22,879 research outputs found
Really Cool Stars and the Star Formation History at the Galactic Center
We present R=550 to 1200 near infrared H and K spectra for a magnitude
limited sample of 79 asymptotic giant branch and cool supergiant stars in the
central ~ 5 pc (diameter) of the Galaxy. We use a set of similar spectra
obtained for solar neighborhood stars with known Teff and Mbol that is in the
same range as the Galactic center (GC) sample to derive Teff and Mbol for the
GC sample. We then construct the Hertzsprung--Russell (HRD) diagram for the GC
sample. Using an automated maximum likelihood routine, we derive a coarse star
formation history of the GC. We find (1) roughly 75% of the stars formed in the
central few pc are older than 5 Gyr; (2) the star formation rate (SFR) is
variable over time, with a roughly 4 times higher star formation rate in the
last 100 Myr compared to the average SFR; (3) our model can only match
dynamical limits on the total mass of stars formed by limiting the IMF to
masses above 0.7 M. This could be a signature of mass segregation or of
the bias toward massive star formation from the unique star formation
conditions in the GC; (4) blue supergiants account for 12 % of the total sample
observed, and the ratio of red to blue supergiants is roughly 1.5; (5) models
with isochrones with [Fe/H] = 0.0 over all ages fit the stars in our HRD better
than models with lower [Fe/H] in the oldest age bins, consistent with the
finding of Ramirez et al. (2000) that stars with ages between 10 Myr and 1 Gyr
have solar [Fe/H].Comment: ApJ, accepted. Latex, 65 pages including 19 figure
Experimental comparison of icing cloud instruments
Icing cloud instruments were tested in the spray cloud Icing Research Tunnel (IRT) in order to determine their relative accuracy and their limitations over a broad range of conditions. It was found that the average of the readings from each of the liquid water content (LWC) instruments tested agreed closely with each other and with the IRT calibration; but all have a data scatter (+ or - one standard deviation) of about + or - 20 percent. The effect of this + or - 20 percent uncertainty is probably acceptable in aero-penalty and deicer experiments. Existing laser spectrometers proved to be too inaccurate for LWC measurements. The error due to water runoff was the same for all ice accretion LWC instruments. Any given laser spectrometer proved to be highly repeatable in its indications of volume median drop size (DVM), LWC and drop size distribution. However, there was a significant disagreement between different spectrometers of the same model, even after careful standard calibration and data analysis. The scatter about the mean of the DVM data from five Axial Scattering Spectrometer Probes was + or - 20 percent (+ or - one standard deviation) and the average was 20 percent higher than the old IRT calibration. The + or - 20 percent uncertainty in DVM can cause an unacceptable variation in the drag coefficient of an airfoil with ice; however, the variation in a deicer performance test may be acceptable
Tripartite entanglement from interlinked parametric interactions
We examine the tripartite entanglement properties of an optical system using
interlinked interactions, recently studied experimentally in terms
of its phase-matching properties by Bondani et al [M. Bondani, A. Allevi, E.
Gevinti, A. Agliati, and A. Andreoni, arXiv:quant-ph/0604002.]. We show that
the system does produce output modes which are genuinely tripartite entangled
and that detection of this entanglement depends crucially on the correlation
functions which are measured, with a three-mode Einstein-Podolsky-Rosen
inequality being the most sensitive.Comment: 15 pages, 5 figure
Travelling waves in wound healing
We illustrate the role of travelling waves in wound healing by considering three different cases. Firstly, we review a model for surface wound healing in the cornea and focus on the speed of healing as a function of the application of growth factors. Secondly, we present a model for scar tissue formation in deep wounds and focus on the role of key chemicals in determining the quality of healing. Thirdly, we propose a model for excessive healing disorders and investigate how abnormal healing may be controlled
Quantum field effects in coupled atomic and molecular Bose-Einstein condensates
This paper examines the parameter regimes in which coupled atomic and
molecular Bose-Einstein condensates do not obey the Gross-Pitaevskii equation.
Stochastic field equations for coupled atomic and molecular condensates are
derived using the functional positive-P representation. These equations
describe the full quantum state of the coupled condensates and include the
commonly used Gross-Pitaevskii equation as the noiseless limit. The model
includes all interactions between the particles, background gas losses,
two-body losses and the numerical simulations are performed in three
dimensions. It is found that it is possible to differentiate the quantum and
semiclassical behaviour when the particle density is sufficiently low and the
coupling is sufficiently strong.Comment: 4 postscript figure
Einstein-Podolsky-Rosen correlations via dissociation of a molecular Bose-Einstein condensate
Recent experimental measurements of atomic intensity correlations through
atom shot noise suggest that atomic quadrature phase correlations may soon be
measured with a similar precision. We propose a test of local realism with
mesoscopic numbers of massive particles based on such measurements. Using
dissociation of a Bose-Einstein condensate of diatomic molecules into bosonic
atoms, we demonstrate that strongly entangled atomic beams may be produced
which possess Einstein-Podolsky-Rosen (EPR) correlations in field quadratures,
in direct analogy to the position and momentum correlations originally
considered by EPR.Comment: Final published version (corrections in Ref. [32], updated
references
Thermodynamic properties of Pb determined from pressure-dependent critical-field measurements
We have carried out extensive low-temperature (1.5 to 10 K) measurements of
the critical field, , for the element Pb up to a pressure of GPa.
From this data the electronic entropy, specific heat, thermal expansion
coefficient and compressibility is calculated as a function of temperature,
pressure and magnetic field. The zero-field data is consistent with direct
thermodynamic measurements and the -dependence of and specific heat
coefficient, allows the determination of the -dependence of
the pairing interaction.Comment: 5 pages, 6 figures, in press Phys. Rev.
Electronic Database Support Systems for Strategic Planning Activities in the Hospitality Industry
An electronic database support system for strategic planning activities can be built by providing conceptual and system specific information. The design and development of this type of system center around the information needs of strategy planners. Data that supply information on the organization\u27s internal and external environments must be originated, evaluated, collected, organized, managed, and analyzed. Strategy planners may use the resulting information to improve their decision making
A comparison of observed and simulated site response in the RhĂ´ne valley
Site effects in the city of Sion in the RhĂ´ne valley are analysed from weak motion signals recorded on a dense temporary array. We simulate the recorded events with a 3-D finite difference method for frequencies up to 4 Hz using a recently developed velocity model of the Sion basin. Site-to-reference Fourier spectral ratios are computed from 16 local and regional events. All sites exhibit amplification factors of up to 12 between 0.5 and 0.6 Hz, which can be reproduced by the numerical simulations. By rotating the weak motion to directions parallel and perpendicular to the valley axis, we show that this low-frequency amplification is caused by the SH00 and SV0 fundamental modes of 2-D resonance. Additional peaks of amplification can be observed at higher frequencies, with amplification factors of up to 20 at some sites. Application of the high-resolution frequency-wavenumber and the multiple signal characterization method to the vertical component of recorded and simulated signals show that edge-generated surface waves arriving from almost all directions dominate the wavefield at 1.25 and 2.50 Hz. Peak ground velocities computed from the simulated ground motion show interference patterns that depend strongly on the incidence direction, and the computed amplification of peak ground velocities are generally in agreement with the observations. We conclude that the complex 3-D geometry of the basin needs to be considered to evaluate site effects up to at least 2.5 H
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Direct imaging of long-range ferromagnetic and antiferromagnetic order in a dipolar metamaterial
Magnetic metamaterials such as artificial spin ice offer a route to tailor magnetic properties. Such materials can be fabricated by lithographically defining arrays of nanoscale magnetic islands. The magnetostatic interactions between the elements are influenced by their shape and geometric arrangement and can lead to long-range ordering. We demonstrate how the magnetic order in a two-dimensional periodic array of circular disks is controlled by the lattice symmetry. Antiferromagnetic and ferromagnetic order extending through the entire array is observed for the square and hexagonal lattice, respectively. Furthermore, we show that a minute deviation from perfect circularity of the elements along a preferred direction results in room-temperature blocking and favors collinear spin textures
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