113,811 research outputs found
The stellar correlation function from SDSS - A statistical search for wide binary stars
We study the statistical properties of the wide binary population in the
Galaxy field with projected separations larger than 200 AU by constructing the
stellar angular two-point correlation function (2PCF) from a homogeneous sample
of nearly 670'000 main sequence stars. The selected stars lie within a
rectangular region around the Northern Galactic Pole and have apparent r-band
magnitudes between 15 and 20.5 mag and spectral classes later than G5 (g-r >
0.5 mag). The data were taken from the Sixth Data Release of the Sloan Digital
Sky Survey. We model the 2PCF by means of the Wasserman-Weinberg technique
including several assumptions on the distribution of the binaries' orbital
parameters, luminosity function, and density distribution in the Galaxy. In
particular, we assume that the semi-major axis distribution is described by a
single powerlaw. The free model parameters - the local wide binary number
density and the power-law index of the semi-major axis distribution - are
inferred simultaneously by least-square fitting. We find the separation
distribution to follow Oepik's law up to the Galactic tidal limit, without any
break and a local density of 5 wide binaries per 1'000 cubic parsec with both
components having spectral type later than G5. This implies that about 10% of
all stars in the solar neighbourhood are members of such a late-type wide
binary system. With a relative statistical (2 sigma) error of about 10%, our
findings are in general agreement with previous studies of wide binaries. The
data suggest that about 800 very wide pairs with projected separations larger
than 0.1 pc exist in our sample, whereas none are found beyond 0.8 pc.Comment: 18 pages, 14 figures, 7 tables; added reference for section
Big Black Hole, Little Neutron Star: Magnetic Dipole Fields in the Rindler Spacetime
As a black hole and neutron star approach during inspiral, the field lines of
a magnetized neutron star eventually thread the black hole event horizon and a
short-lived electromagnetic circuit is established. The black hole acts as a
battery that provides power to the circuit, thereby lighting up the pair just
before merger. Although originally suggested as a promising electromagnetic
counterpart to gravitational-wave detection, the luminous signals are promising
more generally as potentially detectable phenomena, such as short gamma-ray
bursts. To aid in the theoretical understanding, we present analytic solutions
for the electromagnetic fields of a magnetic dipole in the presence of an event
horizon. In the limit that the neutron star is very close to a Schwarzschild
horizon, the Rindler limit, we can solve Maxwell's equations exactly for a
magnetic dipole on an arbitrary worldline. We present these solutions here and
investigate a proxy for a small segment of the neutron star orbit around a big
black hole. We find that the voltage the black hole battery can provide is in
the range ~10^16 statvolts with a projected luminosity of 10^42 ergs/s for an
M=10M_sun black hole, a neutron star with a B-field of 10^12 G, and an orbital
velocity ~0.5c at a distance of 3M from the horizon. Larger black holes provide
less power for binary separations at a fixed number of gravitational radii. The
black hole/neutron star system therefore has a significant power supply to
light up various elements in the circuit possibly powering jets, beamed
radiation, or even a hot spot on the neutron star crust.Comment: Published in Physical Review D:
http://link.aps.org/doi/10.1103/PhysRevD.88.06405
Wage and employment decisions in the Russian economy : an analysis of developments in 1992
The authors analyze changes in the Russian labor market in 1992. They focus on the path of wages and employment in a context of partial price liberalization and considerable ambiguity about government and central bank policy. Under the former Soviet economy, the firm was the bedrock of the centrally planned system. The relaxation of centralized controls did not result in substantial employment losses partly because of the implicit moral economy of the system and partly because of continuing constraints on wages. In 1992, the wage structure and employment levels in the economy's state sector exhibited surprising stability, reflecting the system's immense inertia. Despite announced regime changes, at the end of 1992 the number of jobseekers was no more than 1.5 percent of the labor force. But significant changes have been made: wage and employment decisions have been widely liberalized; some restraints on labor mobility have been removed; changes have also been made in ownership title; and there has been some expansion in the private sector, as yet largely concentrated in services. These substantive changes are important for future expectations about entitlements to jobs and incomes, but the changes remain restricted and the sources of these restrictions imply significant economic costs. The underpinning of the current stagflation is the inability to break the soft budget constraint on state firms and to impose realistically a systematic, transparent set of constraints on the firms'financing demands. This has combined with the firms'continuing ability to exercise market power alongside weak controls on wage claims. Employment transitions have been dominated by high levels of quits at the base of the skill structure. Involuntary separations have been limited, involving mostly women and white collar workers. Firms commonly provide de facto unemployment compensation to workers in the form of minimum wage payments with little or no work requirement. There is evidence of some increase in the proportion of laid-off workers among the unemployed, but firms seem to prefer hoarding labor in light of uncertainty about policy, firm, or product-specific market prospects. Wages have been more volatile. Wages initially bore almost all of the adjustment costs, but have shown mild recovery thereafter. Lax monetary policy and decentralized insider power, giving rise to relative employment stability and real wage rigidity, are powerful ingredients for hyperinflation.Economic Theory&Research,Environmental Economics&Policies,Health Monitoring&Evaluation,Banks&Banking Reform,Municipal Financial Management
Optimisation of autoselective plasma regeneration of wall-flow diesel particulate filters
The increase in number of diesel powered vehicles has led to greater concern for the
effects of their exhaust emissions. Engine manufacturers must now consider using
diesel particulate filters to make their engines meet the legislated limits. Diesel
particulate filters can remove more than 95% of the particulates from the exhaust
flow but require cleaning, known as regeneration.
This thesis describes the research and optimisation of the Autoselective regeneration
system for cordierite wall flow diesel particulate filters. The novel Autoselective
technology uses an atmospheric pressure glow discharge plasma to selectively
oxidise particulate matter (soot) trapped within the filter. The aim of this research was
to produce a regeneration system that can operate under all exhaust conditions with
a low energy demand and no precious metal dependence to compete with the
numerous pre-existing technologies.
The effect of discharge electrode type and position on regeneration performance has
been investigated in terms of regeneration uniformity, power requirement and
regeneration rate. The results showed that the electrode orientation had a large
effect on regeneration distribution and energy demand.
The electrode capacitance and breakdown voltage was shown to affect the choice of
power supply circuit because not all power supply topologies were suitable for
powering electrodes with >100 pF capacitance. A number of power supplies were
designed and tested, a voltage driven resonant transformer type supply was shown
to be optimal when used in conjunction with a swept frequency.
The current and frequency ranges of electrical discharges were continuously
variable, and their effect on discharge regeneration performance was studied. The
results showed that the discharge frequency had no effect on the regeneration
process but did affect spatial distribution. An optimised resonant transformer power
supply was designed that was ideally suited for the electrodes used. A novel power
modulation strategy, which used a switching frequency phase locked to the
~ iii ~
modulating frequency, was employed which extended the operating range of the
discharge to below 10 mA for electrode separations > 7.5 mm.
The heat flows within the filter and discharge during regeneration were analysed and
the filter damage process was linked to the heat released by the discharge inside the
filter wall. Other filter materials were compared based on the findings and Mullite
ceramic was identified as a potentially better filter material for Autoselective
regeneration.
The filtration efficiency is important and was observed to be affected by the
Autoselective process. The effect of the discharge on filtration efficiency was studied
and the mechanism of particulate re-entrainment was identified as a combination of
electrostatic and electro-acoustic forces.
The Autoselective technology was successfully implemented in both flow-rig and
on-engine tests. Results showed significant reduction in back-pressure for power
inputs of ~ 500 W. The understanding of the Autoselective regeneration system has
been improved and the research resulted in a novel method of filter regeneration
The mean surface density of companions in a stellar-dynamical context
Applying the mean surface density of companions, Sigma(r), to the dynamical
evolution of star clusters is an interesting approach to quantifying structural
changes in a cluster. It has the advantage that the entire density structure,
ranging from the closest binary separations, over the core-halo structure
through to the density distribution in moving groups that originate from
clusters, can be analysed coherently as one function of the stellar separations
r.
This contribution assesses the evolution of Sigma(r) for clusters with
different initial densities and binary populations. The changes in the binary,
cluster and halo branches as the clusters evolve are documented using direct
N-body calculations, and are correlated with the cluster core and half-mass
radius. The location of breaks in the slope of Sigma(r) and the possible
occurrence of a binary gap can be used to infer dynamical cluster properties.Comment: 12 pages including 7 figures, accepted for publication in A&
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