15,032 research outputs found
A comparison of star formation characteristics in different types of irregular galaxies
Two regions of recent star formation in blue irregular galaxies were observed with the IUE in the short wavelength, low dispersion mode. The spectra indicates that the massive star content is similar in these regions and is best fit by massive stars formed in a burst and now are approximately 2.5 to 3.0 million years old
Mixed Non-Uniform Width / Evanescent Mode Ceramic Resonator Waveguide Filter With Wide Spurious Free Bandwidth
This paper presents a method to improve the spurious performance of integrated ceramic waveguide filters. Nonuniform width ceramic waveguide resonator and evanescent mode ceramic resonators are employed together to the resonant frequencies of higher order modes. The proposed designs give 75% improvement in stop band response when compared to uniform width ceramic waveguide filter. Simulated results of two six pole chebyshev filters are presented here with improved stop band performance
Spurious Stopband Improvement of Dual-Mode Dielectric Resonator Filters Using T-shaped Coupling Probe
This paper presents a new design technique for dual-mode dielectric loaded resonator filter operating at 2.18GHz by using T-shape coupling probe. A cylindrical dielectric puck is suspended in the middle of metallic cavity and short-circuited on the side walls. Four vertically etched-slots through the dielectric puck from the top to the base of the ceramic puck offer a high unloaded quality factor of 4300 at the fundamental T E11δ mode and allows the T-shape coupling to be located close to the centre of the dielectric puck. The size reduction ratio in this resonator was 11% compared with air filled coaxial resonator. Fourth-poles Chebyshev bandpass filter (BPF) with planar configuration was designed and implemented by employing two dielectric-loaded cavities. The experimental results show that the spurious-free window of approximately 820 MHz from the fundamental frequency of 2.18GHz and the operational bandwidth of 77 MHz were achieved
The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
Rotation has become an important element in evolutionary models of massive
stars, specifically via the prediction of rotational mixing. Here, we study a
sample of stars, including rapid rotators, to constrain such models and use
nitrogen enrichments as a probe of the mixing process. Chemical compositions
(C, N, O, Mg and Si) have been estimated for 135 early B-type stars in the
Large Magellanic Cloud with projected rotational velocities up to ~300km/s
using a non-LTE TLUSTY model atmosphere grid. Evolutionary models, including
rotational mixing, have been generated attempting to reproduce these
observations by adjusting the overshooting and rotational mixing parameters and
produce reasonable agreement with 60% of our core hydrogen burning sample. We
find (excluding known binaries) a significant population of highly nitrogen
enriched intrinsic slow rotators vsini less than 50km/s incompatible with our
models ~20% of the sample). Furthermore, while we find fast rotators with
enrichments in agreement with the models, the observation of evolved (log g
less than 3.7dex) fast rotators that are relatively unenriched (a further ~20%
of the sample) challenges the concept of rotational mixing. We also find that
70% of our blue supergiant sample cannot have evolved directly from the
hydrogen burning main-sequence. We are left with a picture where invoking
binarity and perhaps fossil magnetic fields are required to understand the
surface properties of a population of massive main sequence stars.Comment: ApJL. 10 pages, 1 figure. Updated to match accepted versio
Liquid friction on charged surfaces: from hydrodynamic slippage to electrokinetics
Hydrodynamic behavior at the vicinity of a confining wall is closely related
to the friction properties of the liquid/solid interface. Here we consider,
using Molecular Dynamics simulations, the electric contribution to friction for
charged surfaces, and the induced modification of the hydrodynamic boundary
condition at the confining boundary. The consequences of liquid slippage for
electrokinetic phenomena, through the coupling between hydrodynamics and
electrostatics within the electric double layer, are explored. Strong
amplification of electro-osmotic effects is revealed, and the non-trivial
effect of surface charge is discussed. This work allows to reconsider existing
experimental data, concerning Zeta potentials of hydrophobic surfaces and
suggest the possibility to generate ``giant'' electro-osmotic and
electrophoretic effects, with direct applications in microfluidics
The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds
Aims. We compare the predictions of evolutionary models for early-type stars
with atmospheric parameters, projected rotational velocities and nitrogen
abundances estimated for a sample of Be-type stars. Our targets are located in
4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11
region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.
Methods. Atmospheric parameters and photospheric abundances have been
determined using the non-LTE atmosphere code tlusty. Effective temperature
estimates were deduced using three different methodologies depending on the
spectral features observed; in general they were found to yield consistent
estimates. Gravities were deduced from Balmer line profiles and
microturbulences from the Si iii spectrum. Additionally the contributions of
continuum emission from circumstellar discs were estimated. Given its
importance in constraining stellar evolutionary models, nitrogen abundances (or
upper limits) were deduced for all the stars analysed. Results. Our nitrogen
abundances are inconsistent with those predicted for targets spending most of
their main sequence life rotating near to the critical velocity. This is
consistent with the results we obtain from modelling the inferred rotational
velocity distribution of our sample and of other investigators. We consider a
number of possibilities to explain the nitrogen abundances and rotational
velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&
Light elements in massive single and binary stars
We highlight the role of the light elements (Li, Be, B) in the evolution of
massive single and binary stars, which is largely restricted to a diagnostic
value, and foremost so for the element boron. However, we show that the boron
surface abundance in massive early type stars contains key information about
their foregoing evolution which is not obtainable otherwise. In particular, it
allows to constrain internal mixing processes and potential previous mass
transfer event for binary stars (even if the companion has disappeared). It may
also help solving the mystery of the slowly rotating nitrogen-rich massive main
sequence stars.Comment: 10 pages, 8 figures, to appear in proc. IAU-Symp. 268. C. Charbonnel
et al., eds
Ferromagnetism in the Mott insulator Ba2NaOsO6
Results are presented of single crystal structural, thermodynamic, and
reflectivity measurements of the double-perovskite Ba2NaOsO6. These
characterize the material as a 5d^1 ferromagnetic Mott insulator with an
ordered moment of ~0.2 Bohr magnetons per formula unit and TC = 6.8(3) K. The
magnetic entropy associated with this phase transition is close to Rln2,
indicating that the quartet groundstate anticipated from consideration of the
crystal structure is split, consistent with a scenario in which the
ferromagnetism is associated with orbital ordering.Comment: 5 pages, 5 figures, added reference
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Benchmarking 2D hydraulic models for urban flood simulations
This paper describes benchmark testing of six two-dimensional (2D) hydraulic models (DIVAST, DIVASTTVD, TUFLOW, JFLOW, TRENT and LISFLOOD-FP) in terms of their ability to simulate surface flows in a densely urbanised area. The models are applied to a 1·0 km × 0·4 km urban catchment within the city of Glasgow, Scotland, UK, and are used to simulate a flood event that occurred at this site on 30 July 2002. An identical numerical grid describing the underlying topography is constructed for each model, using a combination of airborne laser altimetry (LiDAR) fused with digital map data, and used to run a benchmark simulation. Two numerical experiments were then conducted to test the response of each model to topographic error and uncertainty over friction parameterisation. While all the models tested produce plausible results, subtle differences between particular groups of codes give considerable insight into both the practice and science of urban hydraulic modelling. In particular, the results show that the terrain data available from modern LiDAR systems are sufficiently accurate and resolved for simulating urban flows, but such data need to be fused with digital map data of building topology and land use to gain maximum benefit from the information contained therein. When such terrain data are available, uncertainty in friction parameters becomes a more dominant factor than topographic error for typical problems. The simulations also show that flows in urban environments are characterised by numerous transitions to supercritical flow and numerical shocks. However, the effects of these are localised and they do not appear to affect overall wave propagation. In contrast, inertia terms are shown to be important in this particular case, but the specific characteristics of the test site may mean that this does not hold more generally
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