22,908 research outputs found
Digital Availability of Product Information for Collaborative Engineering of Spacecraft
In this paper, we introduce a system to collect product information from
manufacturers and make it available in tools that are used for concurrent
design of spacecraft. The planning of a spacecraft needs experts from different
disciplines, like propulsion, power, and thermal. Since these different
disciplines rely on each other there is a high need for communication between
them, which is often realized by a Model-Based Systems Engineering (MBSE)
process and corresponding tools. We show by comparison that the product
information provided by manufacturers often does not match the information
needed by MBSE tools on a syntactic or semantic level. The information from
manufacturers is also currently not available in machine-readable formats.
Afterwards, we present a prototype of a system that makes product information
from manufacturers directly available in MBSE tools, in a machine-readable way.Comment: accepted at CDVE201
Nonequilibrium Precursor Model for the Onset of Percolation in a Two-Phase System
Using a Boltzmann equation, we investigate the nonequilibrium dynamics of
nonperturbative fluctuations within the context of Ginzburg-Landau models. As
an illustration, we examine how a two-phase system initially prepared in a
homogeneous, low-temperature phase becomes populated by precursors of the
opposite phase as the temperature is increased. We compute the critical value
of the order parameter for the onset of percolation, which signals the
breakdown of the conventional dilute gas approximation.Comment: 4 pages, 4 eps figures (uses epsf), Revtex. Replaced with version in
press Physical Review
Starburst radio galaxies: general properties, evolutionary histories and triggering
In this paper we discuss the results of a programme of spectral synthesis
modelling of a sample of starburst radio galaxies in the context of scenarios
for the triggering of the activity and the evolution of the host galaxies. The
starburst radio galaxies -- comprising ~15 - 25% of all powerful extragalactic
radio sources -- frequently show disturbed morphologies at optical wavelengths,
and unusual radio structures, although their stellar masses are typical of
radio galaxies as a class. In terms of the characteristic ages of their young
stellar populations (YSP), the objects can be divided into two groups: those
with YSP ages t_ysp < 0.1 Gyr, in which the radio source has been triggered
quasi-simultaneously with the main starburst episode, and those with older YSP
in which the radio source has been triggered or re-triggered a significant
period after the starburst episode. Combining the information on the YSP with
that on the optical morphologies of the host galaxies, we deduce that the
majority of the starburst radio galaxies have been triggered in galaxy mergers
in which at least one of the galaxies is gas rich. However, the triggering (or
re-triggering) of the radio jets can occur immediately before, around, or a
significant period after the final coalescence of the merging nuclei,
reflecting the complex gas infall histories of the merger events. Overall, our
results provide further evidence that powerful radio jet activity can be
triggered via a variety of mechanisms, including different evolutionary stages
of major galaxy mergers; clearly radio-loud AGN activity is not solely
associated with a particular stage of a unique type of gas accretion event.Comment: 16 pages, 3 Figures, accepted for publication in MNRA
Bulge RR Lyrae stars in the VVV tile
The VISTA Variables in the V\'ia L\'actea (VVV) Survey is one of the six ESO
public surveys currently ongoing at the VISTA telescope on Cerro Paranal,
Chile. VVV uses near-IR () filters that at present provide
photometry to a depth of mag in up to 36 epochs spanning
over four years, and aim at discovering more than 10 variable sources as
well as trace the structure of the Galactic bulge and part of the southern
disk. A variability search was performed to find RR Lyrae variable stars. The
low stellar density of the VVV tile , which is centered at
() (), makes it suitable to search for
variable stars. Previous studies have identified some RR Lyrae stars using
optical bands that served to test our search procedure. The main goal is to
measure the reddening, interstellar extinction, and distances of the RR Lyrae
stars and to study their distribution on the Milky Way bulge. A total of 1.5 sq
deg were analyzed, and we found 39 RR Lyrae stars, 27 of which belong to the
ab-type and 12 to the c-type. Our analysis recovers all the previously
identified RR Lyrae variables in the field and discovers 29 new RR Lyrae stars.
The reddening and extinction toward all the RRab stars in this tile were
derived, and distance estimations were obtained through the period--luminosity
relation. Despite the limited amount of RR Lyrae stars studied, our results are
consistent with a spheroidal or central distribution around and
kpc. for either the Cardelli or Nishiyama extinction law.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Comparison of coherence times in three dc SQUID phase qubits
We report measurements of spectroscopic linewidth and Rabi oscillations in
three thin-film dc SQUID phase qubits. One device had a single-turn Al loop,
the second had a 6-turn Nb loop, and the third was a first order gradiometer
formed from 6-turn wound and counter-wound Nb coils to provide isolation from
spatially uniform flux noise. In the 6 - 7.2 GHz range, the spectroscopic
coherence times for the gradiometer varied from 4 ns to 8 ns, about the same as
for the other devices (4 to 10 ns). The time constant for decay of Rabi
oscillations was significantly longer in the single-turn Al device (20 to 30
ns) than either of the Nb devices (10 to 15 ns). These results imply that
spatially uniform flux noise is not the main source of decoherence or
inhomogenous broadening in these devices.Comment: 4 pages, 5 figures, accepted for publication in IEEE Trans. Appl.
Supercon
EU-Rotate_N – a decision support system – to predict environmental and economic consequences of the management of nitrogen fertiliser in crop rotations
A model has been developed which assesses the economic and environmental performance of crop rotations, in both conventional and organic cropping, for over 70 arable and horticultural crops, and a wide range of growing conditions in Europe. The model, though originally based on the N_ABLE model, has been completely rewritten and contains new routines to simulate root development, the mineralisation and release of nitrogen (N) from soil organic matter and crop residues, and water dynamics in soil. New routines have been added to estimate the effects of sub-optimal rates of N and spacing on the marketable outputs and gross margins. The model provides a mechanism for generating scenarios to represent a range of differing crop and fertiliser management strategies which can be used to evaluate their effects on yield, gross margin and losses of nitrogen through leaching. Such testing has revealed that nitrogen management can be improved and that there is potential to increase gross margins whilst reducing nitrogen losses
Neutron and proton spectra from the decay of hypernuclei
We have determined the spectra of neutrons and protons following the decay of
hypernuclei through the one- and two-nucleon induced mechanisms. The
momentum distributions of the primary nucleons are calculated and a Monte Carlo
simulation is used to account for final state interactions. From the spectra we
calculate the number of neutrons () and protons () per
decay and show how the measurement of these quantities, particularly , can
lead to a determination of , the ratio of neutron to
proton induced decay. We also show that the consideration of the
two-nucleon induced channel has a repercussion in the results, widening the
band of allowed values of with respect to what is
obtained neglecting this channel.Comment: 30 pages, 12 Postscript figures, uuencoded file, ReVTeX, epsf.st
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