4,296 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
Zero Dynamics for Port-Hamiltonian Systems
The zero dynamics of infinite-dimensional systems can be difficult to
characterize. The zero dynamics of boundary control systems are particularly
problematic. In this paper the zero dynamics of port-Hamiltonian systems are
studied. A complete characterization of the zero dynamics for a
port-Hamiltonian systems with invertible feedthrough as another
port-Hamiltonian system on the same state space is given. It is shown that the
zero dynamics for any port-Hamiltonian system with commensurate wave speeds are
well-defined, and are also a port-Hamiltonian system. Examples include wave
equations with uniform wave speed on a network. A constructive procedure for
calculation of the zero dynamics, that can be used for very large system order,
is provided.Comment: 17 page
Reconstructing the Arches I: Constraining the Initial Conditions
We have performed a series of N-body simulations to model the Arches cluster.
Our aim is to find the best fitting model for the Arches cluster by comparing
our simulations with observational data and to constrain the parameters for the
initial conditions of the cluster. By neglecting the Galactic potential and
stellar evolution, we are able to efficiently search through a large parameter
space to determine e.g. the IMF, size, and mass of the cluster. We find, that
the cluster's observed present-day mass function can be well explained with an
initial Salpeter IMF. The lower mass-limit of the IMF cannot be well
constrained from our models. In our best models, the total mass and the virial
radius of the cluster are initially (5.1 +/- 0.8) 10^4 Msun and 0.76 +/- 0.12
pc, respectively. The concentration parameter of the initial King model is w0 =
3-5.Comment: 12 pages, 14 Figures, revised and accepted for publication in MNRA
The solar siblings in the Gaia era
We perform realistic simulations of the Sun's birth cluster in order to
predict the current distribution of solar siblings in the Galaxy. We study the
possibility of finding the solar siblings in the Gaia catalogue by using only
positional and kinematic information. We find that the number of solar siblings
predicted to be observed by Gaia will be around 100 in the most optimistic
case, and that a phase space only search in the Gaia catalogue will be
extremely difficult. It is therefore mandatory to combine the chemical tagging
technique with phase space selection criteria in order to have any hope of
finding the solar siblings.Comment: To be published in the proceedings of the GREAT-ITN conference "The
Milky Way Unravelled by Gaia: GREAT Science from the Gaia Data Releases", 1-5
December 2014, University of Barcelona, Spain, EAS Publications Series, eds
Nicholas Walton, Francesca Figueras, and Caroline Soubira
Rice Intensification in a Changing Environment: Impact on Water Availability in Inland Valley Landscapes in Benin
This study assesses the impact of climate change on hydrological processes under rice intensification in three headwater inland valley watersheds characterized by different land conditions. The Soil and Water Assessment Tool was used to simulate the combined impacts of two land use scenarios defined as converting 25% and 75% of lowland savannah into rice cultivation, and two climate scenarios (A1B and B1) of the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. The simulations were performed based on the traditional and the rainfed-bunded rice cultivation systems and analyzed up to the year 2049 with a special focus on the period of 2030–2049. Compared to land use, climate change impact on hydrological processes was overwhelming at all watersheds. The watersheds with a high portion of cultivated areas are more sensitive to changes in climate resulting in a decrease of water yield of up to 50% (145 mm). Bunded fields cause a rise in surface runoff projected to be up to 28% (18 mm) in their lowlands, while processes were insignificantly affected at the vegetation dominated-watershed. Analyzing three watersheds instead of one as is usually done provides further insight into the natural variability and therefore gives more evidence of possible future processes and management strategie
On the origin of hyperfast neutron stars
We propose an explanation for the origin of hyperfast neutron stars (e.g. PSR
B1508+55, PSR B2224+65, RX J0822-4300) based on the hypothesis that they could
be the remnants of a symmetric supernova explosion of a high-velocity massive
star (or its helium core) which attained its peculiar velocity (similar to that
of the neutron star) in the course of a strong three- or four-body dynamical
encounter in the core of a young massive star cluster. This hypothesis implies
that the dense cores of star clusters (located either in the Galactic disk or
near the Galactic centre) could also produce the so-called hypervelocity stars
-- the ordinary stars moving with a speed of ~1000 km/s.Comment: 2 pages, to appear in Dynamical Evolution of Dense Stellar Systems,
Proceed. of the IAU Symp. 246 (Capri, Sept. 2007), eds. E.Vesperini, M.
Giersz, and A. Sill
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