arXiv subjects

Eiichiro Kokubo

Publications and source records attributed to Eiichiro Kokubo.

At least 55 records · Page 3Linked to original sources

arXiv2021

Elemental abundances of nearby M dwarfs based on high-resolution near-infrared spectra obtained by the Subaru/IRD survey: Proof of concept

Detailed chemical analyses of M dwarfs are scarce but necessary to constrain the formation environment and internal structure of planets being found around them. We present elemental abundances of 13 M dwarfs (2900 < Teff < 3500 K) observed in the Subaru/IRD planet search project. They are mid-to-late M dwarfs whose abundance of individual elements has not been well studied. We use the high-resolution (~70,000) near-infrared (970-1750 nm) spectra to measure the abundances of Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Sr by the line-by-line analysis based on model atmospheres, with typical errors ranging from 0.2 dex for [Fe/H] to 0.3-0.4 dex for other [X/H]. We measure radial velocities from the spectra and combine them with Gaia astrometry to calculate the Galactocentric space velocities UVW. The resulting [Fe/H] values agree with previous estimates based on medium-resolution K-band spectroscopy, showing a wide distribution of metallicity (-0.6 < [Fe/H] < +0.4). The abundance ratios of individual elements [X/Fe] are generally aligned with the solar values in all targets. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin disk population, the most metal-poor object, GJ 699, could be a thick disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and Galactic stellar population of nearby M dwarfs.

astro-ph.SR↗

arXiv2021

Early Initiation of Inner Solar System Formation at Dead-Zone Inner Edge

The inner solar system possesses a unique orbital structure in which there are no planets inside the Mercury orbit and the mass is concentrated around the Venus and Earth orbits. The origins of these features still remain unclear. We propose a novel concept that the building blocks of the inner solar system formed at the dead-zone inner edge in the early phase of the protosolar disk evolution, where the disk is effectively heated by the disk accretion. First, we compute the dust evolution in a gas disk with a dead zone and obtain the spatial distribution of rocky planetesimals. The disk is allowed to evolve both by a viscous diffusion and magnetically-driven winds. We find that the rocky planetesimals are formed in concentrations around $\sim$ 1 au with a total mass comparable to the mass of the current inner solar system in the early phase of the disk evolution within $\lesssim0.1$ Myr. Based on the planetesimal distribution and the gas disk structure, we subsequently perform \textit{N}-body simulations of protoplanets to investigate the dynamical configuration of the planetary system. We find that the protoplanets can grow into planets without significant orbital migration because of the rapid clearing of the inner disk by the magnetically-driven disk winds. Our model can explain the origins of the orbital structure of the inner solar system. Several other features such as the rocky composition can also be explained by the early formation of rocky planetesimals.

astro-ph.EP↗

arXiv2021

Size evolution of close-in super-Earths through giant impacts and photoevaporation

The Kepler transit survey with follow-up spectroscopic observations has discovered numerous super-Earth sized planets and revealed intriguing features of their sizes, orbital periods, and their relations between adjacent planets. For the first time, we investigate the size evolution of planets via both giant impacts and photoevaporation to compare with these observed features. We calculate the size of a protoplanet, which is the sum of its core and envelope sizes, by analytical models. $N$-body simulations are performed to evolve planet sizes during the giant impact phase with envelope stripping via impact shocks. We consider the initial radial profile of the core mass and the initial envelope mass fractions as parameters. Inner planets can lose their whole envelopes via giant impacts, while outer planets can keep their initial envelopes since they do not experience giant impacts. Photoevaporation is simulated to evolve planet sizes afterward. Our results suggest that the period-radius distribution of the observed planets would be reproduced if we perform simulations in which the initial radial profile of the core mass follows a wide range of power-law distributions and the initial envelope mass fractions are $\sim0.1$. Moreover, our model shows that the adjacent planetary pairs have similar sizes and regular spacings, with slight differences from detailed observational results such as the radius gap.

astro-ph.EP↗

arXiv2021

Merging Criteria for Planetesimal Collisions

In the standard scenario of planet formation, terrestrial planets, ice giants, and cores of gas giants are formed by the accumulation of planetesimals. However, there are few N-body simulation studies of planetesimal accretion that correctly take into account the merging condition of planetesimals. In order to investigate a realistic accretion process of planetesimals, it is necessary to clarify the merging criteria of planetesimals at collision. We perform numerical collision experiments using smoothed particle hydrodynamics and obtain the merging criteria as a function of planetesimal mass and impact parameters for undifferentiated rocky and icy planetesimals and differentiated icy planetesimals. We vary the total mass of colliding planetesimals, their mass ratios, and the impact angle and obtain the critical impact velocity as the merging criteria distinguishing merging from hit-and-run collision. We find that the critical impact velocity normalized by the two-body surface escape velocity decreases with increasing impact angle. The critical impact velocity does not depend on the total mass, while it has a weak positive dependence on the mass ratio. These results barely depend on the composition and internal structure of the planetesimals.

astro-ph.EP↗

arXiv2021

Planetesimal Dynamics in the Presence of a Giant Planet

Standard models of planet formation explain how planets form in axisymmetric, unperturbed disks in single star systems. However, it is possible that giant planets could have already formed when other planetary embryos start to grow. We investigate the dynamics of planetesimals under the perturbation of a giant planet in a gaseous disk. Our aim is to understand the effect of the planet's perturbation on the formation of giant planet cores outside the orbit of the planet. We calculate the orbital evolution of planetesimals ranging from $10^{13}$ to $10^{20}$g, with a Jupiter-mass planet located at 5.2 au. We find orbital alignment of planetesimals distributed in $\simeq 9$-15 au, except for the mean motion resonance (MMR) locations. The degree of alignment increases with increasing distance from the planet and decreasing planetesimal mass. Aligned orbits lead to low encounter velocity and thus faster growth. The typical velocity dispersion for identical-mass planetesimals is $\sim \mathcal{O}(10)$ $\rm{m\text{ } s}^{-1}$ except for the MMR locations. The relative velocity decreases with increasing distance from the planet and decreasing mass ratio of planetesimals. When the eccentricity vectors of planetesimals reach equilibrium under the gas drag and secular perturbation, the relative velocity becomes lower when the masses of two planetesimals are both on the larger end of the mass spectrum. Our results show that with a giant planet embedded in the disk, the growth of another planetary core outside the planet orbit might be accelerated in certain locations.

astro-ph.EP↗

arXiv2021

Elementary Process of Galactic Spiral Arm Formation: Phase Synchronization of Epicyclic Motion by Gravitational Scattering

Swing amplification is a model of spiral arm formation in disk galaxies. Previous $N$-body simulations show that the epicycle phases of stars in spiral arms are synchronized. However, the elementary process of the phase synchronization is not well understood. In order to investigate phase synchronization, we investigate the orbital evolution of stars due to gravitational scattering by a perturber under the epicycle approximation and its dependence on orbital elements and a disk parameter. We find that gravitational scattering by the perturber can cause phase synchronization of stellar orbits. The epicycle phases are better synchronized for smaller initial epicycle amplitudes of stars and larger shear rates of galactic disks. The vertical motion of stars does not affect the phase synchronization. The phase synchronization forms trailing dense regions, which may correspond to spiral arms.

astro-ph.GA↗

arXiv2020

Formation of "Blanets" from Dust Grains around the Supermassive Black Holes in Galaxies

In Wada, Tsukamoto, and Kokubo (2019), we proposed for the first time that a new class of planets, "blanets" (i.e., black hole planets), can be formed around supermassive black holes (SMBHs) in the galactic center. Here, we investigate the dust coagulation processes and physical conditions of the blanet formation outside the snow line ($r_{snow} \sim $ several parsecs) in more detail, especially considering the effect of the radial advection of the dust aggregates. We found that a dimensionless parameter $α= v_t^2/c_s^2$, where $v_t$ is the turbulent velocity and $c_s$ is the sound velocity, describing the turbulent viscosity should be smaller than 0.04 in the circumnuclear disk, to prevent the destruction of the aggregates due to collisions. The formation timescale of blanets $τ_{GI}$ at $r_{snow}$ is, $τ_{GI} \simeq$ 70-80 Myr for $α= 0.01-0.04$ and $M_{BH} = 10^6 M_\odot$. The mass of the blanets ranges from $\sim 20 M_E$ to $3000 M_E$ in $r < 4$ pc for $α= 0.02$ ($M_E$ is the Earth mass), which is in contrast with $ 4 M_E-6 M_E$ for the case without the radial advection. Our results suggest that \textit{blanets} could be formed around relatively low-luminosity AGNs ($L_{bol} \sim 10^{42}$ erg s$^{-1}$) during their lifetime ($\lesssim 10^8$ yr).

astro-ph.EP↗

arXiv2020

Ejection of close-in super-Earths around low-mass stars in the giant impact stage

Earth-sized planets were observed in close-in orbits around M dwarfs. While more and more planets are expected to be uncovered around M dwarfs, theories of their formation and dynamical evolution are still in their infancy. We investigate the giant impact growth of protoplanets, which includes strong scattering around low-mass stars. The aim is to clarify whether strong scattering around low-mass stars affects the orbital and mass distributions of the planets. We perform $N$-body simulation of protoplanets by systematically surveying the parameter space of the stellar mass and surface density of protoplanets. We find that protoplanets are often ejected after twice or three times close-scattering around late M dwarfs. The ejection sets the upper limit of the largest planet mass. Adopting the surface density scaling linearly with the stellar mass, we find that as the stellar mass decreases less massive planets are formed in orbits with higher eccentricities and inclinations. Under this scaling, we also find that a few close-in protoplanets are generally ejected. The ejection of protoplanets plays an important role in the mass distribution of super-Earths around late M dwarfs. The mass relation of observed close-in super-Earths and their central star mass is well reproduced by ejection.

astro-ph.EP↗

arXiv2020

Coherent Stellar Motion in Galactic Spiral Arms by Swing Amplification

We perform local $N$-body simulations of disk galaxies and investigate the evolution of spiral arms. We calculate the time autocorrelation of the surface density of spiral arms and find that the typical evolution timescale is described by the epicycle period. We investigate the distribution of the orbital elements of stars and find that in spiral arms the epicycle motions of stars are in phase while the spatial distribution of the guiding center is nearly uniform. These facts clearly show that the phase synchronization of the epicycle motion takes place, which is theoretically predicted by the swing amplification.

astro-ph.GA↗

arXiv2019

Hyperbolic Orbits in the Solar System: Interstellar Origin or Perturbed Oort Cloud Comets?

We study the dynamical properties of objects in hyperbolic orbits passing through the inner Solar system in the context of two different potential sources: interstellar space and the Oort cloud. We analytically derive the probability distributions of eccentricity, $e$, and perihelion distance, $q$, for each source and estimate the numbers of objects produced per unit of time as a function of these quantities. By comparing the numbers from the two sources, we assess which origin is more likely for a hyperbolic object having a given eccentricity and perihelion distance. We find that the likelihood that a given hyperbolic object is of interstellar origin increases with decreasing eccentricity and perihelion. Conversely, the likelihood that a hyperbolic object has been scattered from the Oort cloud by a passing star increases with decreasing eccentricity and increasing perihelion. By carefully considering their orbital elements, we conclude that both 1I/2017 U1 'Oumuamua ($e\simeq$ 1.2 and $q\simeq$ 0.26 au) and 2I/2019 Q4 Borisov ($e\simeq$ 3.3 and $q\simeq$ 2 au) are most likely of interstellar origin, not scattered from the Oort cloud. However, we also find that Oort cloud objects can be scattered into hyperbolic orbits like those of the two known examples, by sub-stellar and even sub-Jovian mass perturbers. This highlights the need for better characterization of the low mass end of the free-floating brown dwarf and planet population.

astro-ph.EP↗

arXiv2019

Planet Formation around Super Massive Black Holes in the Active Galactic Nuclei

As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line", located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier" does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass $M_{BH}$, and it is comparable to the AGN's lifetime ($\sim 10^8$ yr) for low mass ($M_{BH} \sim 10^6 M_\odot$) SMBHs.

astro-ph.GA↗

arXiv2018

Global N-Body Simulation of Galactic Spiral Arms

The origin of galactic spiral arms is one of fundamental problems in astrophysics. Based on the local analysis Toomre (1981) proposed the swing amplification mechanism in which the self-gravity forms spiral arms as leading waves of stars rotate to trailing ones due to galactic shear. The structure of spiral arms is characterized by their number and pitch angle. We perform global $N$-body simulations of spiral galaxies to investigate the dependence of the spiral structure on disk parameters and compare the simulation results with the swing amplification model. We find that the spiral structure in the $N$-body simulations agrees well with that predicted by the swing amplification for the wide range of parameters. The pitch angle decreases with increasing the shear rate and is independent of the disk mass fraction. The number of spiral arms decreases with both increasing the shear rate and the disk mass fraction. If the disk mass fraction is fixed, the pitch angle increases with the number of spiral arms.

astro-ph.GA↗

arXiv2018

Planets around the evolved stars 24 Booties and $γ$ Libra: A 30d-period planet and a double giant-planet system in possible 7:3 MMR

We report the detection of planets around two evolved giant stars from radial velocity measurements at Okayama Astrophysical observatory. 24 Boo (G3IV) has a mass of $0.99\,M_{\odot}$, a radius of $10.64\,R_{\odot}$, and a metallicity of ${\rm [Fe/H]}=-0.77$. The star hosts one planet with a minimum mass of $0.91\,M_{\rm Jup}$ and an orbital period of $30.35{\rm d}$. The planet has one of the shortest orbital periods among those ever found around evolved stars by radial-veloocity methods. The stellar radial velocities show additional periodicity with $150{\rm d}$, which are probably attributed to stellar activity. The star is one of the lowest-metallicity stars orbited by planets currently known. $γ$ Lib (K0III) is also a metal-poor giant with a mass of $1.47\,M_{\odot}$, a radius of $11.1\,R_{\odot}$, and ${\rm [Fe/H]}=-0.30$. The star hosts two planets with minimum masses of $1.02M_{\rm Jup}$ and $4.58\,M_{\rm Jup}$, and periods of $415{\rm d}$ and $964{\rm d}$, respectively. The star has the second lowest metallicity among the giant stars hosting more than two planets. Dynamical stability analysis for the $γ$ Lib system sets a minimum orbital inclination angle to be about $70^{\circ}$ and suggests that the planets are in 7:3 mean-motion resonance, though the current best-fitted orbits to the radial-velocity data are not totally regular.

astro-ph.EP↗

arXiv2018

Formation of the terrestrial planets in the solar system around 1 au via radial concentration of planetesimals

No planets exist inside the orbit of Mercury and the terrestrial planets of the solar system exhibit a localized configuration. According to thermal structure calculation of protoplanetary disks, a silicate condensation line (~ 1300 K) is located around 0.1 au from the Sun except for the early phase of disk evolution, and planetesimals could have formed inside the orbit of Mercury. A recent study of disk evolution that includes magnetically driven disk winds showed that the gas disk obtains a positive surface density slope inside ~ 1 au from the central star. In a region with positive midplane pressure gradient, planetesimals undergo outward radial drift. We investigate the radial drift of planetesimals and type I migration of planetary embryos in a disk that viscously evolves with magnetically driven disk winds. We show a case in which no planets remain in the close-in region. Radial drifts of planetesimals are simulated using a recent disk evolution model that includes effects of disk winds. The late stage of planet formation is also examined by performing N-body simulations of planetary embryos. We demonstrate that in the middle stage of disk evolution, planetesimals can undergo convergent radial drift in a magnetorotational instability (MRI)-inactive disk, in which the pressure maximum is created, and accumulate in a narrow ring-like region with an inner edge at ~ 0.7 au from the Sun. We also show that planetary embryos that may grow from the narrow planetesimal ring do not exhibit significant type I migration in the late stage of disk evolution. The origin of the localized configuration of the terrestrial planets of the solar system, in particular the deficit of close-in planets, can be explained by the convergent radial drift of planetesimals in disks with a positive pressure gradient in the close-in region.

astro-ph.EP↗

arXiv2018

Formation of close-in super-Earths in evolving protoplanetary disks due to disk winds

Planets with masses larger than about 0.1 Earth-masses undergo rapid inward migration (type I migration) in a standard protoplanetary disk. Recent magnetohydrodynamical simulations revealed the presence of magnetically driven disk winds, which would alter the disk profile and the type I migration in the close-in region. We investigate orbital evolution of planetary embryos in disks that viscously evolve under the effects of disk winds. The aim is to discuss effects of altered disk profiles on type I migration. In addition, we aim to examine whether observed distributions of close-in super-Earths can be reproduced by simulations that include effects of disk winds. We perform N-body simulations of super-Earth formation from planetary embryos, in which a recent model for disk evolution is used. We explore a wide range of parameters and draw general trends. We also carry out N-body simulations of close-in super-Earth formation from embryos in such disks under various conditions. We find that the type I migration is significantly suppressed in many cases. Even in cases in which inward migration occurs, the migration timescale is lengthened to 1 Myr, which mitigates the type I migration problem. This is because the gas surface density is decreased and has a flatter profile in the close-in region due to disk winds. We find that when the type I migration is significantly suppressed, planets undergo late orbital instability during the gas depletion, leading to a non-resonant configuration. We also find that observed distributions of close-in super-Earths (e.g., period ratio, mass ratio) can be reproduced. In addition, we show that in some results of simulations, systems with a chain of resonant planets, like the TRAPPIST-1 system, form.

astro-ph.EP↗

arXiv2018

Gravitational instability of a dust layer composed of porous silicate dust aggregates in a protoplanetary disk

Planetesimal formation is one of the most important unsolved problems in planet formation theory. In particular, rocky planetesimal formation is difficult because silicate dust grains are easily broken when they collide. Recently, it has been proposed that they can grow as porous aggregates when their monomer radius is smaller than $\sim$ 10 nm, which can also avoid the radial drift toward the central star. However, the stability of a layer composed of such porous silicate dust aggregates has not been investigated. Therefore, we investigate the gravitational instability of this dust layer. To evaluate the disk stability, we calculate Toomre's stability parameter $Q$, for which we need to evaluate the equilibrium random velocity of dust aggregates. We calculate the equilibrium random velocity considering gravitational scattering and collisions between dust aggregates, drag by mean flow of gas, stirring by gas turbulence, and gravitational scattering by gas density fluctuation due to turbulence. We derive the condition of the gravitational instability using the disk mass, dust-to-gas ratio, turbulent strength, orbital radius, and dust monomer radius. We find that, for the minimum mass solar nebula model at 1 au, the dust layer becomes gravitationally unstable when the turbulent strength $α\lesssim10^{-5}$. If the dust-to-gas ratio is increased twice, the gravitational instability occurs for $α\lesssim10^{-4}$. We also find that the dust layer is more unstable in disks with larger mass, higher dust-to-gas ratio, and weaker turbulent strength, at larger orbital radius, and with a larger monomer radius.

astro-ph.EP↗

arXiv2017

Effects of global gas flows on type I migration

Magnetically-driven disk winds would alter the surface density slope of gas in the inner region of a protoplanetary disk $(r \lesssim 1 {\rm au})$. This in turn affects planet formation. Recently, the effect of disk wind torque has been considered with the suggestion that it would carve out the surface density of the disk from inside and would induce global gas flows (wind-driven accretion). We aim to investigate effects of global gas flows on type I migration and also examine planet formation. A simplified approach was taken to address this issue, and N-body simulations with isolation-mass planets were also performed. In previous studies, the effect of gas flow induced by turbulence-driven accretion has been taken into account for its desaturation effect of the corotation torque. If more rapid gas flows (e.g., wind-driven accretion) are considered, the desaturation effect can be modified. In MRI-inactive disks, in which the wind-driven accretion dominates the disk evolution, the gas flow at the midplane plays an important role. If this flow is fast, the corotation torque is efficiently desaturated. Then, the fact that the surface density slope can be positive in the inner region due to the wind torque can generate an outward migration region extended to super-Earth mass planets. In this case, we observe that no planets fall onto the central star in N-body simulations with migration forces imposed to reproduce such migration pattern. We also see that super-Earth mass planets can undergo outward migration. Relatively rapid gas flows affects type I migration and thus the formation of close-in planets.

astro-ph.EP↗

arXiv2017

Formation of Close-in Super-Earths by Giant Impacts: Effects of Initial Eccentricities and Inclinations of Protoplanets

Recent observations have revealed the eccentricity and inclination distributions of close-in super-Earths. These distributions have the potential to constrain their formation processes. In the in-situ formation scenario, the eccentricities and inclinations of planets are determined by gravitational scattering and collisions between protoplanets on the giant impact stage. We investigate the effect of the initial eccentricities and inclinations of protoplanets on the formation of close-in super-Earths. We perform $N$-body simulations of protoplanets in gas-free disks, changing the initial eccentricities and inclinations systematically. We find that while the eccentricities of protoplanets are well relaxed through their evolution, the inclinations are not. When the initial inclinations are small, they are not generally pumped up since scattering is less effective and collisions occur immediately after orbital crossing. On the other hand, when the initial inclinations are large, they tend to be kept large since collisional damping is less effective. Not only the resultant inclinations of planets, but also their number, eccentricities, angular momentum deficit, and orbital separations are affected by the initial inclinations of protoplanets.

astro-ph.EP↗