2,257 research outputs found
Black hole mergers in the universe
Mergers of black-hole binaries are expected to release large amounts of
energy in the form of gravitational radiation. However, binary evolution models
predict merger rates too low to be of observational interest. In this paper we
explore the possibility that black holes become members of close binaries via
dynamical interactions with other stars in dense stellar systems. In star
clusters, black holes become the most massive objects within a few tens of
millions of years; dynamical relaxation then causes them to sink to the cluster
core, where they form binaries. These black-hole binaries become more tightly
bound by superelastic encounters with other cluster members, and are ultimately
ejected from the cluster. The majority of escaping black-hole binaries have
orbital periods short enough and eccentricities high enough that the emission
of gravitational radiation causes them to coalesce within a few billion years.
We predict a black-hole merger rate of about per year per
cubic megaparsec, implying gravity wave detection rates substantially greater
than the corresponding rates from neutron star mergers. For the first
generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), we
expect about one detection during the first two years of operation. For its
successor LIGO-II, the rate rises to roughly one detection per day. The
uncertainties in these numbers are large. Event rates may drop by about an
order of magnitude if the most massive clusters eject their black hole binaries
early in their evolution.Comment: 12 pages, ApJL in pres
Expected Coalescence Rate of Double Neutron Stars for Ground Based Interferometers
In this paper we present new estimates of the coalescence rate of neutron
star binaries in the local universe and we discuss its consequences for the
first generations of ground based interferometers. Our approach based on both
evolutionary and statistical methods gives a galactic merging rate of 1.7
10 yr, in the range of previous estimates 10 - 10
yr. The local rate which includes the contribution of elliptical
galaxies is two times higher, in the order of 3.4 10 yr. We
predict one detection every 148 and 125 years with initial VIRGO and LIGO, and
up to 6 events per year with their advanced configuration. Our recent detection
rate estimates from investigations on VIRGO future improvements are quoted.Comment: talk given at the GWDAW9 (Annecy, 2004) to be published in CQ
Demand for wool by grade
The aims of the project were to examine the relationships between auction
prices of different types of wool, to identify categories of wool and to investigate substitution effects amongst wools. A modelling framework was developed which enabled these relationships to be analysed. While there are some clear avenues for further research, this study makes some useful first steps towards a conceptualisation of wool markets
The Sandwich algorithm for spatial equilibrium analysis
Recent advances in mathematical programming techniques have
made it possible to provide more realistic solutions to applied
economic problems. Although mathematical programming techniques are
widely used, the economic content of the solutions is often limited by
the assumptions imposed by the algorithms available. This report is
designed to demonstrate the increased flexibility which is currently
available for the solution of a wide range of spatial economic
problems.
Transportation and transhipment models have been widely used in
the analysis of the impact of policy changes on spatial activity,
Borrell & Zwart [l]; Beck, Rathbun and Abbott [2]. One of the major
shortcomings of such models has been an inability to model the impact
of more flexible pricing policies on regional supply and demand, while
maintaining the realistic non linearities which are associated with
processing and transportation costs. In this paper a simplified
version of the transhipment model developed by Borrell & Zwart [l] is
modified to incorporate regional supply response while at the same time
retaining complex processing and handling cost relationships.
This report outlines the general form of the spatial
equilibrium problem and some of the solution techniques available, in a
format easily understood by readers not conversant with operational
research techniques. Initially the problem is defined and solution
methods used in the past are then briefly described. The advantages
and disadvantages of these methods are outlined before showing how a
relatively new solution technique may be able to improve both the scope
and flexibility of the problems being solved
A runaway collision in a young star cluster as the origin of the brightest supernova
Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded
\cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P,
2006CBET..695....1F}. Its progenitor might have been a very massive (
\msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen
in the spectrum of the supernova, because stars \msun are believed to
have shed their hydrogen envelopes several hundred thousand years before the
explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have
arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we
show that the collision frequency of massive stars in a dense and young cluster
(of the kind to be expected near the center of a galaxy) is sufficient to
provide a reasonable chance that SN 2006gy resulted from such a bombardment. If
this is the correct explanation, then we predict that when the supernova fades
(in a year or so) a dense cluster of massive stars becomes visible at the site
of the explosion
A pilgrimage to gravity on GPUs
In this short review we present the developments over the last 5 decades that
have led to the use of Graphics Processing Units (GPUs) for astrophysical
simulations. Since the introduction of NVIDIA's Compute Unified Device
Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body
simulations and is so popular these days that almost all papers about high
precision N-body simulations use methods that are accelerated by GPUs. With the
GPU hardware becoming more advanced and being used for more advanced algorithms
like gravitational tree-codes we see a bright future for GPU like hardware in
computational astrophysics.Comment: To appear in: European Physical Journal "Special Topics" : "Computer
Simulations on Graphics Processing Units" . 18 pages, 8 figure
Modelling Collision Products of Triple-Star Mergers
In dense stellar clusters, binary-single and binary-binary encounters can
ultimately lead to collisions involving two or more stars. A comprehensive
survey of multi-star collisions would need to explore an enormous amount of
parameter space, but here we focus on a number of representative cases
involving low-mass main-sequence stars. Using both Smoothed Particle
Hydrodynamics (SPH) calculations and a much faster fluid sorting software
package (MMAS), we study scenarios in which a newly formed product from an
initial collision collides with a third parent star. By varying the order in
which the parent stars collide, as well as the orbital parameters of the
collision trajectories, we investigate how factors such as shock heating affect
the chemical composition and structure profiles of the collision product. Our
simulations and models indicate that the distribution of most chemical elements
within the final product is not significantly affected by the order in which
the stars collide, the direction of approach of the third parent star, or the
periastron separations of the collisions. We find that the sizes of the
products, and hence their collisional cross sections for subsequent encounters,
are sensitive to the order and geometry of the collisions. For the cases that
we consider, the radius of the product formed in the first (single-single star)
collision ranges anywhere from roughly 2 to 30 times the sum of the radii of
its parent stars. The final product formed in our triple-star collisions can
easily be as large or larger than a typical red giant. We therefore expect the
collisional cross section of a newly formed product to be greatly enhanced over
that of a thermally relaxed star of the same mass.Comment: 20 pages, submitted to MNRA
Gravitational-wave confusion background from cosmological compact binaries: Implications for future terrestrial detectors
Increasing the sensitivity of a gravitational-wave (GW) detector improves our
ability to measure the characteristics of detected sources. It also increases
the number of weak signals that contribute to the data. Because GW detectors
have nearly all-sky sensitivity, they can be subject to a confusion limit: Many
sources which cannot be distinguished may be measured simultaneously, defining
a stochastic noise floor to the sensitivity. For GW detectors operating at
present and for their planned upgrades, the projected event rate is
sufficiently low that we are far from the confusion-limited regime. However,
some detectors currently under discussion may have large enough reach to binary
inspiral that they enter the confusion-limited regime. In this paper, we
examine the binary inspiral confusion limit for terrestrial detectors. We
consider a broad range of inspiral rates in the literature, several planned
advanced gravitational-wave detectors, and the highly advanced "Einstein
Telescope" design. Though most advanced detectors will not be impacted by this
limit, the Einstein Telescope with a very low frequency "seismic wall" may be
subject to confusion noise. At a minimum, careful data analysis will be require
to separate signals which will appear confused. This result should be borne in
mind when designing highly advanced future instruments.Comment: 19 pages, 6 figures and 3 tables; accepted for publication in Phys.
Rev.
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