We developed a new direct-tree hybrid N -body algorithm for fully self-consistent N -body simulations of star clusters in their parent galaxies. In such simulations, star clusters need high accuracy, while galaxies need a fast scheme because of the large number of the particles required to model it. In our new algorithm, the internal motion of the star cluster is calculated accurately using the direct Hermite scheme with individual timesteps and all other motions are calculated using the tree code with second-order leapfrog integrator. The direct and tree schemes are combined using an extension of the mixed variable symplectic (MVS) scheme. Thus, the Hamiltonian corresponding to everything other than the internal motion of the star cluster is integrated with the leapfrog, which is symplectic. Using this algorithm, we performed fully self-consistent N -body simulations of star clusters in their parent galaxy. The internal and orbital evolutions of the star cluster agreed well with those obtained using the direct scheme. We also performed fully self-consistent N -body simulation for large-N models (N = 2 × 10 6 ). In this case, the calculation speed was seven times faster than what would be if the direct scheme was used.

We present the basic idea, implementation, measured performance and performance model of FDPS (Framework for developing particle simulators). FDPS is an application-development framework which helps the researchers to develop simulation programs using particle methods for large-scale distributed-memory parallel supercomputers. A particle-based simulation program for distributed-memory parallel computers needs to perform domain decomposition, exchange of particles which are not in the domain of each computing node, and gathering of the particle information in other nodes which are necessary for interaction calculation. Also, even if distributed-memory parallel computers are not used, in order to reduce the amount of computation, algorithms such as Barnes-Hut tree algorithm or Fast Multipole Method should be used in the case of long-range interactions. For short-range interactions, some methods to limit the calculation to neighbor particles are necessary. FDPS provides all of these functions which are necessary for efficient parallel execution of particle-based simulations as "templates", which are independent of the actual data structure of particles and the functional form of the particle-particle interaction. By using FDPS, researchers can write their programs with the amount of work necessary to write a simple, sequential and unoptimized program of O(N 2 ) calculation cost, and yet the program, once compiled with FDPS, will run efficiently on largescale parallel supercomputers. A simple gravitational N -body program can be written in around 120 lines. We report the actual performance of these programs and the performance model. The weak scaling performance is very good, and almost linear speedup was obtained for up to the full system of K computer. The minimum calculation time per timestep is in the range of 30 ms (N = 10 7 ) to 300 ms (N = 10 9 ). These are currently limited by the time for the calculation of the domain decomposition and communication necessary for the interaction calculation. We discuss how we can overcome these bottlenecks.

Background: Mature osteoclasts with a spontaneous tendency toward apoptosis resorb bone efficiently during their short lifespan. Results: Released ATP from intracellular stores has a negative impact on the bone resorption activity of osteoclasts by altering their cytoskeletal structures. Conclusion: ATP depletion leads to osteoclastic bone resorption. Significance: This study provides a new direction for investigating the mechanisms involved in physiological and pathological bone resorption.

We present the result of N-body simulations of dynamical evolution of triple massive blackhole (BH) systems in galactic nuclei. We found that in most cases two of the three BHs merge through gravitational wave (GW) radiation in the timescale much shorter than the Hubble time, before ejecting one BH through a slingshot. In order for a binary BH to merge before ejecting out the third one, it has to become highly eccentric since the gravitational wave timescale would be much longer than the Hubble time unless the eccentricity is very high. We found that two mechanisms drive the increase of the eccentricity of the binary. One is the strong binary-single BH interaction resulting in the thermalization of the eccentricity. The second is the Kozai mechanism which drives the cyclic change of the inclination and eccentricity of the inner binary of a stable hierarchical triple system. Our result implies that many of supermassive blackholes are binaries.

We performed, for the first time, the simulation of spiral-in of a star cluster formed close to the Galactic center (GC) using a fully self-consistent N -body model. In our model, the central super-massive black hole (SMBH) is surrounded by stars and the star cluster. Not only are the orbits of stars and the cluster stars integrated self-consistently, but the stellar evolution, collisions and merging of the cluster stars are also included. We found that an intermediate-mass black hole (IMBH) is formed in the star cluster and stars escaped from the cluster are captured into a 1:1 mean motion resonance with the IMBH. These "Trojan" stars are brought close to the SMBH by the IMBH, which spirals into the GC due to the dynamical friction. Our results show that, once the IMBH is formed, it brings the massive stars to the vicinity of the central SMBH even after the star cluster itself is disrupted. Stars carried by the IMBH form a disk similar to the observed disks and the core of the cluster including the IMBH has properties similar to those of IRS13E, which is a compact assembly of several young stars.

In recent numerical simulations (Matsubayashi et al. 2007;Löckmann, & Baumgardt 2008), it has been found that the eccentricity of supermassive black hole(SMBH) -intermediate black hole(IMBH) binaries grows toward unity through interactions with stellar background. This increase of eccentricity reduces the merging timescale of the binary through the gravitational radiation to the value well below the Hubble Time. It also gives the theoretical explanation of the existence of eccentric binary such as that in OJ287 (Lehto, & Valtonen 1996;Valtonen et al. 2008). In self-consistent N-body simulations, this increase of eccentricity is always observed. On the other hand, the result of scattering experiment between SMBH binaries and field stars (Quinlan 1996) indicated no increase of eccentricity. This discrepancy leaves the high eccentricity of the SMBH binaries in N-body simulations unexplained. Here we present a stellar-dynamical mechanism that drives the increase of the eccentricity of an SMBH binary with large mass ratio. There are two key processes involved. The first one is the Kozai mechanism under non-axisymmetric potential, which effectively randomizes the angular momenta of surrounding stars. The other is the selective ejection of stars with prograde orbits. Through these two mechanisms, field stars extract the orbital angular momentum of the SMBH binary. Our proposed mechanism causes the increase in the eccentricity of most of SMBH binaries, resulting in the rapid merger through gravitational wave radiation. Our result has given a definite solution to the "last-parsec problem"

Abstract 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 supercomputer. 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 nuclear star clusters, becomes possible to be solved.

The anti-apoptotic molecule Bcl-2 inhibits apoptosis by preventing cytochrome c release from mitochondria. Although several studies have indicated the importance of Bcl-2 in maintaining skeletal integrity, the detailed cellular and molecular mechanisms remain elusive. Bcl-2 ؊/؊ mice are growth-retarded and exhibit increased bone volume of the primary spongiosa, mainly due to the decreased number and dysfunction of osteoclasts. Osteoblast function is also impaired in Bcl-2 ؊/؊ mice.Ex vivo studies on osteoblasts and osteoclasts showed that Bcl-2 promoted the differentiation, activation, and survival of both cell types. Because Bcl-2 ؊/؊ mice die before 6 weeks of age due to renal failure and cannot be compared with adult wild type mice, we generated Bcl-2 ؊/؊ Bim ؉/؊ mice, in which a single Bim allele was inactivated, and compared them with theirBcl-2 ؉/؊ Bim ؉/؊ littermates. Loss of a single Bim allele restored normal osteoclast function in Bcl-2 ؊/؊ mice but did not restore the impaired function of osteoblasts, and the mice exhibited osteopenia. These data demonstrate that Bcl-2 promotes the differentiation, activity, and survival of both osteoblasts and osteoclasts. The balance between Bcl-2 and Bim regulates osteoclast apoptosis and function, whereas other pro-apoptotic members are important for osteoblasts.