4,750 research outputs found
On participatory service provision at the network edge with community home gateways
Edge computing is considered as a technology to enable new types of services which operate at the network edge. There are important use cases in ambient intelligence and the Internet of Things (IoT) for edge computing driven by huge business potentials. Most of today's edge computing platforms, however, consist of proprietary gateways, which are either closed or fairly restricted to deploy any third-party services. In this paper we discuss a participatory edge computing system running on home gateways to serve as an open environment to deploy local services. We present first motivating use cases and review existing approaches and design considerations for the proposed system. Then we show our platform which materializes the principles of an open and participatory edge environment, to lower the entry barriers for service deployment at the network edge. By using containers, our platform can flexibly enable third-party services, and may serve as an infrastructure to support several application domains of ambient intelligence.Peer ReviewedPostprint (author's final draft
Introduction to Library Trends 23 (3) Winter 1975: Music and Fine Arts in the General Library
published or submitted for publicatio
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
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
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
Runaway collisions in young star clusters. II. Numerical results
We present a new study of the collisional runaway scenario to form an
intermediate-mass black hole (IMBH, MBH > 100 Msun) at the centre of a young,
compact stellar cluster. The first phase is the formation of a very dense
central core of massive stars (Mstar =~ 30-120 Msun) through mass segregation
and gravothermal collapse. Previous work established the conditions for this to
happen before the massive stars evolve off the main sequence (MS). In this and
a companion paper, we investigate the next stage by implementing direct
collisions between stars. Using a Monte Carlo stellar dynamics code, we follow
the core collapse and subsequent collisional phase in more than 100 models with
varying cluster mass, size, and initial concentration. Collisions are treated
either as ideal, ``sticky-sphere'' mergers or using realistic prescriptions
derived from 3-D hydrodynamics computations. In all cases for which the core
collapse happens in less than the MS lifetime of massive stars (~3 Myr), we
obtain the growth of a single very massive star (VMS, Mstar =~ 400-4000 Msun)
through a runaway sequence of mergers. Mass loss from collisions, even for
velocity dispersions as high as sigma1D ~ 1000 km/s, does not prevent the
runaway. The region of cluster parameter space leading to runaway is even more
extended than predicted in previous work because, in clusters with sigma1D >
300 km/s, collisions accelerate (and, in extreme cases, drive) core collapse.
Although the VMS grows rapidly to > 1000 Msun in models exhibiting runaway, we
cannot predict accurately its final mass. This is because the termination of
the runaway process must eventually be determined by a complex interplay
between stellar dynamics, hydrodynamics, and the stellar evolution of the VMS.
[abridged]Comment: 23 pages, 24 figures. For publication in MNRAS. Paper revised to
follow requests and suggestions of referee. Companion paper to Freitag, Rasio
& Baumgardt 200
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
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
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
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