336 research outputs found
Conjugate times and regularity of the minimum time function with differential inclusions
This paper studies the regularity of the minimum time function, ,
for a control system with a general closed target, taking the state equation in
the form of a differential inclusion. Our first result is a sensitivity
relation which guarantees the propagation of the proximal subdifferential of
along any optimal trajectory. Then, we obtain the local regularity of
the minimum time function along optimal trajectories by using such a relation
to exclude the presence of conjugate times
Towards A Census of Earth-mass Exo-planets with Gravitational Microlensing
Thirteen exo-planets have been discovered using the gravitational
microlensing technique (out of which 7 have been published). These planets
already demonstrate that super-Earths (with mass up to ~10 Earth masses) beyond
the snow line are common and multiple planet systems are not rare. In this
White Paper we introduce the basic concepts of the gravitational microlensing
technique, summarise the current mode of discovery and outline future steps
towards a complete census of planets including Earth-mass planets. In the
near-term (over the next 5 years) we advocate a strategy of automated follow-up
with existing and upgraded telescopes which will significantly increase the
current planet detection efficiency. In the medium 5-10 year term, we envision
an international network of wide-field 2m class telescopes to discover
Earth-mass and free-floating exo-planets. In the long (10-15 year) term, we
strongly advocate a space microlensing telescope which, when combined with
Kepler, will provide a complete census of planets down to Earth mass at almost
all separations. Such a survey could be undertaken as a science programme on
Euclid, a dark energy probe with a wide-field imager which has been proposed to
ESA's Cosmic Vision Programme.Comment: 10 pages. White Paper submission to the ESA Exo-Planet Roadmap
Advisory Team. See also "Inferring statistics of planet populations by means
of automated microlensing searches" by M. Dominik et al. (arXiv:0808.0004
An effective mass theorem for the bidimensional electron gas in a strong magnetic field
We study the limiting behavior of a singularly perturbed
Schr\"odinger-Poisson system describing a 3-dimensional electron gas strongly
confined in the vicinity of a plane and subject to a strong uniform
magnetic field in the plane of the gas. The coupled effects of the confinement
and of the magnetic field induce fast oscillations in time that need to be
averaged out. We obtain at the limit a system of 2-dimensional Schr\"odinger
equations in the plane , coupled through an effective selfconsistent
electrical potential. In the direction perpendicular to the magnetic field, the
electron mass is modified by the field, as the result of an averaging of the
cyclotron motion. The main tools of the analysis are the adaptation of the
second order long-time averaging theory of ODEs to our PDEs context, and the
use of a Sobolev scale adapted to the confinement operator
Inelastic lifetimes of confined two-component electron systems in semiconductor quantum wire and quantum well structures
We calculate Coulomb scattering lifetimes of electrons in two-subband quantum
wires and in double-layer quantum wells by obtaining the quasiparticle
self-energy within the framework of the random-phase approximation for the
dynamical dielectric function. We show that, in contrast to a single-subband
quantum wire, the scattering rate in a two-subband quantum wire contains
contributions from both particle-hole excitations and plasmon excitations. For
double-layer quantum well structures, we examine individual contributions to
the scattering rate from quasiparticle as well as acoustic and optical plasmon
excitations at different electron densities and layer separations. We find that
the acoustic plasmon contribution in the two-component electron system does not
introduce any qualitatively new correction to the low energy inelastic
lifetime, and, in particular, does not produce the linear energy dependence of
carrier scattering rate as observed in the normal state of high-
superconductors.Comment: 16 pages, RevTeX, 7 figures. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Orbital and physical parameters of eclipsing binaries from the ASAS catalogue - IV. A 0.61 + 0.45 M_sun binary in a multiple system
We present the orbital and physical parameters of a newly discovered low-mass
detached eclipsing binary from the All-Sky Automated Survey (ASAS) database:
ASAS J011328-3821.1 A - a member of a visual binary system with the secondary
component separated by about 1.4 seconds of arc. The radial velocities were
calculated from the high-resolution spectra obtained with the 1.9-m
Radcliffe/GIRAFFE, 3.9-m AAT/UCLES and 3.0-m Shane/HamSpec
telescopes/spectrographs on the basis of the TODCOR technique and positions of
H_alpha emission lines. For the analysis we used V and I band photometry
obtained with the 1.0-m Elizabeth and robotic 0.41-m PROMPT telescopes,
supplemented with the publicly available ASAS light curve of the system.
We found that ASAS J011328-3821.1 A is composed of two late-type dwarfs
having masses of M_1 = 0.612 +/- 0.030 M_sun, M_2 = 0.445 +/- 0.019 M_sun and
radii of R_1 = 0.596 +/- 0.020 R_sun, R_2 = 0.445 +/- 0.024 R_sun, both show a
substantial level of activity, which manifests in strong H_alpha and H_beta
emission and the presence of cool spots. The influence of the third light on
the eclipsing pair properties was also evaluated and the photometric properties
of the component B were derived. Comparison with several popular stellar
evolution models shows that the system is on its main sequence evolution stage
and probably is more metal rich than the Sun. We also found several clues which
suggest that the component B itself is a binary composed of two nearly
identical ~0.5 M_sun stars.Comment: 12 pages, 7 figures, 7 tables, to appear in MNRA
On Secure Workflow Decentralisation on the Internet
Decentralised workflow management systems are a new research area, where most
work to-date has focused on the system's overall architecture. As little
attention has been given to the security aspects in such systems, we follow a
security driven approach, and consider, from the perspective of available
security building blocks, how security can be implemented and what new
opportunities are presented when empowering the decentralised environment with
modern distributed security protocols. Our research is motivated by a more
general question of how to combine the positive enablers that email exchange
enjoys, with the general benefits of workflow systems, and more specifically
with the benefits that can be introduced in a decentralised environment. This
aims to equip email users with a set of tools to manage the semantics of a
message exchange, contents, participants and their roles in the exchange in an
environment that provides inherent assurances of security and privacy. This
work is based on a survey of contemporary distributed security protocols, and
considers how these protocols could be used in implementing a distributed
workflow management system with decentralised control . We review a set of
these protocols, focusing on the required message sequences in reviewing the
protocols, and discuss how these security protocols provide the foundations for
implementing core control-flow, data, and resource patterns in a distributed
workflow environment
OGLE-2005-BLG-018: Characterization of Full Physical and Orbital Parameters of a Gravitational Binary Lens
We present the analysis result of a gravitational binary-lensing event
OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent
strong features and a single weak feature separated from the strong features.
The light curve exhibits noticeable deviations from the best-fit model based on
standard binary parameters. To explain the deviation, we test models including
various higher-order effects of the motions of the observer, source, and lens.
From this, we find that it is necessary to account for the orbital motion of
the lens in describing the light curve. From modeling of the light curve
considering the parallax effect and Keplerian orbital motion, we are able to
measure not only the physical parameters but also a complete orbital solution
of the lens system. It is found that the event was produced by a binary lens
located in the Galactic bulge with a distance kpc from the Earth.
The individual lens components with masses and are separated with a semi-major axis of AU and
orbiting each other with a period yr. The event demonstrates
that it is possible to extract detailed information about binary lens systems
from well-resolved lensing light curves.Comment: 19 pages, 6 figure
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