2,389 research outputs found
Recalibrating the Wide-field Infrared Survey Explorer (WISE) W4 Filter
We present a revised effective wavelength and photometric calibration for the
Wide-field Infrared Survey Explorer (WISE) W4 band, including tests of
empirically motivated modifications to its pre-launch laboratory-measured
relative system response curve. We derived these by comparing measured W4
photometry with photometry synthesised from spectra of galaxies and planetary
nebulae. The difference between measured and synthesised photometry using the
pre-launch laboratory-measured W4 relative system response can be as large as
0.3 mag for galaxies and 1 mag for planetary nebulae. We find the W4 effective
wavelength should be revised upward by 3.3%, from 22.1 micron to 22.8 micron,
and the W4 AB magnitude of Vega should be revised from m = 6.59 to m = 6.66. In
an attempt to reproduce the observed W4 photometry, we tested three
modifications to the pre-launch laboratory-measured W4 relative system response
curve, all of which have an effective wavelength of 22.8 micron. Of the three
relative system response curve models tested, a model that matches the
laboratory-measured relative system response curve, but has the wavelengths
increased by 3.3% (or 0.73 micron) achieves reasonable agreement between the
measured and synthesised photometry.Comment: Accepted for publication in Publications of the Astronomical Society
of Australia, 6 pages, 4 figures, 1 tabl
Survey Simulations of a New Near-Earth Asteroid Detection System
We have carried out simulations to predict the performance of a new
space-based telescopic survey operating at thermal infrared wavelengths that
seeks to discover and characterize a large fraction of the potentially
hazardous near-Earth asteroid (NEA) population. Two potential architectures for
the survey were considered: one located at the Earth-Sun L1 Lagrange point, and
one in a Venus-trailing orbit. A sample cadence was formulated and tested,
allowing for the self-follow-up necessary for objects discovered in the daytime
sky on Earth. Synthetic populations of NEAs with sizes >=140 m in effective
spherical diameter were simulated using recent determinations of their physical
and orbital properties. Estimates of the instrumental sensitivity, integration
times, and slew speeds were included for both architectures assuming the
properties of new large-format 10 um detector arrays capable of operating at
~35 K. Our simulation included the creation of a preliminary version of a
moving object processing pipeline suitable for operating on the trial cadence.
We tested this pipeline on a simulated sky populated with astrophysical sources
such as stars and galaxies extrapolated from Spitzer and WISE data, the catalog
of known minor planets (including Main Belt asteroids, comets, Jovian Trojans,
etc.), and the synthetic NEA model. Trial orbits were computed for simulated
position-time pairs extracted from the synthetic surveys to verify that the
tested cadence would result in orbits suitable for recovering objects at a
later time. Our results indicate that the Earth-Sun L1 and Venus-trailing
surveys achieve similar levels of integral completeness for potentially
hazardous asteroids larger than 140 m; placing the telescope in an interior
orbit does not yield an improvement in discovery rates. This work serves as a
necessary first step for the detailed planning of a next-generation NEA survey.Comment: AJ accepted; corrected typ
From Spitzer Galaxy Photometry to Tully-Fisher Distances
This paper involves a data release of the observational campaign: Cosmicflows
with Spitzer (CFS). Surface photometry of the 1270 galaxies constituting the
survey is presented. An additional ~ 400 galaxies from various other Spitzer
surveys are also analyzed. CFS complements the Spitzer Survey of Stellar
Structure in Galaxies, that provides photometry for an additional 2352
galaxies, by extending observations to low galactic latitudes (|b|<30 degrees).
Among these galaxies are calibrators, selected in K band, of the Tully-Fisher
relation. The addition of new calibrators demonstrate the robustness of the
previously released calibration. Our estimate of the Hubble constant using
supernova host galaxies is unchanged, H0 = 75.2 +/- 3.3 km/s/Mpc.
Distance-derived radial peculiar velocities, for the 1935 galaxies with all the
available parameters, will be incorporated into a new data release of the
Cosmicflows project. The size of the previous catalog will be increased by 20%,
including spatial regions close to the Zone of Avoidance.Comment: Accepted for publication in MNRAS, 16 pages, 14 figures, 6 table
Interplay between function and structure in complex networks
We show that abrupt structural transitions can arise in functionally optimal
networks, driven by small changes in the level of transport congestion. Our
results offer an explanation as to why so many diverse species of network
structure arise in Nature (e.g. fungal systems) under essentially the same
environmental conditions. Our findings are based on an exactly solvable model
system which mimics a variety of biological and social networks. We then extend
our analysis by introducing a novel renormalization scheme involving cost
motifs, to describe analytically the average shortest path across
multiple-ring-and-hub networks. As a consequence, we uncover a 'skin effect'
whereby the structure of the inner multi-ring core can cease to play any role
in terms of determining the average shortest path across the network.Comment: Expanded version of physics/0508228 with additional new result
Variability Flagging in the Wide-field Infrared Survey Explorer Preliminary Data Release
The Wide-field Infrared Survey Explorer Preliminary Data Release Source Catalog contains over 257 million objects. We describe the method used to flag variable source candidates in the Catalog. Using a method based on the chi-square of single-exposure flux measurements, we generated a variability flag for each object, and have identified almost 460,000 candidate sources that exhibit significant flux variability with greater than ~7σ confidence. We discuss the flagging method in detail and describe its benefits and limitations. We also present results from the flagging method, including example light curves of several types of variable sources including Algol-type eclipsing binaries, RR Lyr, W UMa, and a blazar candidate
Multi-Objective Optimisation of Aero-Engine Compressors
The design of a new aero-engine compressor is a complex
task: design objectives are almost always conflicting, the
design space is large, nonlinear and highly constrained, and
the effects of some geometrical changes can be difficult to
predict.
Computational fluid dynamics (CFD) is now widely used
in real-world applications and especially in the design of turbomachinery.
However, the large design space and the time
required for the numerical simulation of the whole turbomachine
make the use of CFD in the early phases of the design
process infeasible: preliminary design relies on a number of
physical and empirical relations, still quite similar to those
used in the early history of turbomachinery design.
In this study, 87 independent parameters were used to
define the geometry of a 7-stage compressor, the performance
of which was evaluated using proprietary design
codes for mean-line, multi-stage analysis. The effects on
efficiency and surge margin of changing 44 design variables
were analysed and their optimal values found by means
of deterministic (gradient-based) and meta-heuristic (Tabu
Search [TS]) optimisation methods.
The results show clearly how the use of meta-heuristic
optimisation tools can improve the preliminary design of
turbomachinery, allowing a more thorough but still rapid
exploration of the design space to identify the most promising
regions that will then be verified and further analysed
with higher fidelity tools.
The results also reveal the impact of introducing various
constraints into the design process, highlighting the effects
of design decomposition
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