3,967 research outputs found
The origin of very wide binary systems
The majority of stars in the Galactic field and halo are part of binary or
multiple systems. A significant fraction of these systems have orbital
separations in excess of thousands of astronomical units, and systems wider
than a parsec have been identified in the Galactic halo. These binary systems
cannot have formed through the 'normal' star-formation process, nor by capture
processes in the Galactic field. We propose that these wide systems were formed
during the dissolution phase of young star clusters. We test this hypothesis
using N-body simulations of evolving star clusters and find wide binary
fractions of 1-30%, depending on initial conditions. Moreover, given that most
stars form as part of a binary system, our theory predicts that a large
fraction of the known wide 'binaries' are, in fact, multiple systems.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 266,
eds. R. de Grijs & J.R.D. Lepin
A Comparison of Soviet and US Industrial Performance: 1928-90
This paper contains estimates of comparative labour productivity levels in manufacturing for the Soviet Union and the USA. Value added was converted to a common currency by using an expenditure based unit value ratio (or purchasing power parity). Time series for value added and labour inputs were used to estimate relative Soviet/US productivity levels back to 1928. The main findings are that in 1987, Soviet value added per person employed was 24.8 per cent of American levels. Value added per hour was 26.3 per cent of the USA. Combining US and Soviet time series for output and employment together with the results of the detailed comparison of labour productivity for 1987, brought me to the conclusion that over the period 1928-89 Soviet labour productivity inproved very little relative to the US. The results of this study are partly based on Soviet datasets that were formerly not available to researchers.
Binaries and the dynamical mass of star clusters
The total mass of a distant star cluster is often derived from the virial
theorem, using line-of-sight velocity dispersion measurements and half-light
radii, under the implicit assumption that all stars are single (although it is
known that most stars form part of binary systems). The components of binary
stars exhibit orbital motion, which increases the measured velocity dispersion,
resulting in a dynamical mass overestimation. In this article we quantify the
effect of neglecting the binary population on the derivation of the dynamical
mass of a star cluster. We find that the presence of binaries plays an
important role for clusters with total mass M < 10^5 Msun; the dynamical mass
can be significantly overestimated (by a factor of two or more). For the more
massive clusters, with Mcl > 10^5 Msun, binaries do not affect the dynamical
mass estimation significantly, provided that the cluster is significantly
compact (half-mass radius < 5 pc).Comment: Comments: 2 pages. Conference proceedings for IAUS246 'Dynamical
Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief Editor), M.
Giersz, A. Sills, Capri, Sept. 200
Spatial filtering in multichannel magnetoencephalography
Partial differential equations in boundary-value problems have been studied in order to estimate the influence of several geometrical and physical parameters involved in the outward transmission of the brain's magnetic field. Explicit Green kernels are used to obtain integral forms of generalized solutions which can be deduced from each other, as expressed over concentric spherical surfaces. That leads to numerical applications dealing with the radial component of the magnetic field. From this study, a new spatial filtering is proposed as a possible improvement in two-dimensional magnetoencephalographic mapping using large multisensors
Pairing mechanisms for binary stars
Knowledge of the binary population in stellar groupings provides important
information about the outcome of the star forming process in different
environments. Binarity is also a key ingredient in stellar population studies
and is a prerequisite to calibrate the binary evolution channels. In these
proceedings we present an overview of several commonly used methods to pair
individual stars into binary systems, which we refer to as the pairing
function. Many pairing functions are frequently used by observers and
computational astronomers, either for the mathematical convenience, or because
they roughly describe the expected outcome of the star forming process. We
discuss the consequences of each pairing function for the interpretation of
observations and numerical simulations. The binary fraction and mass ratio
distribution generally depend strongly on the selection of the range in primary
spectral type in a sample. These quantities, when derived from a binary survey
with a mass-limited sample of target stars, are thus not representative for the
population as a whole.Comment: 4 pages, 2 figures, in: Galactic and stellar dynamics in the era of
high-resolution surveys, Boily C., Combes F., Hensler G., eds., Strasbourg
(France), March 2008, in press (Astron. Nachr.
Charge noise analysis of an AlGaAs/GaAs quantum dot using transmission-type radio-frequency single-electron transistor technique
Radio-frequency (rf)- operated single-electron transistors (SETs) are
high-sensitivity, fast-response electrometers, which are valuable for
developing new insights into single-charge dynamics. We investigate
high-frequency (up to 1 MHz) charge noise in an AlGaAs/GaAs quantum dot using a
transmission-type rf-SET technique. The electron capture and emission kinetics
on a trap in the vicinity of the quantum dot are dominated by a Poisson
process. The maximum bandwidth for measuring single trapping events is about 1
MHz, which is the same as that required for observing single-electron tunneling
oscillations in a measurable current (~0.1pA).Comment: 4 pages, 4 figures, to be published in Appl. Phys. Let
Electron Pair Resonance in the Coulomb Blockade
We study many-body corrections to the cotunneling current via a localized
state with energy at large bias voltages . We show that the
transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the
localized level, results in ionization resonance peaks in the third derivative
of the current with respect to , centered at . Our
results predict the existence of previously unnoticed structure within
Coulomb-blockade diamonds.Comment: 5 pages, 4 figure
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