of giant impacts, and the timing of these impacts relative to the dissipation of disk gas significantly influences the volatile budget. In particular, initial impacts, occurring while nebular gas is still present, can lead to excess hydrogen incorporation into the protoplanet's core. Subsequent impacts with hydrogen-poor bodies, after gas dispersal, can dilute this hydrogen content. This process allows for the formation of a planet with a hydrogen inventory consistent with Earth's current core. Our findings suggest that late giant impacts, occurring after the depletion of nebular gas, provide a viable mechanism for producing Earth-like interior compositions near 1 AU.

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arXiv2024

Modifications of SPH towards three-dimensional simulations of an icy moon with internal ocean

There are some traces of the existence of internal ocean in some icy moons, such as the vapor plumes of Europa and Enceladus. This implies a region of liquid water beneath the surface ice shell. Since liquid water would be essential for the origin of life, it is important to understand the development of these internal oceans, particularly their temperature distribution and evolution. The balance between tidal heating and radiative cooling is believed to sustain liquid water beneath an icy moon's surface. We aim to simulate the tidal heating of an internal ocean in an icy moon using 3-dimensional numerical fluid calculations with the Smoothed Particle Hydrodynamics (SPH) method. We incorporated viscosity and thermal conduction terms into the governing equations of SPH. However, we encountered two issues while calculating rigid body rotation using SPH with a viscous term: (1) conventional viscosity formulations generated unphysical forces that hindered rotation, and (2) there was artificial internal energy partitioning within the layered structure, which was due to the standard SPH formulations. To address the first issue, we modified the viscosity formulation.For the second, we adopted Density Independent SPH (DISPH) developed in previous studies to improve behavior at discontinuous surfaces. Additionally, we implemented radiative cooling using an algorithm to define fluid surfaces via the particle method. We also introduced an equation of state accounting for phase transitions. With these modifications, we have refined the SPH method to encompass all necessary physical processes for simulating the evolution of icy moons with internal oceans.

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arXiv2022

Orbital Evolution of Close-in Super-Earths Driven by Atmospheric Escape

The increasing number of super-Earths close to their host stars revealed a scarcity of close-in small planets with 1.5-2.0$\,R_\oplus$ in the radius distribution of ${\it Kepler}$ planets. The atmospheric escape of super-Earths by photoevaporation can explain the origin of the observed "radius gap." Many theoretical studies considered the in-situ mass loss of a close-in planet. Planets that undergo the atmospheric escape, however, move outward due to the change in the orbital angular momentum of their star-planet systems. In this study, we calculate the orbital evolution of an evaporating super-Earth with a H$_2$/He atmosphere around FGKM-type stars under a stellar X-ray and extreme UV irradiation (XUV). The rate of increase in the orbital radius of an evaporating planet is approximately proportional to that of the atmospheric mass loss during a high stellar XUV phase. We show that super-Earths with a rocky core of $\lesssim$ 10$\,M_\oplus$ and a H$_2$/He atmosphere at $\lesssim$ 0.03-0.1$\,$au ($\lesssim$ 0.01-0.03$\,$au) around G-type stars (M-type stars) are prone to the outward migration driven by photoevaporation. Although the changes in the orbits of the planets would be small, they would rearrange the orbital configurations of compact, multi-planet systems, such as the TRAPPIST-1 system. We also find that the radius gap and the so-called "Neptune desert" in the observed population of close-in planets around FGK-type stars still appear in our simulations. On the other hand, the observed planet population around M-type stars can be reproduced only by a high stellar XUV luminosity model.

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arXiv2020

N-body Simulations of Ring Formation Process around the Dwarf Planet Haumea

Haumea is the only known trans-Neptunian object with a ring. The ring is in a position that causes a 3:1 spin-orbit resonance with the rotational period of Haumea, which has a triaxial shape. The non-axisymmetric gravitational field around Haumea is thought to affect the dynamics of the ring; however, the process of ring formation has not been elucidated. In this study, we analyze in some detail a potential ring formation scenario for Haumea. We first calculated the gravitational field around the triaxial ellipsoid and estimated the distance at which an object revolving around Haumea can exist stably using simulation that incorporated the time-varying gravitational field. The results of this simulation showed that the trajectory of the object became unstable just inside its current ring position. Next, we analytically derived the Roche radius for a rigid body revolving around a triaxial ellipsoid and showed that the Roche radius could be near the current ring position. Furthermore, as a parameter study, we performed N-body simulations using the coefficient of rubble pile restitution as a variable. Results demonstrated that, according to numerous parameters, the position of the Roche radius was near the current position of Haumea's ring. Based on these findings, we can assume that there is a high possibility that the ring formed in the region between the boundary of the unstable region of the orbit and the Roche radius. The scenario presented in this study could help explain the process by which Haumea's ring formed.

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arXiv2020

Uranian Satellite Formation by Evolution of a Water Vapor Disk Generated by a Giant Impact

The ice-giant planet Uranus likely underwent a giant impact, given that its spin axis is tilted by 98 degrees. That its satellite system is equally inclined and prograde suggests that it was formed as a consequence of the impact. However, the disks predicted by the impact simulations generally have sizes one order smaller and masses two orders larger than those of the observed system at present. Here we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. Because the vaporization temperature of water ice is low and both Uranus and the impactor are assumed to be ice-dominated, we can conclude that the impact-generated disk has mostly vaporized. We predict that the disk lost a significant amount of water vapour mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin. From the predicted distribution of condensed ices, our N-body simulation is able to reproduce the observed mass-orbit configuration of Uranian satellites. This scenario contrasts with the giant-impact model for the Earth's Moon, in which about half of the compact, impact-generated, solid or liquid disk is immediately incorporated into the Moon on impact.

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arXiv2019

The Galilean Satellites Formed Slowly from Pebbles

It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter's formation. However, understanding the specifics of the formation process is challenging as both small particles (pebbles) as well as the satellites are subject to fast migration processes. Here, we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede's mass is determined by pebble isolation. Our slow-pebble-accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties.

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arXiv2019

Can the Uranian Satellites Form from a Debris Disk Generated by a Giant Impact?

Hydrodynamic simulations of a giant impact to proto-Uranus indicated that such an impact could tilt its rotational axis and produce a circumplanetary debris disk beyond the corotation radius of Uranus. However, whether Uranian satellites can actually be formed from such a wide disk remains unclear. Herein, we modeled a wide debris disk of solids with several initial conditions inferred from the hydrodynamic simulations, and performed $N$-body simulations to investigate in-situ satellite formation from the debris disk. We also took account of orbital evolutions of satellites due to the planetary tides after the growth of satellites. We found that, in any case, the orbital distribution of the five major satellites could not be reproduced from the disk as long as the power index of its surface density is similar to that of the disk generated just after the giant impact: Satellites in the middle region obtained much larger masses than Ariel or Umbriel, while the outermost satellite did not grow to the mass of Oberon. Our results indicate that we should consider the thermal and viscous evolution of the evaporated disk after the giant impact to form the five major satellites through the in-situ formation scenario. On the other hand, the small inner satellites would be formed from the rings produced by the disrupted satellites which migrated from around the corotation radius of Uranus due to the planetary tides.

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arXiv2019

Orbital Evolution of a Circumbinary Planet in a Gaseous Disk

Sub-Jupiter classed circumbinary planets discovered in close-in binary systems have orbits just beyond the dynamically unstable region, which is determined by the eccentricity and mass ratio of the host binary stars. These planets are assumed to have formed beyond the snow line and migrated to the current orbits rather than forming in situ. We propose a scenario in which a planet formed beyond the snow line and migrated to the inner edge of the circumbinary disk, which was within the unstable area, and then moved to the current orbit through outward transportation. This outward transportation is driven by the balance of orbital excitation of the central stars inside the gravitationally unstable region and damping by the gas-drag force. We carried out N-body simulations with a dissipating circumbinary protoplanetary disk for binary systems with different eccentricities and mass ratios. Planets are more likely to achieve a stable orbit just beyond the unstable region in less eccentric binary systems. This result is not as sensitive to mass ratio as it is to eccentricity. These dependencies are consistent with the data from observed binary systems hosting circumbinary planets. We find CBPs' orbits close to the instability boundaries are explained by our orbital evolution scenario.

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arXiv2019

Impact of Stellar Superflares on Planetary Habitability

High-energy radiation caused by exoplanetary space weather events from planet-hosting stars can play a crucial role in conditions promoting or destroying habitability in addition to the conventional factors. In this paper, we present the first quantitative impact evaluation system of stellar flares on the habitability factors with an emphasis on the impact of Stellar Proton Events. We derive the maximum flare energy from stellar starspot sizes and examine the impacts of flare associated ionizing radiation on CO$_2$, H$_2$, N$_2$+O$_2$ --rich atmospheres of a number of well-characterized terrestrial type exoplanets. Our simulations based on the Particle and Heavy Ion Transport code System [PHITS] suggest that the estimated ground level dose for each planet in the case of terrestrial-level atmospheric pressure (1 bar) for each exoplanet does not exceed the critical dose for complex (multi-cellular) life to persist, even for the planetary surface of Proxima Centauri b, Ross-128 b and TRAPPIST-1 e. However, when we take into account the effects of the possible maximum flares from those host stars, the estimated dose reaches fatal levels at the terrestrial lowest atmospheric depth on TRAPPIST-1 e and Ross-128 b. Large fluxes of coronal XUV radiation from active stars induces high atmospheric escape rates from close-in exoplanets suggesting that the atmospheric depth can be substantially smaller than that on the Earth. In a scenario with the atmospheric thickness of 1/10 of Earth's, the radiation dose from close-in planets including Proxima Centauri b and TRAPPIST-1 e reach near fatal dose levels with annual frequency of flare occurrence from their hoststars.

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arXiv2018

Particle Number Dependence of The $N$-Body Simulations of Moon Formation

The formation of the Moon from the circumterrestrial disk has been investigated by using $N$-body simulations with the number $N$ of particles limited from $10^4$ to $10^5$. We develop an $N$-body simulation code on multiple Pezy-SC processors and deploy FDPS (Framework for Developing Particle Simulators) to deal with large number of particles. We execute several high- and extra-high-resolution $N$-body simulations of lunar accretion from a circumterrestrial disk of debris generated by a giant impact on Earth. The number of particles is up to $10^7$, in which 1 particle corresponds to a 10 km-size satellitesimal. We find that the spiral structures inside the Roche limit radius differ between low-resolution simulations ($N \leq10^5$) and high-resolution simulations ($N \geq10^6$). According to this difference, angular momentum fluxes, which determine the accretion timescale of the Moon also depend on the numerical resolution.

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arXiv2017

Satellitesimal Formation via Collisional Dust Growth in Steady Circumplanetary Disks

The icy satellites around Jupiter are considered to have formed in a circumplanetary disk. While previous models focused on the formation of satellites starting from satellitesimals, the question of how satellitesimals form from smaller dust particles has not been addressed so far. In this work, we study the possibility that satellitesimals form in situ in a circumplanetary disk. We calculate the radial distribution of the surface density and representative size of icy dust particles that grow by colliding with each other and drift toward the central planet in a steady circumplanetary disk with a continuous supply of gas and dust from the parent protoplanetary disk. The radial drift barrier is overcome if the ratio of the dust to gas accretion rates onto the circumplanetary disk, $\dot{M}_{\mathrm{d}}/\dot{M}_{\mathrm{g}}$, is high and the strength of turbulence, $α$, is not too low. The collision velocity is lower than the critical velocity of fragmentation when $α$ is low. Taken together, we find that the conditions for satellitesimal formation via dust coagulation are given by $\dot{M}_{\mathrm{d}}/\dot{M}_{\mathrm{g}}\ge1$ and $10^{-4}\leα<10^{-2}$. The former condition is generally difficult to achieve, suggesting that the in-situ satellitesimal formation via particle sticking is viable only under an extreme condition. We also show that neither satellitesimal formation via the collisional growth of porous aggregates nor via streaming instability is viable as long as $\dot{M}_{\mathrm{d}}/\dot{M}_{\mathrm{g}}$ is low.

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arXiv2017

Ejection of iron-bearing giant-impact fragments and the dynamical and geochemical influence of the fragment re-accretion

The Earth was born in violence. Many giant collisions of protoplanets are thought to have occurred during the terrestrial planet formation. Here we investigated the giant impact stage by using a hybrid code that consistently deals with the orbital evolution of protoplanets around the Sun and the details of processes during giant impacts between two protoplanets. A significant amount of materials (up to several tens of percent of the total mass of the protoplanets) is ejected by giant impacts. We call these ejected fragments the giant-impact fragments (GIFs). In some of the erosive hit-and-run and high-velocity collisions, metallic iron is also ejected, which comes from the colliding protoplanets' cores. From ten numerical simulations for the giant impact stage, we found that the mass fraction of metallic iron in GIFs ranges from ~ 1wt% to ~ 25wt%. We also discussed the effects of the GIFs on the dynamical and geochemical characteristics of formed terrestrial planets. We found that the GIFs have the potential to solve the following dynamical and geochemical conflicts: (1) The Earth, currently in a near circular orbit, is likely to have had a highly eccentric orbit during the giant impact stage. The GIFs are large enough in total mass to lower the eccentricity of the Earth to its current value via their dynamical friction. (2) The concentrations of highly siderophile elements (HSEs) in the Earth's mantle are greater than what was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the Earth can contribute to the excess of HSEs. In addition, the estimated amount of iron-bearing GIFs provides significant reducing agent that could transform primitive CO2-H2O atmosphere and ocean into more reducing H2-bearing atmosphere. Thus, GIFs are important for the origin of Earth's life and its early evolution.

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arXiv2016

Population synthesis of planet formation using a torque formula with dynamic effects

Population synthesis studies into planet formation have suggested that distributions consistent with observations can only be reproduced if the actual Type I migration timescale is at least an order of magnitude longer than that deduced from linear theories. Although past studies considered the effect of the Type I migration of protoplanetary embryos, in most cases they used a conventional formula based on static torques in isothermal disks, and employed a reduction factor to account for uncertainty in the mechanism details. However, in addition to static torques, a migrating planet experiences dynamic torques that are proportional to the migration rate. These dynamic torques can impact on planet migration and predicted planetary populations. In this study, we derived a new torque formula for Type I migration by taking into account dynamic corrections. This formula was used to perform population synthesis simulations with and without the effect of dynamic torques. In many cases, inward migration was slowed significantly by the dynamic effects. For the static torque case, gas giant formation was effectively suppressed by Type I migration; however, when dynamic effects were considered, a substantial fraction of cores survived and grew into gas giants.

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arXiv2015

Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

No Hadean rocks have ever been found on Earth's surface except for zircons---evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent α as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of α. Our results show that LHB does not excavate/melt all of Hadean continental crust directly, but over 70% of the Earth's surface area can be covered by subsequent melts in a broad range of α. If there have been no overturns of the continental crust until today, LHB could be responsible for the absence of Hadean rocks because most of Hadean continental crust is not be exposed on the Earth's surface in this case.

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arXiv2014

Formation, Habitability, and Detection of Extrasolar Moons

The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon's stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1 - 0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.

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arXiv2010

Origin of the Different Architectures of the Jovian and Saturnian Satellite Systems

The Jovian regular satellite system mainly consists of four Galilean satellites that have similar masses and are trapped in mutual mean motion resonances except for the outer satellite, Callisto. On the other hand, the Saturnian regular satellite system has only one big icy body, Titan, and a population of much smaller icy moons. We have investigated the origin of these major differences between the Jovian and Saturnian satellite systems by semi-analytically simulating the growth and orbital migration of proto-satellites in an accreting proto-satellite disk. We set up two different disk evolution/structure models that correspond to Jovian and Saturnian systems, by building upon previously developed models of an actively-supplied proto-satellite disk, the formation of gas giants, and observations of young stars. Our simulations extend previous models by including the (1) different termination timescales of gas infall onto the proto-satellite disk and (2) different evolution of a cavity in the disk, between the Jovian and Saturnian systems. We have performed Monte Carlo simulations and show that in the case of the Jovian systems, four to five similar-mass satellites are likely to remain trapped in mean motion resonances. This orbital configuration is formed by type I migration, temporal stopping of the migration near the disk inner edge, and quick truncation of gas infall caused by Jupiter opening a gap in the Solar nebula. The Saturnian systems tend to end up with one dominant body in the outer regions caused by the slower decay of gas infall associated with global depletion of the Solar nebula. The total mass and compositional zoning of the predicted Jovian and Saturnian satellite systems are consistent with the observed satellite systems.

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