The influence of initial mass segregation on the runaway merging of stars

We have investigated the effect of initial mass segregation on the runaway merging of stars. The evolution of multi-mass, dense star clusters was followed by means of direct N-body simulations of up to 131.072 stars. All clusters started from King models with dimensionless central potentials of 3.0 <= W_0 <= 9.0. Initial mass segregation was realized by varying the minimum mass of a certain fraction of stars whose either (1) distances were closest to the cluster center or (2) total energies were lowest. The second case is more favorable to promote the runaway merging of stars by creating a high-mass core of massive, low-energy stars. Initial mass segregation could decrease the central relaxation time and thus help the formation of a high-mass core. However, we found that initial mass segregation does not help the runaway stellar merger to happen if the overall mass density profile is kept constant. This is due to the fact that the collision rate of stars is not increased due to initial mass segregation. Our simulations show that initial mass segregation is not sufficient to allow runaway merging of stars to occur in clusters with central densities typical for star clusters in the Milky Way.Comment: 25 pages, 9 figures, 3 tables, accepted for publication in Ap

Dynamical Interactions and the Black Hole Merger Rate of the Universe

Binary black holes can form efficiently in dense young stellar clusters, such as the progenitors of globular clusters, via a combination of gravitational segregation and cluster evaporation. We use simple analytic arguments supported by detailed N-body simulations to determine how frequently black holes born in a single stellar cluster should form binaries, be ejected from the cluster, and merge through the emission of gravitational radiation. We then convolve this ``transfer function'' relating cluster formation to black hole mergers with (i) the distribution of observed cluster masses and (ii) the star formation history of the universe, assuming that a significant fraction gcl of star formation occurs in clusters and that a significant fraction gcand of clusters undergo this segregation and evaporation process. We predict future ground--based gravitational wave (GW) detectors could observe ~500 (gcl/0.5) (gcand/0.1) double black hole mergers per year, and the presently operating LIGO interferometer would have a chance (50%) at detecting a merger during its first full year of science data. More realistically, advanced LIGO and similar next-generation gravitational wave observatories provide unique opportunities to constrain otherwise inaccessible properties of clusters formed in the early universe.Comment: 4 pages, 2 figures. To appear in PRD Rapid Communication

On Convergence of the Inexact Rayleigh Quotient Iteration with the Lanczos Method Used for Solving Linear Systems

For the Hermitian inexact Rayleigh quotient iteration (RQI), the author has established new local general convergence results, independent of iterative solvers for inner linear systems. The theory shows that the method locally converges quadratically under a new condition, called the uniform positiveness condition. In this paper we first consider the local convergence of the inexact RQI with the unpreconditioned Lanczos method for the linear systems. Some attractive properties are derived for the residuals, whose norms are $\xi_{k+1}$'s, of the linear systems obtained by the Lanczos method. Based on them and the new general convergence results, we make a refined analysis and establish new local convergence results. It is proved that the inexact RQI with Lanczos converges quadratically provided that $\xi_{k+1}\leq\xi$ with a constant $\xi\geq 1$. The method is guaranteed to converge linearly provided that $\xi_{k+1}$ is bounded by a small multiple of the reciprocal of the residual norm $\|r_k\|$ of the current approximate eigenpair. The results are fundamentally different from the existing convergence results that always require $\xi_{k+1}<1$, and they have a strong impact on effective implementations of the method. We extend the new theory to the inexact RQI with a tuned preconditioned Lanczos for the linear systems. Based on the new theory, we can design practical criteria to control $\xi_{k+1}$ to achieve quadratic convergence and implement the method more effectively than ever before. Numerical experiments confirm our theory.Comment: 20 pages, 8 figures. arXiv admin note: text overlap with arXiv:0906.223

Merger of Black Holes in the Galactic Center

We present the results of three body simulations focused on understanding the fates of intermediate mass black holes (IBH) that drift within the central 0.5 pc of the Galaxy. In particular, we modeled the interactions between pairs of $4000 {\rm M}_{\odot}$ black holes as they orbit a central blac k hole of mass $4 \times 10^6 {\rm M}_{\odot}$. The simulations performed assume a Schwarzschild geometry and account for Chandrasekhar dynamical friction as well as acceleration resulting from energy lost due to gravitational radiation. We found the branching ratio for one of the orbiting IBHs to merge with the CBH was 0.95 and is independent of the inner IBH's initial eccentricity as well as the rate of sinking. This, coupled with an infall rate of $\sim 10^7$ yrs for an IBH to drift into the Galactic center, results in an IBH-CBH merger every $\lesssim 11$ Myrs. Lastly we found that the IBH-IBH-CBH triple body system ``resets'' itself, in the sense that a system with an inner I BH with an initially circular orbit generally left behind an IBH with a large eccentricity, whereas a system in which the inner IBH had a high eccentricity ($e_0 \sim 0.9$) usually left a remnant with low eccentricity. Branching ratios for different outcomes are also similar in the two cases.Comment: Official paper to appear in November 2008 issue of Ap

Rapid formation of exponential disks and bulges at high redshift from the dynamical evolution of clump cluster and chain galaxies

Many galaxies at high redshift have peculiar morphologies dominated by 10^8-10^9 Mo kpc-sized clumps. Using numerical simulations, we show that these "clump clusters" can result from fragmentation in gravitationally unstable primordial disks. They appear as "chain galaxies" when observed edge-on. In less than 1 Gyr, clump formation, migration, disruption, and interaction with the disk cause these systems to evolve from initially uniform disks into regular spiral galaxies with an exponential or double-exponential disk profile and a central bulge. The inner exponential is the initial disk size and the outer exponential is from material flung out by spiral arms and clump torques. A nuclear black hole may form at the same time as the bulge from smaller black holes that grow inside the dense cores of each clump. The properties and lifetimes of the clumps in our models are consistent with observations of the clumps in high redshift galaxies, and the stellar motions in our models are consistent with the observed velocity dispersions and lack of organized rotation in chain galaxies. We suggest that violently unstable disks are the first step in spiral galaxy formation. The associated starburst activity gives a short timescale for the initial stellar disk to form.Comment: ApJ Accepted, 13 pages, 9 figure

Decay Resistance of Saltwater-Exposed Douglas-Fir Piles

Wood that is submerged in fresh water for long periods has increased permeability and other changed properties. Wood that is submerged in salt water may absorb considerable quantities of salts, which may render the wood resistant to microbial colonization. In this report, we describe decay tests on Douglas-fir sapwood and heartwood after long-term exposure in the Great Salt Lake of Utah. This wood was generally resistant to fungal attack, but was susceptible to leaching. Scanning electron microscopic examination revealed that salt crystals in the wood were primarily sodium chloride, which was readily removed in a leaching procedure. Decay resistance attributed to saltwater exposure declined with prolonged leaching

Extreme mass ratio inspiral rates: dependence on the massive black hole mass

We study the rate at which stars spiral into a massive black hole (MBH) due to the emission of gravitational waves (GWs), as a function of the mass M of the MBH. In the context of our model, it is shown analytically that the rate approximately depends on the MBH mass as M^{-1/4}. Numerical simulations confirm this result, and show that for all MBH masses, the event rate is highest for stellar black holes, followed by white dwarfs, and lowest for neutron stars. The Laser Interferometer Space Antenna (LISA) is expected to see hundreds of these extreme mass ratio inspirals per year. Since the event rate derived here formally diverges as M->0, the model presented here cannot hold for MBHs of masses that are too low, and we discuss what the limitations of the model are.Comment: Accepted to CQG, special LISA issu

The role of contacts in graphene transistors: A scanning photocurrent study

A near-field scanning optical microscope is used to locally induce photocurrent in a graphene transistor with high spatial resolution. By analyzing the spatially resolved photo-response, we find that in the n-type conduction regime a p-n-p structure forms along the graphene device due to the doping of the graphene by the metal contacts. The modification of the electronic structure is not limited only underneath the metal electrodes, but extends 0.2-0.3 um into the graphene channel. The asymmetric conduction behavior of electrons and holes that is commonly observed in graphene transistors is discussed in light of the potential profiles obtained from this photocurrent imaging approach. Furthermore, we show that photocurrent imaging can be used to probe single- / multi-layer graphene interfaces

A Lightweight Service Placement Approach for Community Network Micro-Clouds

Community networks (CNs) have gained momentum in the last few years with the increasing number of spontaneously deployed WiFi hotspots and home networks. These networks, owned and managed by volunteers, offer various services to their members and to the public. While Internet access is the most popular service, the provision of services of local interest within the network is enabled by the emerging technology of CN micro-clouds. By putting services closer to users, micro-clouds pursue not only a better service performance, but also a low entry barrier for the deployment of mainstream Internet services within the CN. Unfortunately, the provisioning of these services is not so simple. Due to the large and irregular topology, high software and hardware diversity of CNs, a "careful" placement of micro-clouds services over the network is required to optimize service performance. This paper proposes to leverage state information about the network to inform service placement decisions, and to do so through a fast heuristic algorithm, which is critical to quickly react to changing conditions. To evaluate its performance, we compare our heuristic with one based on random placement in Guifi.net, the biggest CN worldwide. Our experimental results show that our heuristic consistently outperforms random placement by 2x in bandwidth gain. We quantify the benefits of our heuristic on a real live video-streaming service, and demonstrate that video chunk losses decrease significantly, attaining a 37% decrease in the packet loss rate. Further, using a popular Web 2.0 service, we demonstrate that the client response times decrease up to an order of magnitude when using our heuristic. Since these improvements translate in the QoE (Quality of Experience) perceived by the user, our results are relevant for contributing to higher QoE, a crucial parameter for using services from volunteer-based systems and adapting CN micro-clouds as an eco-system for service deployment