H. Daisaka

Kobe University, Kōbe, Hyōgo, Japan

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Publications (25)30.3 Total impact

  • Keiji Ohtsuki, Yuki Yasui, Hiroshi Daisaka
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    ABSTRACT: We examine the gravitational capture probability of colliding particles in circumplanetary particle disks and accretion rates of small particles onto an embedded moonlet, using analytic calculation, three-body orbital integrations, and N-body simulations. Expanding our previous work, we take into account the Rayleigh distribution of particles' orbital eccentricities and inclinations in our analytic calculation and orbital integration and confirm agreement between them when the particle velocity dispersion is comparable to or larger than their mutual escape velocity and the ratio of the sum of the physical radii of colliding particles to their mutual Hill radius () is much smaller than unity. As shown by our previous work, the capture probability decreases significantly when the velocity dispersion is larger than the escape velocity and/or . Rough surfaces of particles can enhance the capture probability. We compare the results of three-body calculations with N-body simulations for accretion of small particles by an embedded moonlet and find agreement at the initial stage of accretion. However, when particles forming an aggregate on the moonlet surface nearly fill the Hill sphere, the aggregate reaches a quasi-steady state with a nearly constant number of particles covering the moonlet, and the accretion rate is significantly reduced compared to the three-body results.
    The Astronomical Journal 06/2013; 146(2):25. · 4.97 Impact Factor
  • Y. Yasui, K. Ohtsuki, H. Daisaka
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    ABSTRACT: Using local N-body simulations, we investigate the process of accretion of small ring particles onto a larger moonlet in Saturn's rings.
    03/2013;
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    Keiji Ohtsuki, H. Daisaka, H. Tanaka
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    ABSTRACT: Viscosity in planetary rings arises from collisional and gravitational interactions between constituent particles. Angular momentum is transferred mainly through mutual collisions and gravitational encounters when the optical depth of the ring is low, while the formation of gravitational wakes significantly enhances the viscosity in dense rings (Daisaka, Tanaka, Ida 2001, Icarus 154, 296). On the basis of the formulation derived by Tanaka, Ohtsuki, and Daisaka (2003, Icarus 161, 144) and using analytic calculation, three-body orbital integration and N-body simulation, we investigate viscosity in self-gravitating planetary rings, both with and without the effect of particle spins. In the case of rings with low optical depth, we confirmed agreement between results of three-body calculation and N-body simulation. When the optical depth is low and the effect of particles' mutual gravity is weak, particles' surface friction with a reasonable range of a friction parameter tends to reduce their random velocity and, consequently, viscosity as well. However, in dense rings with gravitational wakes, the effect of self-gravity is dominant. We obtain ring viscosity for a wide range of parameter values, and derive an approximate expression which reproduces our numerical results. This work was supported by NASA's Outer Planets Research Program and the Cassini Project.
    The Astronomical Journal 05/2012; 143(5). · 4.97 Impact Factor
  • Y. Yasui, K. Ohtsuki, H. Daisaka
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    ABSTRACT: Using local N-body simulation, we examine viscosity of planetary rings consisting of spinning, self-gravitating particles for a wide range of parameters, including the cases of dense rings with temporary aggregate formation.
    03/2012;
  • J. Makino, H. Daisaka
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    ABSTRACT: In this paper, we describe the design and performance of GRAPE-8 accelerator processor for gravitational N-body simulations. It is designed to evaluate gravitational interaction with cutoff between particles. The cutoff function is useful for schemes like TreePM or Particle-Particle Particle-Tree, in which gravitational force is divided to short-range and longrange components. A single GRAPE-8 processor chip integrates 48 pipeline processors. The effective number of floating-point operations per interaction is around 40. Thus the peak performance of a single GRAPE-8 processor chip is 480 Gflops. A GRAPE-8 processor card houses two GRAPE-8 chips and one FPGA chip for PCI-Express interface. The total power consumption of the board is 46W. Thus, theoretical peak performance per watt is 20.5 Gflops/W. The effective performance of the total system, including the host computer, is around 5Gflops/W. This is more than a factor of two higher than the highest number in the current Green500 list.
    High Performance Computing, Networking, Storage and Analysis (SC), 2012 International Conference for; 01/2012
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    ABSTRACT: We describe the design and performance of the GRAPE-MPs, a series of SIMD accelerator boards for quadruple/hexuple/octuple-precision arithmetic operations. Basic design of GRAPE-MPs is that it consists of a number of processing elements (PE) and memory components which handle data with quadruple/hexuple/octuple-precision. A GRAPE-MPs processor is implemented on a structured ASIC chip and an FPGA chip. GRAPE-MP (quadruple-precision) uses a structured ASIC chip from eASIC corp., which has 6 PE and operates with 100MHz clock cycle. The theoretical peak quadruple-precision performance of the single board is 1.2 Gflops and the achieved performance for the Feynman loop integrals is about 0.5 Gflops. GRAPE-MP4/6/8 (quadruple/hexuple/octuple-precision) uses an FPGA chip from Aletra corporation. For example, in the current implementation, MP8 has 10 PE with 70MHz operation clock cycle. We also present the performance results with the multiple GRAPE-MPs boards. The achieved performance of four MP8 boards is about 1.6 Gflops. It is roughly 90 times faster than the performance of a single core of a CPU with comparable precision. We show that our hardware based approach to evaluate the Feynman loop integrals in high precision arithmetic operations is highly effective.
    Embedded Multicore Socs (MCSoC), 2012 IEEE 6th International Symposium on; 01/2012
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    ABSTRACT: Ultraluminous infrared galaxies (ULIRGs) with multiple ($\ge 3$) nuclei are frequently observed. It has been suggested that these nuclei are produced by multiple major mergers of galaxies. The expected rate of such mergers is, however, too low to reproduce the observed number of ULIRGs with multiple nuclei. We have performed high-resolution simulations of the merging of two gas-rich disk galaxies. We found that extremely massive and compact star clusters form from the strongly disturbed gas disks after the first or second encounter between the galaxies. The mass of such clusters reaches $\sim 10^8 M_{\odot}$, and their half-mass radii are $20-30 \rm{pc}$. Since these clusters consist of young stars, they appear to be several bright cores in the galactic central region ($\sim \rm{kpc}$). The peak luminosity of these clusters reaches $\sim 10%$ of the total luminosity of the merging galaxy. These massive and compact clusters are consistent with the characteristics of the observed multiple nuclei in ULIRGs. Multiple mergers are not necessary to explain multiple nuclei in ULIRGs.
    The Astrophysical Journal 11/2011; 746(1). · 6.73 Impact Factor
  • Y. Yasui, K. Ohtsuki, H. Daisaka
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    ABSTRACT: Saturn's rings are composed of many icy particles, and angular momentum is transported due to collision and gravitational interaction between these particles. Viscosity in the rings arising from such interactions between particles governs the rate of dynamical evolution and structure formation in the rings. We examine the effect of surface friction on the viscosity and the dependence of the viscosity on optical depth and distance from Saturn, using local N-body simulation. We derive semi-analytic formula that approximately reproduces our numerical results.
    10/2011;
  • Y. Yasui, K. Ohtsuki, H. Daisaka
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    ABSTRACT: Using local N-body simulation for planetary rings consisting of self-gravitating particles with surface friction, we examine the dependence of viscosity on various parameters such as optical depth and normal and tangential restitution coefficients.
    03/2011;
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    ABSTRACT: As to April 2010, 48 TNO (trans-Neptunian Object) binaries have been found. This is about 6% of known TNOs. However, in previous theoretical studies of planetary formation in the TNO region, the effect of binary formation has been neglected. TNO binaries can be formed through a variety of mechanisms, such as three-body process, dynamical friction on two massive bodies, inelastic collisions between two bodies etc. Most of these mechanisms become more effective as the distance from the Sun increases. In this paper, we studied three-body process using direct N-body simulations. We systematically changed the distance from the Sun, the number density of planetesimals, and the radius of the planetesimals and studied the effect of the binaries on the collision rate of planetesimals. In the TNO region, binaries are involved in 1/3 - 1/2 of collisions, and the collision rate is increased by about a factor of a few compared to the theoretical estimate for the direct two-body collisions. Thus, it is possible that the binaries formed through three-body process significantly enhance the collision rate and reduce the growth time scale. In the terrestrial planet region, binaries are less important, because the ratio between the Hill radius and physical size of the planetesimals is relatively small. Although the time scale of our simulations is short, they clearly demonstrated that the accretion process in the TNO region is quite different from that in the terrestrial planet region. Simulations which cover longer time scale are required to obtain more accurate estimate for the accretion enhancement.
    Publications- Astronomical Society of Japan 02/2011; · 2.44 Impact Factor
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    ABSTRACT: We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamcs simulations of colliding galaxies. The total number of particles is 1.2x10^8 for our high resolution run. The gravitational softening is 5 pc and we allow gas to cool down to \sim 10 K. During the first encounter of the collision, a giant filament consists of cold and dense gas found between the progenitors by shock compression. A vigorous starburst took place in the filament, resulting in the formation of star clusters. The mass of these star clusters ranges from 10^{5-8} Msun. These star clusters formed hierarchically: at first small star clusters formed, and then they merged via gravity, resulting in larger star clusters.
    Proceedings of the International Astronomical Union 01/2011; 270.
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    ABSTRACT: We describe the design and performance of the GRAPE-MP board, an SIMD accelerator board for quadrupleprecision arithmetic operations. A GRAPE-MP board houses one GRAPE-MP processor chip and an FPGA chip which handles the communication with the host computer. A GRAPE-MP chip has 6 processing elements (PE) and operates with 100MHz clock cycle. Each PE can perform one addition and one multiplication in every clock cycle. The architecture of the GRAPE-MP is similar to that of the GRAPE-DR. It is implemented using the structured ASIC chip from eASIC corp. A GRAPE-MP processor board has the theoretical peak quadruple-precision performance of 1.2 Gflops. As a preliminary result, we present the performance of the GRAPE-MP board for two target applications. The performance of the numerical integration of Feynman loop is 0.53 Gflops. The performance of a N-body simulation with the second order leapfrog schema is 0.505 Gflops for N = 1984, which is more than 10 times faster than the performance of the host computer.
    Procedia CS. 01/2011; 4:878-887.
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    ABSTRACT: We describe the implementation and performance of dense matrix multiplication and LU decomposition on the GRAPE-DR SIMD accelerator board. A GRAPE-DR card, with 4 GRAPE-DR chips, has the theoretical peak DP performance of 819 Gflops. Each GRAPE-DR chip has 512 processing elements and operates with 400MHz clock cycle. each PE can perform one addition and one multiplication in every two clock cycles. The measured performance of matrix multiplication is 730 Gflops for the multiplication of matrices with size 51200 by 2048 and 2048 by 51200. The performance of LU decomposition is 480 Gflops for the problem size of 51200.
    Procedia CS. 01/2011; 4:888-897.
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    ABSTRACT: We performed high resolution simulations of galaxy-galaxy merging in order to investigate the mechanism of a starburst. The multiphase nature of the interstellar medium is correctly taken into account in our model, which allows the use of a realistic star formation model. In these simulations, we for the first time found that the shock-induced starburst involving star cluster (SC) formation takes place during the first encounter. Detailed analyses show that the SC formation is mainly driven by hierarchical mergings of proto (smaller) SCs. Our result implies that SCs can become much more massive than the local Jeans mass of the SC forming region.
    06/2010;
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    ABSTRACT: We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of gas density showed clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at $n_{\rm H} \sim 10{\rm cm^{-3}}$ in the PDF, and then the excess moves to higher densities ($n_{\rm H} \gtrsim 100{\rm cm^{-3}}$) in a few times $10^7$ years where starburst takes place. After the starburst, the PDF goes back to the quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters.
    Publications- Astronomical Society of Japan 06/2008; · 2.44 Impact Factor
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    ABSTRACT: We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T < 100 K & n_H > 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged)
    Publications- Astronomical Society of Japan 03/2008; · 2.44 Impact Factor
  • H. Daisaka, J. Makino
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    ABSTRACT: We investigate the formation and maintenance of the Uranian eccentric rings by N-body simulation of a ring-satellite system. Our simulation demonstrates the formation and evolution of an eccentric ring from an initially circular ring.
    01/2003;
  • Hidekazu Tanaka, Keiji Ohtsuki, Hiroshi Daisaka
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    ABSTRACT: We present a new formulation of the viscosity in planetary rings, where particles interact through their gravitational forces and direct collisions. In the previous studies on the viscosity in self-gravitating rings, the viscosity consists of three components, which are defined separately in different ways. The complex definitions make it difficult to evaluate the viscosity in N-body simulation of rings. In our new formulation, the viscosity is expressed in terms of changes in orbital elements of particles due to particle interactions. This makes the expression of the viscosity simple. The new formulation gives a simple way to evaluate the viscosity in N-body simulation. We find that for practical evaluation of the viscosity of planetary rings, only energy dissipation at direct inelastic collisions is needed.For tenuous particle disks (i.e., optically thin disks), we further derive a formula of the viscosity. The formula requires only a numerical coefficient that can be obtained from three-body calculation. Since planetesimal disks are also tenuous, the viscosity in planetesimal disks can be also obtained from this formula. In a subsequent paper, we will evaluate this coefficient through three-body calculation and obtain the viscosity for a wide range of parameters such as the restitution coefficient and the radial location in rings.
    Icarus 01/2003; 161(1):144-156. · 3.16 Impact Factor
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    H. Daisaka
    Proceedings of the International Astronomical Union 01/2003; 208:387.
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    ABSTRACT: As an entry for the 2002 Gordon Bell performance prize, we report the performance achieved on the GRAPE-6 system for a simulation of the early evolution of the protoplanet-planetesimal system of the Uranus-Neptune region. GRAPE-6 is a special-purpose computer for astrophysical N-body calculations. The present configuration has 2048 custom pipeline chips, each containing six pipeline processors for the calculation of gravitational interactions between particles. Its theoretical peak performance is 63.4 Tflops. The actual performance obtained was 29.5 Tflops, for a simulation of the early evolution of outer Solar system with 1.8 million planetesimals and two massive protoplanets.
    Supercomputing, ACM/IEEE 2002 Conference; 12/2002

Publication Stats

108 Citations
30.30 Total Impact Points

Institutions

  • 2013
    • Kobe University
      • Department of Earth and Planetary Sciences
      Kōbe, Hyōgo, Japan
  • 2008–2012
    • Hitotsubashi University
      Edo, Tōkyō, Japan
  • 2002–2003
    • The University of Tokyo
      • • Department of Astronomy
      • • College of Art and Science & Graduate School of Arts and Sciences
      Edo, Tōkyō, Japan
  • 2000–2001
    • Tokyo Institute of Technology
      • Earth and Planetary Sciences Department
      Tokyo, Tokyo-to, Japan