arXiv subjects

Junichiro Makino

Publications and source records attributed to Junichiro Makino.

At least 19 recordsLinked to original sources

arXiv2026

Self-consistent $N$-body simulation of Planetesimal-Driven Migration. II. The effect of PDM on planet formation from a planetesimal disk

According to the canonical planet formation theory, planets form "in-situ" within a planetesimal disk via runaway and oligarchic growth. This theory, however, cannot naturally account for the formation timescale of ice giants or the existence of diverse exoplanetary systems. Planetary migration is a key to resolving these problems. One well-known mechanism of planetary migration is planetesimal-driven migration (PDM), which can let planets undergo significant migration through gravitational scattering of planetesimals. In our previous paper (Jinno et al. 2024, PASJ, 76, 1309), we investigated the migration of a single planet through PDM, addressing previously unexplored aspects of both the gravitational interactions among planetesimals and the interactions with disk gas. Here we perform the first high-resolution simulations of planet formation from a large-scale planetesimal disk, incorporating planet-gas disk interactions, planet-planetesimal interactions, gravitational interactions among all planetesimals, and physical collisions between planetesimals to investigate the role of PDM in the planet formation process. Our results show that protoplanets undergo dynamic inward/outward migrations during the runaway growth stage via PDM. Moreover, orbital repulsion combined with PDM tends to make two groups of protoplanets, outer ones going outward and inner ones going inward. Such dynamic migration significantly influences the early stages of planetary formation. These findings provide a viable pathway for the formation of Earth-like planets and ice giants' cores. Furthermore, they suggest that a standard protoplanetary disk model can account for the planetary migration necessary to explain diverse exoplanetary systems without the need for additional hypotheses.

astro-ph.EP↗

arXiv2025

The First Star-by-star $N$-body/Hydrodynamics Simulation of Our Galaxy Coupling with a Surrogate Model

A major goal of computational astrophysics is to simulate the Milky Way Galaxy with sufficient resolution down to individual stars. However, the scaling fails due to some small-scale, short-timescale phenomena, such as supernova explosions. We have developed a novel integration scheme of $N$-body/hydrodynamics simulations working with machine learning. This approach bypasses the short timesteps caused by supernova explosions using a surrogate model, thereby improving scalability. With this method, we reached 300 billion particles using 148,900 nodes, equivalent to 7,147,200 CPU cores, breaking through the billion-particle barrier currently faced by state-of-the-art simulations. This resolution allows us to perform the first star-by-star galaxy simulation, which resolves individual stars in the Milky Way Galaxy. The performance scales over $10^4$ CPU cores, an upper limit in the current state-of-the-art simulations using both A64FX and X86-64 processors and NVIDIA CUDA GPUs.

astro-ph.GA↗

arXiv2025

A new approach for predicting the stability of hierarchical triple systems -- I. Coplanar Cases

Hierarchical triple systems play a crucial role in various astrophysical contexts, and therefore the understanding of their stability is important. Traditional empirical stability criteria rely on a threshold value of $Q$, the ratio between the outer orbit's pericenter distance and the inner orbit's semi-major axis. However, determining a single critical value of $Q$ is impossible because there is a range of the value of $Q$ for which both stable and unstable systems exist, referred to as the mixed-region. In this study, we introduce a novel method to assess the stability of triple systems within this mixed-region. We numerically integrate equal-mass, coplanar hierarchical triples within the mixed-region. By performing Fourier analysis of the time evolution of the semi-major axes ratio during the first 1000 inner orbital periods of the systems, we find notable features in stable systems: if the main peaks are periodically spaced in the frequency domain and the continuous components and irregularly spaced peaks are small, the system tends to be stable. This observation indicates that the evolution of stable triples is more periodic than that of unstable ones. We quantified the periodicity of the triples and investigated the correlation between the Fourier power distribution and the system's lifetime. Using this correlation, we show that it is possible to determine if a triple system in the mixed-region is stable or not with very high accuracy. These findings suggest that periodicity in orbital evolution can serve as a robust indicator of stability for hierarchical triples.

astro-ph.SR↗

arXiv2024

An independent hybrid imaging of Sgr A* from the data in EHT 2017 observations

We propose that the ring structure found by the Event Horizon Telescope Collaboration (EHTC) as the black hole shadow of Sgr A*is an artifact by the bumpy PSF (Point Spread Function) of the EHT2017. The imaging using sparse u-v data requires detailed scrutiny of the PSF. The estimated shadow diameter (48.7 +- 7 muas) is equal to the spacing between the main beam and the first sidelobe of the PSF (49.09 muas), which immediately suggests a potential problem in the deconvolution of the PSF. We show that the ring image can be derived from non-ring simulated datasets (noise only; point source) with a narrow Field-of-View (FOV) and an assumed self-calibration suggesting the EHT2017's u-v coverage is insufficient for reliable imaging. The EHTC analysis, based on calibrations with assumptions about the source's size and properties, selected the final image by prioritizing appearance rate of the similar structure from a large imaging parameter space over data consistency. Our independent analysis with the conventional hybrid mapping reveals an elongated east-west structure, consistent with previous observations. We believe it to be more reliable than the EHTC image, due to half the residuals in normalized visibility amplitude. The eastern half is brighter, possibly due to a Doppler boost from the rapid rotating disk. We hypothesize our image shows a portion of the accretion disk about 2 to a few Rs away from the black hole, rotating with nearly 60 % of the speed of light viewed from an angle of 40 -45 degrees.

astro-ph.HE↗

arXiv2024

Self-consistent $N$-body simulation of Planetesimal-Driven Migration I. The trajectories of single planets in the uniform background

Recent exoplanet observations have revealed a diversity of exoplanetary systems, which suggests the ubiquity of radial planetary migration. One powerful known mechanism of planetary migration is planetesimal-driven migration (PDM), which can let planets undergo significant migration through gravitational scattering with planetesimals. In this series of papers, we present the results of our high-resolution self-consistent $N$-body simulations of PDM, in which gravitational interactions among planetesimals, the gas drag, and Type-I migration are all taken into account. In this first paper (Paper I), we investigate the migration of a single planet through PDM within the framework of the classical standard disk model (the Minimum-Mass Solar Nebula model). Paper I aims to improve our understanding of planetary migration through PDM, addressing previously unexplored aspects of both the gravitational interactions among planetesimals and the interactions with disk gas. Our results show that even small protoplanets can actively migrate through PDM. Such active migration can act as a rapid radial diffusion mechanism for protoplanets and significantly influence the early stages of planetary formation (i.e., during the runaway growth phase). Moreover, a fair fraction of planets migrate outward. This outward migration may offer a potential solution for the ``planet migration problem" caused by Type-I migration and gives a natural mechanism for outward migration assumed in many recent scenarios for the formation of outer planets.

astro-ph.EP↗

arXiv2024

Black hole ring images from PSF structures

Two critical aspects of radio interferometric imaging analysis are data calibration and deconvolution of the point spread function (PSF) structure. Both of these are particularly important for high-frequency observations using a VLBI network consisting of a small number of stations, such as those conducted by the Event Horizon Telescope (EHT). The Event Horizon Telescope Collaboration (EHTC) has presented images of ring-shaped black holes from observations of M 87 (d = 42 +- 3 muas)(EHTC2019a) and the Galactic Center (d = 51.8 +- 2.3 muas)(EHTC2022a). The ring structures seen in the EHTC images are consistent with the estimated shadow diameter of the black hole based on its mass and distance. However, these black hole ring sizes are also the same with the typical up and down spacings (e.g., the intervals between the main beam and nearby 1st-sidelobes) seen in the point spread function (PSF; dirty beam) for each observation. These facts suggest that the EHTC ring structures are artifacts derived from the shape of the PSFs rather than the intrinsic structure of the SMBHs in M 87 and the Galactic Center. The EHTC utilizes novel imaging techniques in addition to the standard CLEAN algorithm. The CLEAN method was designed for PSF shape deconvolution in mind, yet in practice, it may not always be able to completely remove the PSF shape. In the imaging analysis of data from interferometers with a small number of antennas like the EHT, it is crucial to assess the PSF shape and compare it with the imaging results. The novel imaging methods employed by the EHTC have not yet been fully evaluated for PSF deconvolution performance, and it is highly recommended that their performance in this regard be thoroughly examined. It is also important to investigate the data calibration capability, i.e., the ability to separate error noise from the observed data.

astro-ph.IM↗

arXiv2024

The jet and resolved features of the central supermassive black hole of M 87 observed with EHT in 2017 -- Comparison with the GMVA 86 GHz results

M 87 is the best target for studying black hole accretion and jet formation. Reanalysis of the EHT public data at 230 GHz shows a core-knots structure at the center and jet features (Miyoshi et al. 2022a). We here compare this with the new results of GMVA at 86 GHz showing a spatially resolved central core (Lu et al. 2023a). There are similarities and differences between the two. At 86 GHz,"two bright regions" are seen on the ring in the core."Core-Knot-Westknot", triple structure in the 230 GHz image shows apparent appearance of two peaks similar to the "two bright regions" when convolved with the GMVA beam. This similarity suggests that both frequencies reveal the same objects in the core area. Protrusions are observed on both the south and north sides of the core at both frequencies, becoming prominent and wing-like at 230 GHz. The 86 GHz image shows a triple ridge jet structure, while the 230 GHz image shows only a bright central ridge with two roots. Both frequencies show a shade between the core and the central ridge. To detect the faint features from the EHT2017 data, we found that the use of all baseline data is essential. Using all including the ultrashort baseline data, revealed the jet and faint structures. Without the ultrashort baselines, these structures were not detectable. The lack of detection of any faint structures other than the ring in the M 87 data by the EHTC is presumably due to the exclusion of ultrashort baselines from their analysis.

astro-ph.HE↗

arXiv2023

Surrogate Modeling for Computationally Expensive Simulations of Supernovae in High-Resolution Galaxy Simulations

Some stars are known to explode at the end of their lives, called supernovae (SNe). The substantial amount of matter and energy that SNe release provides significant feedback to star formation and gas dynamics in a galaxy. SNe release a substantial amount of matter and energy to the interstellar medium, resulting in significant feedback to star formation and gas dynamics in a galaxy. While such feedback has a crucial role in galaxy formation and evolution, in simulations of galaxy formation, it has only been implemented using simple {\it sub-grid models} instead of numerically solving the evolution of gas elements around SNe in detail due to a lack of resolution. We develop a method combining machine learning and Gibbs sampling to predict how a supernova (SN) affects the surrounding gas. The fidelity of our model in the thermal energy and momentum distribution outperforms the low-resolution SN simulations. Our method can replace the SN sub-grid models and help properly simulate un-resolved SN feedback in galaxy formation simulations. We find that employing our new approach reduces the necessary computational cost to $\sim$ 1 percent compared to directly resolving SN feedback.

astro-ph.GA↗

arXiv2023

3D-Spatiotemporal Forecasting the Expansion of Supernova Shells Using Deep Learning toward High-Resolution Galaxy Simulations

Supernova (SN) plays an important role in galaxy formation and evolution. In high-resolution galaxy simulations using massively parallel computing, short integration timesteps for SNe are serious bottlenecks. This is an urgent issue that needs to be resolved for future higher-resolution galaxy simulations. One possible solution would be to use the Hamiltonian splitting method, in which regions requiring short timesteps are integrated separately from the entire system. To apply this method to the particles affected by SNe in a smoothed-particle hydrodynamics simulation, we need to detect the shape of the shell on and within which such SN-affected particles reside during the subsequent global step in advance. In this paper, we develop a deep learning model, 3D-MIM, to predict a shell expansion after a SN explosion. Trained on turbulent cloud simulations with particle mass $m_{\rm gas}=1$M$_\odot$, the model accurately reproduces the anisotropic shell shape, where densities decrease by over 10 per cent by the explosion. We also demonstrate that the model properly predicts the shell radius in the uniform medium beyond the training dataset of inhomogeneous turbulent clouds. We conclude that our model enables the forecast of the shell and its interior where SN-affected particles will be present.

astro-ph.GA↗

arXiv2023

$N$-body simulation of planetary formation through pebble accretion in a radially structured protoplanetary disk

In the conventional theory of planet formation, it is assumed that protoplanetary disks are axisymmetric and have a smooth radial profile. However, recent radio observations of protoplanetary disks have revealed that many of them have complex radial structures. In this study, we perform a series of $N$-body simulations to investigate how planets are formed in protoplanetary disks with radial structures. For this purpose, we consider the effect of continuous pebble accretion onto the discontinuity boundary within the terrestrial planet-forming region ($\sim0.6$ AU). We found that protoplanets grow efficiently at the discontinuity boundary, reaching the Earth mass within $\sim10^4$ years. We confirmed that giant collisions of protoplanets occur universally in our model. Moreover, we found that multiple planet-sized bodies form at regular intervals in the vicinity of the discontinuity boundary. These results indicate the possibility of the formation of solar system-like planetary systems in radially structured protoplanetary disks.

astro-ph.EP↗

arXiv2022

Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy

The $r$-process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way and the nucleosynthesis of the heaviest elements. Observations by the $R$-Process Alliance (RPA) and others have confirmed that many RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf galaxies of the Milky Way (MW). However, we do not know how RPE stars are formed. Here, we present the result of a cosmological zoom-in simulation of an MW-like galaxy with $r$-process enrichment, performed with the highest resolution in both time and mass. Thanks to this advancement, unlike previous simulations, we find that most highly RPE ($r$-II; [Eu/Fe] $> +0.7$) stars are formed in low-mass dwarf galaxies that have been enriched in $r$-process elements for [Fe/H] $\,<-2.5$, while those with higher metallicity are formed in situ, in locally enhanced gas clumps that were not necessarily members of dwarf galaxies. This result suggests that low-mass accreted dwarf galaxies are the main formation site of $r$-II stars with [Fe/H] $\,<-2.5$. We also find that most low-metallicity $r$-II stars exhibit halo-like kinematics. Some $r$-II stars formed in the same halo show low dispersions in [Fe/H] and somewhat larger dispersions of [Eu/Fe], similar to the observations. The fraction of simulated $r$-II stars is commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo $r$-II stars matches that observed. These results demonstrate that RPE stars can be valuable probes of the accretion of dwarf galaxies in the early stages of their formation.

astro-ph.GA↗

arXiv2022

The jet and resolved features of the central supermassive black hole of M 87 observed with EHT

We report our independent image reconstruction of the M 87 from the public data of the Event Horizon Telescope Collaborators (EHTC). Our result is different from the image published by the EHTC. Our analysis shows that (a) the structure at 230 GHz is consistent with those of lower frequency VLBI observations, (b) the jet structure is evident at 230 GHz extending from the core to a few mas, though the intensity rapidly decreases along the axis, and (c) the unresolved core is resolved into bright three features presumably showing an initial jet with a wide opening angle of about 70 deg. The ring-like structures of the EHTC can be created not only from the public data, but also from the simulated data of a point image. Also, the rings are very sensitive to the FOV size. The u-v coverage of EHT lack about 40 micro-asec fringe spacings. Combining with a very narrow FOV, it created the 40 micro-asec ring structure. We conclude that the absence of the jet and the presence of the ring in the EHTC result are both artifacts owing to the narrow FOV setting and the u-v data sampling bias effect of the EHT array. Because the EHTC's simulations only take into account the reproduction of the input image models, and not those of the input noise models, their optimal parameters can enhance the effects of sampling bias and produce artifacts such as the 40 micro-asec ring structure, rather than reproducing the correct image.

astro-ph.HE↗

arXiv2021

Particle-particle Particle-tree Code for Planetary System Formation with Individual Cut-off Method: GPLUM

In a standard theory of the formation of the planets in our Solar System, terrestrial planets and cores of gas giants are formed through accretion of kilometer-sized objects (planetesimals) in a protoplanetary disk. Gravitational $N$-body simulations of a disk system made up of numerous planetesimals are the most direct way to study the accretion process. However, the use of $N$-body simulations has been limited to idealized models (e.g. perfect accretion) and/or narrow spatial ranges in the radial direction, due to the limited number of simulation runs and particles available. We have developed new $N$-body simulation code equipped with a particle-particle particle-tree (${\rm P^3T}$) scheme for studying the planetary system formation process: GPLUM. For each particle, GPLUM uses the fourth-order Hermite scheme to calculate gravitational interactions with particles within cut-off radii and the Barnes-Hut tree scheme for particles outside the cut-off radii. In existing implementations, ${\rm P^3T}$ schemes use the same cut-off radius for all particles, making a simulation become slower when the mass range of the planetesimal population becomes wider. We have solved this problem by allowing each particle to have an appropriate cut-off radius depending on its mass, its distance from the central star, and the local velocity dispersion of planetesimals. In addition to achieving a significant speed-up, we have also improved the scalability of the code to reach a good strong-scaling performance up to 1024 cores in the case of $N=10^6$. GPLUM is freely available from https://github.com/YotaIshigaki/GPLUM with MIT license.

astro-ph.EP↗

arXiv2020

PeTar: a high-performance N-body code for modeling massive collisional stellar systems

The numerical simulations of massive collisional stellar systems, such as globular clusters (GCs), are very time-consuming. Until now, only a few realistic million-body simulations of GCs with a small fraction of binaries (5%) have been performed by using the NBODY6++GPU code. Such models took half a year computational time on a GPU based super-computer. In this work, we develop a new N-body code, PeTar, by combining the methods of Barnes-Hut tree, Hermite integrator and slow-down algorithmic regularization (SDAR). The code can accurately handle an arbitrary fraction of multiple systems (e.g. binaries, triples) while keeping a high performance by using the hybrid parallelization methods with MPI, OpenMP, SIMD instructions and GPU. A few benchmarks indicate that PeTar and NBODY6++GPU have a very good agreement on the long-term evolution of the global structure, binary orbits and escapers. On a highly configured GPU desktop computer, the performance of a million-body simulation with all stars in binaries by using PeTar is 11 times faster than that of NBODY6++GPU. Moreover, on the Cray XC50 supercomputer, PeTar well scales when number of cores increase. The ten million-body problem, which covers the region of ultra compact dwarfs and nuclearstar clusters, becomes possible to be solved.

astro-ph.IM↗

arXiv2020

A slow-down time-transformed symplectic integrator for solving the few-body problem

An accurate and efficient method dealing with the few-body dynamics is important for simulating collisional N-body systems like star clusters and to follow the formation and evolution of compact binaries. We describe such a method which combines the time-transformed explicit symplectic integrator (Preto & Tremaine 1999; Mikkola & Tanikawa 1999) and the slow-down method (Mikkola & Aarseth 1996). The former conserves the Hamiltonian and the angular momentum for a long-term evolution, while the latter significantly reduces the computational cost for a weakly perturbed binary. In this work, the Hamilton equations of this algorithm are analyzed in detail. We mathematically and numerically show that it can correctly reproduce the secular evolution like the orbit averaged method and also well conserve the angular momentum. For a weakly perturbed binary, the method is possible to provide a few order of magnitude faster performance than the classical algorithm. A publicly available code written in the c++ language, SDAR, is available on GitHub (https://github.com/lwang-astro/SDAR). It can be used either as a stand alone tool or a library to be plugged in other $N$-body codes. The high precision of the floating point to 62 digits is also supported.

astro-ph.EP↗

arXiv2020

Are long-term $N$-body simulations reliable?

$N$-body integrations are used to model a wide range of astrophysical dynamics, but they suffer from errors which make their orbits diverge exponentially in time from the correct orbits. Over long time-scales, their reliability needs to be established. We address this reliability by running a three-body planetary system over about $200$ e-folding times. Using nearby initial conditions, we can construct statistics of the long-term phase-space structure and compare to rough estimates of resonant widths of the system. We compared statistics for a wide range of numerical methods, including a Runge--Kutta method, Wisdom--Holman method, symplectic corrector methods, and a method by Laskar and Robutel. "Improving" an integrator did not increase the phase space accuracy, but simply increasing the number of initial conditions did. In fact, the statistics of a higher order symplectic corrector method were inconsistent with the other methods in one test.

astro-ph.EP↗

arXiv2019

Accelerated FDPS --- Algorithms to Use Accelerators with FDPS

In this paper, we describe the algorithms we implemented in FDPS to make efficient use of accelerator hardware such as GPGPUs. We have developed FDPS to make it possible for many researchers to develop their own high-performance parallel particle-based simulation programs without spending large amount of time for parallelization and performance tuning. The basic idea of FDPS is to provide a high-performance implementation of parallel algorithms for particle-based simulations in a "generic" form, so that researchers can define their own particle data structure and interparticle interaction functions and supply them to FDPS. FDPS compiled with user-supplied data type and interaction function provides all necessary functions for parallelization, and using those functions researchers can write their programs as though they are writing simple non-parallel program. It has been possible to use accelerators with FDPS, by writing the interaction function that uses the accelerator. However, the efficiency was limited by the latency and bandwidth of communication between the CPU and the accelerator and also by the mismatch between the available degree of parallelism of the interaction function and that of the hardware parallelism. We have modified the interface of user-provided interaction function so that accelerators are more efficiently used. We also implemented new techniques which reduce the amount of work on the side of CPU and amount of communication between CPU and accelerators. We have measured the performance of N-body simulations on a systems with NVIDIA Volta GPGPU using FDPS and the achieved performance is around 27 \% of the theoretical peak limit. We have constructed a detailed performance model, and found that the current implementation can achieve good performance on systems with much smaller memory and communication bandwidth.

astro-ph.IM↗

arXiv2018

Hermite integrator for high-order mesh-free schemes

In most of mesh-free methods, the calculation of interactions between sample points or particles is the most time consuming. When we use mesh-free methods with high spatial orders, the order of the time integration should also be high. If we use usual Runge-Kutta schemes, we need to perform the interaction calculation multiple times per one time step. One way to reduce the number of interaction calculations is to use Hermite schemes, which use the time derivatives of the right hand side of differential equations, since Hermite schemes require smaller number of interaction calculations than RK schemes do to achieve the same order. In this paper, we construct a Hermite scheme for a mesh-free method with high spatial orders. We performed several numerical tests with fourth-order Hermite schemes and Runge-Kutta schemes. We found that, for both of Hermite and Runge-Kutta schemes, the overall error is determined by the error of spatial derivatives, for timesteps smaller than the stability limit. The calculation cost at the timestep size of the stability limit is smaller for Hermite schemes. Therefore, we conclude that Hermite schemes are more efficient than Runge-Kutta schemes and thus useful for high-order mesh-free methods for Lagrangian Hydrodynamics.

physics.comp-ph↗