arXiv subjects

Junichiro Makino

Publications and source records attributed to Junichiro Makino.

At least 109 records · Page 6Linked to original sources

arXiv2003

A Dynamical Model for the Globular Cluster G1

We present a comparison between the observational data on the kinematical structure of G1 in M31, obtained with the Hubble WFPC2 and STIS instruments, and the results of dynamical simulations carried out using the special-purpose computer GRAPE-6. We have obtained good fits for models starting with single cluster King-model initial conditions and even better fits when starting our simulations with a dynamically constructed merger remnant of two star clusters. In the latter case, the results from our simulations are in excellent agreement with the observed profiles of luminosity, velocity dispersion, rotation, and ellipticity. We obtain a mass-to-light ratio of $M/L = 4.0 \pm 0.4$ and a total cluster mass of $M=(8\pm 1)\times 10^6 M_\odot$. Given that our dynamical model can fit all available observational data very well, there seems to be no need to invoke the presence of an intermediate-mass black hole in the center of G1.

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arXiv2003

Kuiper-belt Binary Formation through Exchange Reactions

Recent observations (Burnes et al 2002,Veillet et al 2002, Margot et al 2002a) have revealed an unexpectedly high binary fraction among the Trans-Neptunian Objects (TNOs) that populate the Kuiper Belt. The TNO binaries are strikingly different from asteroid binaries in four respects (Veillet et al 2002): their frequency is an order of magnitude larger, the mass ratio of their components is closer to unity, and their orbits are wider and highly eccentric. Two explanations have been proposed for their formation, one assuming large numbers of massive bodies (Weidenschilling 2002), and one assuming large numbers of light bodies (Goldreich et al 2002). We argue that both assumptions are unwarranted, and we show how TNO binaries can be produced from a modest number of intermediate-mass bodies of the type predicted by the gravitational instability theory for the formation of planetesimals (Goldreich and Ward 1973). We start with a TNO binary population similar to the asteroid binary population, but subsequently modified by three-body exchange reactions, a process that is far more efficient in the Kuiper belt, because of the much smaller tidal perturbations by the Sun. Our mechanism can naturally account for all four characteristics that distinguish TNO binaries from main-belt asteroid binaries.

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arXiv2003

Parameters of core-collapse

This paper considers the phenomenon of deep core collapse in collisional stellar systems, with stars of equal mass. The collapse takes place on some multiple, $ξ^{-1}$, of the central relaxation time, and produces a density profile in which $ρ\propto r^{-α}$, where $α$ is a constant. The parameters $α$ and $ξ$ have usually been determined from simplified models, such as gas and Fokker-Planck models, often with the simplification of isotropy. Here we determine the parameters directly from N-body simulations carried out using the newly completed GRAPE-6.

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arXiv2002

Dynamical evolution of star clusters in tidal fields

We report results of a large set of N-body calculations aimed to study the evolution of multi-mass star clusters in external tidal fields. Our clusters start with the same initial mass-functions, but varying particle numbers, orbital types and density profiles. Our main focus is to study how the stellar mass-function and other cluster parameters change under the combined influence of stellar evolution, two-body relaxation and the external tidal field. We find that the lifetimes of star clusters moving on similar orbits scale as T sim T_RH^x, where T_RH is the relaxation time, and the exponent x depends on the initial concentration of the cluster and is around x approx 0.75. From the results for the lifetimes, we predict that between 53% to 67% of all galactic globular clusters will be destroyed within the next Hubble time. Low-mass stars are preferentially lost and the depletion is strong enough to turn initially increasing mass-functions into mass-functions which decrease towards the low-mass end. The details of this depletion are insensitive to the starting condition of the cluster. The preferential depletion of low-mass stars from star clusters leads to a decrease of their mass-to-light ratios except for a short period close to final dissolution. The fraction of compact remnants is increasing throughout the evolution and they are more strongly concentrated towards the cluster cores than main-sequence stars. For a sample of galactic globular clusters with well observed parameters, we find a correlation between the observed slope of the mass-function and the lifetimes predicted by us. It seems possible that galactic globular clusters started with a mass-function similar to what one observes for the average mass-function of the galactic disc and bulge. (Abridged)

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arXiv2002

MODEST-1: Integrating Stellar Evolution and Stellar Dynamics

We summarize the main results from MODEST-1, the first workshop on MOdeling DEnse STellar systems. Our goal is to go beyond traditional population synthesis models, by introducing dynamical interactions between single stars, binaries, and multiple systems. The challenge is to define and develop a software framework to enable us to combine in one simulation existing computer codes in stellar evolution, stellar dynamics, and stellar hydrodynamics. With this objective, the workshop brought together experts in these three fields, as well as other interested astrophysicists and computer scientists. We report here our main conclusions, questions and suggestions for further steps toward integrating stellar evolution and stellar (hydro)dynamics.

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arXiv2002

To Circularize or Not to Circularize? -- Orbital Evolution of Satellite Galaxies

We investigated the orbital evolution of satellite galaxies using numerical simulations. It has been long believed that the orbit suffers circularization due to the dynamical friction from the galactic halo during orbital decay. This circularization was confirmed by numerous simulations where dynamical friction is added as external force. However, some of the resent N-body simulations demonstrated that circularization is much slower than expected from approximate calculations. We found that the dominant reason for this discrepancy is the assumption that Coulomb logarithm log Lambda is constant, which has been used in practically all recent calculations. Since the size of the satellite is relatively large, accurate determination of the outer cutoff radius is crucial to obtain good estimate for the dynamical friction. An excellent agreement between $N$-body simulations and approximate calculations was observed when the outer cutoff radius is taken to be the distance of the satellite to the center of the galaxy. When satellite is at the perigalacticon, the distance to the center is smaller and therefore log Lambda becomes smaller. As a result, the dynamical friction becomes less effective. We apply our result to the Large Magellanic Cloud. We found that the expected lifetime of the LMC is twice as long as that would be predicted with previous calculations. Previous study predicts that the LMC will merge into the Milky Way after 7 G years, while we found that the merging will take place after 14 G years from now. Our result suggests that generally satellites formed around a galaxy have longer lifetime than previous estimates.

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arXiv2001

The lives and deaths of star clusters near the Galactic center

We study the evolution and observability of young, compact star clusters near the Galactic center, such as the Arches and Quintuplet systems. The clusters are modeled by integrating the equations of motion of all stars while accounting for the internal evolution of stars and binaries, as well as the effect of the Galactic tidal field. We find that clusters within 150 pc of the Galactic center dissolve within ~55 Myr, but their projected densities drop below the background density in the direction of the Galactic center within only a few Myr, effectively making these clusters undetectable after that time. Detailed observations of the Arches cluster, taken at face value, suggest that its mass function is unusually flat and that the cluster contains an overabundance of stars more massive than 20 Msun. Our dynamical analysis, however, shows that the observed characteristics of the Arches cluster are consistent with a perfectly normal initial mass function. The observed anomalies are then caused by a combination of observational selection effects and the dynamical evolution of the cluster. We calibrate the current parameters of the Arches cluster using a normal initial mass function and conclude that the cluster is more massive than 40000 Msun, has a half mass radius of about 0.35 pc and is located between 50 and 90 pc from the Galactic center.

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arXiv2001

An efficient parallel algorithm for O(N^2) direct summation method and its variations on distributed-memory parallel machines

We present a novel, highly efficient algorithm to parallelize O(N^2) direct summation method for N-body problems with individual timesteps on distributed-memory parallel machines such as Beowulf clusters. Previously known algorithms, in which all processors have complete copies of the N-body system, has the serious problem that the communication-computation ratio increases as we increase the number of processors, since the communication cost is independent of the number of processors. In the new algorithm, p processors are organized as a $\sqrt{p}\times \sqrt{p}$ two-dimensional array. Each processor has $N/\sqrt{p}$ particles, but the data are distributed in such a way that complete system is presented if we look at any row or column consisting of $\sqrt{p}$ processors. In this algorithm, the communication cost scales as $N /\sqrt{p}$, while the calculation cost scales as $N^2/p$. Thus, we can use a much larger number of processors without losing efficiency compared to what was practical with previously known algorithms.

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arXiv2001

Structure of Dark Matter Halos From Hierarchical Clustering: II. Universality and Self-Similarity in Cluster-Sized Halos

We investigate the structure of the dark matter halo formed in three different cold dark matter scenarios. We performed N-body simulations of formation of 13 cluster-sized halos. In all runs, density cusps proportional to $r^{-1.5}$ developed at the center. This result was independent of the cosmological models we simulated. We could not reproduce the cusp shallower than $r^{-1.5}$, which was obtained in some of previous studies. We also found that in all runs the density structure evolves in a self-similar way, even in $Ω\ne 1$ universes. These results show that the formation of structural form is a process decoupled from a background cosmology.

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arXiv2001

Missing Link Found? --- The ``runaway'' path to supermassive black holes

Observations of stellar kinematics, gas dynamics and masers around galactic nuclei have now firmly established that many galaxies host central supermassive black holes (SMBHs) with masses in the range $10^6 \sim 10^9$M$_{\odot}$. However, how these SMBHs formed is not well understood. One reason for this situation is the lack of observations of intermediate-mass BHs (IMBHs), which could bridge the gap between stellar-mass BHs and SMBHs. Recently, this missing link (i.e., an IMBH) has been found in observations made by the ASCA and the Chandra of the central region of the starburst galaxy M82 \citep{MT99, PG99, MT01, Ka01}. Subsequent observations by SUBARU have revealed that this IMBH apparently coincides with a young compact star cluster. Based on these findings, we suggest a new formation scenario for SMBHs. In this scenario, IMBHs first form in young compact star clusters through runaway merging of massive stars. While these IMBHs are forming, the host star clusters sink toward the galactic nucleus through dynamical friction, and upon evaporation deposit their IMBHs near the galactic center. The IMBHs then form binaries and eventually merge via gravitational radiation, forming an SMBH.

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arXiv2001

Structure of Dark Matter Halos From Hierarchical Clustering

We investigate the structure of the dark matter halo formed in the cold dark matter scenario using $N$-body simulations. We simulated 12 halos with the mass of $6.6\times 10^{11}M_{\odot}$ to $8.0\times 10^{14}M_{\odot}$. In almost all runs, the halos have density cusps proportional to $r^{-1.5}$ developed at the center, which is consistent with the results of recent high-resolution calculations. The density structure evolves in a self-similar way, and is universal in the sense that it is independent of the halo mass and initial random realization of density fluctuation. The density profile is in good agreement with the profile proposed by Moore et al. (1999), which has central slope proportional to $r^{-1.5}$ and outer slope proportional to $r^{-3}$. The halo grows through repeated accretion of diffuse smaller halos. We argue that the cusp is understood as a convergence slope for the accretion of tidally disrupted matter.

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arXiv2000

Runaway Collisions in Star Clusters

We study the occurrence of physical collisions between stars in young and compact star cluster. The calculations are performed on the GRAPE-4 with the starlab software environment which include the dynamical evolution and the nuclear evolution of all stars and binaries. The selection of the initial conditions is based on existing and well observed star clusters, such as R136 in the 30 Doradus region in the Large Magellanic Cloud and the Arches and Quintuplet star clusters in the vicinity of the Galactic center. Collisions between stars occurred rather frequently in our models. At any time a single star dominates the collision history of the system. The collision rate of this runaway merger scales with the initial relaxation time of the cluster and is independent on other cluster parameters, such as the initial mass function or the initial density profile of the cluster. Subsequent encounters result in a steady grow in mass of the coagulating star, until it escapes or explodes in a supernova. The collision rate in these models is about 0.00022 collisions per star per Myr for a cluster with an initial relaxation time of 1 Myr.

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arXiv2000

Pseudoparticle Multipole Method: A Simple Method to Implement High-Accuracy Treecode

In this letter we describe the pseudoparticle multipole method (P2M2), a new method to express multipole expansion by a distribution of pseudoparticles. We can use this distribution of particles to calculate high order terms in both the Barnes-Hut treecode and FMM. The primary advantage of P2M2 is that it works on GRAPE. GRAPE is a special-purpose hardware for the calculation of gravitational force between particles. Although the treecode has been implemented on GRAPE, we could handle terms only up to dipole, since GRAPE can calculate forces from point-mass particles only. Thus the calculation cost grows quickly when high accuracy is required. With P2M2, the multipole expansion is expressed by particles, and thus GRAPE can calculate high order terms. Using P2M2, we implemented an arbitrary-order treecode on GRAPE-4. Timing result shows GRAPE-4 accelerates the calculation by a factor between 10 (for low accuracy) to 150 (for high accuracy). Even on general-purpose programmable computers, our method offers the advantage that the mathematical formulae and therefore the actual program is much simpler than that of the direct implementation of multipole expansion.

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arXiv2000

How many young star clusters exist in the Galactic center?

We study the evolution and observability of young compact star clusters within about 200pc of the Galactic center. Calculations are performed using direct N-body integration on the GRAPE-4, including the effects of both stellar and binary evolution and the external influence of the Galaxy. The results of these detailed calculations are used to calibrate a simplified model applicable over a wider range of cluster initial conditions. We find that clusters within 200 pc from the Galactic center dissolve within about 70 Myr. However, their projected densities drop below the background density in the direction of the Galactic center within 20 Myr, effectively making these clusters undetectable after that time. Clusters farther from the Galactic center but at the same projected distance are more strongly affected by this selection effect, and may go undetected for their entire lifetimes. Based on these findings, we conclude that the region within 200 pc of the Galactic center could easily harbor some 50 clusters with properties similar to those of the Arches or the Quintuplet systems.

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arXiv2000

Direct Simulation of Dense Stellar Systems with GRAPE-6

In this paper we describe the current status of the GRAPE-6 project to develop a special-purpose computer with a peak speed exceeding 100 Tflops for the simulation of astrophysical N-body problems. One of the main targets of the GRAPE-6 project is the simulation of dense stellar systems. In this paper, therefore, we overview the basic algorithms we use for the simulation of dense stellar systems and their characteristics. We then describe how we designed GRAPE hardwares to meet the requirements of these algorithms. GRAPE-6 will be completed by the year 2001. As an example of what science can be done on GRAPE-6, we describe our work on the galactic center with massive black holes performed on GRAPE-4, the predecessor of GRAPE-6.

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arXiv2000

Star cluster ecology IVa: Dissection of an open star cluster---photometry

The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account. Initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations include a realistic mass function, primordial binaries and the external potential of the parent Galaxy. Our model clusters are generally significantly flattened in the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially due to the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries. At late times, the cluster core is quite rich in giants and white dwarfs. There is no evidence for preferential evaporation of old white dwarfs, on the contrary the formed white dwarfs are likely to remain in the cluster. Stars tend to escape from the cluster through the first and second Lagrange points, in the direction of and away from the Galactic center. Mass segregation manifests itself in our models well within an initial relaxation time. As expected, giants and white dwarfs are much more strongly affected by mass segregation than main-sequence stars. Open clusters are dynamically rather inactive. However, the combined effect of stellar mass loss and evaporation of stars from the cluster potential drives its dissolution on a much shorter timescale than if these effects are neglected. The often-used argument that a star cluster is barely older than its relaxation time and therefore cannot be dynamically evolved is clearly in error for the majority of star clusters.

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arXiv1999

7.0/Mflops Astrophysical N-Body Simulation with Treecode on GRAPE-5

As an entry for the 1999 Gordon Bell price/performance prize, we report an astrophysical N-body simulation performed with a treecode on GRAPE-5 (Gravity Pipe 5) system, a special-purpose computer for astrophysical N-body simulations. The GRAPE-5 system has 32 pipeline processors specialized for the gravitational force calculation. Other operations, such as tree construction, tree traverse and time integration, are performed on a general purpose workstation. The total cost for the GRAPE-5 system is 40,900 dollars. We performed a cosmological N-body simulation with 2.1 million particles, which sustained a performance of 5.92 Gflops averaged over 8.37 hours. The price per performance obtained is 7.0 dollars per Mflops.

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arXiv1999

On the Cusp around Central Black Holes in Luminous Elliptical Galaxies

In this letter, we show that a massive black hole (MBH) which falls into the center of a galaxy in dynamical timescale leaves a weak cusp ($ρ\propto r^{-1/2}$) around it, which is in good agreement with the recent observations of luminous ellipticals by Hubble Space Telescope. Such event is a natural outcome of merging of two galaxies which have central MBHs. This is the only known mechanism to form weak cusps in luminous ellipticals. Therefore, the existence of the weak cusps indicates the central BHs of luminous ellipticals have fallen to the center from outside, most likely during a major merger event.

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