3,376 research outputs found
Stellar collisions in accreting protoclusters: a Monte Carlo dynamical study
We explore the behaviour of accreting protoclusters with a Monte Carlo
dynamical code in order to evaluate the relative roles of accretion, two body
relaxation and stellar collisions in the cluster evolution. We corroborate the
suggestion of Clarke & Bonnell that the number of stellar collisions should
scale as (independent of other cluster parameters, where
N is the number of stars in the cluster and the rate of mass
accretion) and thus strengthen the argument that stellar collisions are more
likely in populous (large N) clusters. We however find that the estimates of
Clarke & Bonnell were pessimistic in the sense that we find that more than 99 %
of the stellar collisions occur within the post-adiabatic regime as the cluster
evolves towards core collapse, driven by a combination of accretion and
two-body relaxation. We discuss how the inclusion of binaries may reduce the
number of collisions through the reversal of core collapse but also note that
it opens up another collisional channel involving the merger of stars within
hard binaries; future Nbody simulations are however required in order to
explore this issue.Comment: 9 pages, 9 figures; accepted for publication in MNRAS. This version
contains minor revisions after referee's comments
Prototyping Incentive-based Resource Assignment for Clouds in Community Networks
Wireless community networks are a successful example of a collective where communities operate ICT infrastructure and provide IP connectivity based on the principle of reciprocal resource sharing of network bandwidth. This sharing, however, has not extended to computing and storage resources, resulting in very few applications and services which are currently deployed within community networks. Cloud computing, as in today's Internet, has made it common to consume resources provided by public clouds providers, but such cloud infrastructures have not materialized within community networks. We analyse in this paper socio-technical characteristics of community networks in order to derive scenarios for community clouds. Based on an architecture for such a community cloud, we implement a prototype for the incentive-driven resource assignment component, deploy it in a testbed of community network nodes, and evaluate its behaviour experimentally. Our evaluation gives insight into how the deployed prototype components regulate the consumption of cloud resources taking into account the users' contributions, and how this regulation affects the system usage. Our results suggest a further integration of this regulation component into current cloud management platforms in order to open them up for the operation of an ecosystem of community cloud
Support Service for Reciprocal Computational Resource Sharing in Wireless Community Networks
In community networks, individuals and local organizations from a geographic area team up to create and run a community-owned IP network to satisfy the community's demand for ICT, such as facilitating Internet access and providing services of local interest. Most current community networks use wireless links for the node interconnection, applying off-the-shelf wireless equipment. While IP connectivity over the shared network infrastructure is successfully achieved, the deployment of applications in community networks is surprisingly low. To address the solution of this problem, we propose in this paper a service to incentivize the contribution of computing and storage as cloud resources to community networks, in order to stimulate the deployment of services and applications. Our final goal is the vision that in the long term, the users of community networks will not need to consume applications from the Internet, but find them within the wireless community network
Device modeling of long-channel nanotube electro-optical emitter
We present a simple analytic model of nanotube electro-optical emitters,
along with improved experimental measurements using PMMA-passivated devices
with reduced hysteresis. Both the ambipolar electrical characteristics and the
motion of the infrared-emission spot are well described. The model indicates
that the electric field is strongly enhanced at the emission spot, and that
device performance can be greatly improved by the use of thinner gate oxides
Improved approximate inspirals of test-bodies into Kerr black holes
We present an improved version of the approximate scheme for generating
inspirals of test-bodies into a Kerr black hole recently developed by
Glampedakis, Hughes and Kennefick. Their original "hybrid" scheme was based on
combining exact relativistic expressions for the evolution of the orbital
elements (the semi-latus rectum p and eccentricity e) with approximate,
weak-field, formula for the energy and angular momentum fluxes, amended by the
assumption of constant inclination angle, iota, during the inspiral. Despite
the fact that the resulting inspirals were overall well-behaved, certain
pathologies remained for orbits in the strong field regime and for orbits which
are nearly circular and/or nearly polar. In this paper we eliminate these
problems by incorporating an array of improvements in the approximate fluxes.
Firstly, we add certain corrections which ensure the correct behaviour of the
fluxes in the limit of vanishing eccentricity and/or 90 degrees inclination.
Secondly, we use higher order post-Newtonian formulae, adapted for generic
orbits. Thirdly, we drop the assumption of constant inclination. Instead, we
first evolve the Carter constant by means of an approximate post-Newtonian
expression and subsequently extract the evolution of iota. Finally, we improve
the evolution of circular orbits by using fits to the angular momentum and
inclination evolution determined by Teukolsky based calculations. As an
application of the improved scheme we provide a sample of generic Kerr
inspirals and for the specific case of nearly circular orbits we locate the
critical radius where orbits begin to decircularise under radiation reaction.
These easy-to-generate inspirals should become a useful tool for exploring LISA
data analysis issues and may ultimately play a role in source detection.Comment: 25 pages, 14 figures, some typos corrected, short section on
conservative corrections added, minor changes for consistency with published
versio
Initial Populations of Black Holes in Star Clusters
Using an updated population synthesis code we study the formation and
evolution of black holes (BHs) in young star clusters following a massive
starburst. This study continues and improves on the initial work described by
Belczynski, Sadowski & Rasio (2004). In our new calculations we account for the
possible ejections of BHs and their progenitors from clusters because of natal
kicks imparted by supernovae and recoil following binary disruptions. The
results indicate that the properties of both retained BHs in clusters and
ejected BHs (forming a field population) depend sensitively on the depth of the
cluster potential. In particular, most BHs ejected from binaries are also
ejected from clusters with central escape speeds Vesc < 100 km/s. Conversely,
most BHs remaining in binaries are retained by clusters with Vesc > 50 km/s.
BHs from single star evolution are also affected significantly: about half of
the BHs originating from primordial single stars are ejected from clusters with
Vesc < 50 km/s. Our results lay a foundation for theoretical studies of the
formation of BH X-ray binaries in and around star clusters, including possible
ultra-luminous sources, as well as merging BH--BH binaries detectable with
future gravitational-wave observatories.Comment: 35 pages, 8 tables, 17 figures; resubmitted to ApJ (revised version
Permalloy-based carbon nanotube spin-valve
In this Letter we demonstrate that Permalloy (Py), a widely used Ni/Fe alloy,
forms contacts to carbon nanotubes (CNTs) that meet the requirements for the
injection and detection of spin-polarized currents in carbon-based spintronic
devices. We establish the material quality and magnetization properties of Py
strips in the shape of suitable electrical contacts and find a sharp
magnetization switching tunable by geometry in the anisotropic
magnetoresistance (AMR) of a single strip at cryogenic temperatures. In
addition, we show that Py contacts couple strongly to CNTs, comparable to Pd
contacts, thereby forming CNT quantum dots at low temperatures. These results
form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance
switchings in the tunneling magnetoresistance, which directly correspond to the
magnetization reversals in the individual contacts observed in AMR experiments.Comment: 3 page
On the Applicability of OGSA-BES to D-Grid Community Scheduling Systems
In this paper, we exemplary review the requirements of two Grid communities in the D-Grid project and identify similarities in the addressed scientific applications respectively. To facilitate Grid scheduler interoperability on the underlying heterogeneous middleware systems we extend the standardized OGSA-BES interface and propose a basic concept for the exploitation of collaboration potential in the D-Grid community in general. Compared with existing meta-scheduling architectures there will be no need for a central scheduler instance
Energy dissipation in graphene field-effect transistors
We measure the temperature distribution in a biased single-layer graphene
transistor using Raman scattering microscopy of the 2D-phonon band. Peak
operating temperatures of 1050 K are reached in the middle of the graphene
sheet at 210 KW cm^(-2) of dissipated electric power. The metallic contacts act
as heat sinks, but not in a dominant fashion. To explain the observed
temperature profile and heating rate, we have to include heat-flow from the
graphene to the gate oxide underneath, especially at elevated temperatures,
where the graphene thermal conductivity is lowered due to umklapp scattering.
Velocity saturation due to phonons with about 50 meV energy is inferred from
the measured charge density via shifts in the Raman G-phonon band, suggesting
that remote scattering (through field coupling) by substrate polar surface
phonons increases the energy transfer to the substrate and at the same time
limits the high-bias electronic conduction of graphene.Comment: The pdf-file contains the main manuscript (19 pages, 3 figures) and
the supplement (5 pages, 4 figures
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
'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 with a
constant . The method is guaranteed to converge linearly provided
that is bounded by a small multiple of the reciprocal of the
residual norm of the current approximate eigenpair. The results are
fundamentally different from the existing convergence results that always
require , 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 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
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