209 research outputs found
A Parameter Study of Classical Be Star Disk Models Constrained by Optical Interferometry
We have computed theoretical models of circumstellar disks for the classical
Be stars Dra, Psc, and Cyg. Models were constructed
using a non-LTE radiative transfer code developed by \citet{sig07} which
incorporates a number of improvements over previous treatments of the disk
thermal structure, including a realistic chemical composition. Our models are
constrained by direct comparison with long baseline optical interferometric
observations of the H emitting regions and by contemporaneous H
line profiles. Detailed comparisons of our predictions with H
interferometry and spectroscopy place very tight constraints on the density
distributions for these circumstellar disks.Comment: 10 figures,28 pages, accepted by Ap
Effects of three-body interactions on the structure and thermodynamics of liquid krypton
Large-scale molecular dynamics simulations are performed to predict the
structural and thermodynamic properties of liquid krypton using a potential
energy function based on the two-body potential of Aziz and Slaman plus the
triple-dipole Axilrod-Teller (AT) potential. By varying the strength of the AT
potential we study the influence of three-body contribution beyond the
triple-dipole dispersion. It is seen that the AT potential gives an overall
good description of liquid Kr, though other contributions such as higher order
three-body dispersion and exchange terms cannot be ignored.Comment: 11 pages, 3 figures, LaTeX, to appear in J. Chem. Phy
Order parameter model for unstable multilane traffic flow
We discuss a phenomenological approach to the description of unstable vehicle
motion on multilane highways that explains in a simple way the observed
sequence of the phase transitions "free flow -> synchronized motion -> jam" as
well as the hysteresis in the transition "free flow synchronized motion".
We introduce a new variable called order parameter that accounts for possible
correlations in the vehicle motion at different lanes. So, it is principally
due to the "many-body" effects in the car interaction, which enables us to
regard it as an additional independent state variable of traffic flow. Basing
on the latest experimental data (cond-mat/9905216) we assume that these
correlations are due to a small group of "fast" drivers. Taking into account
the general properties of the driver behavior we write the governing equation
for the order parameter. In this context we analyze the instability of
homogeneous traffic flow manifesting itself in both of the mentioned above
phase transitions where, in addition, the transition "synchronized motion ->
jam" also exhibits a similar hysteresis. Besides, the jam is characterized by
the vehicle flows at different lanes being independent of one another. We
specify a certain simplified model in order to study the general features of
the car cluster self-formation under the phase transition "free flow
synchronized motion". In particular, we show that the main local parameters of
the developed cluster are determined by the state characteristics of vehicle
motion only.Comment: REVTeX 3.1, 10 pages with 10 PostScript figure
First-principles calculation of positron lifetimes and affinities in perfect and imperfect transition-metal carbides and nitrides
New anti-perovskite-type Superconductor ZnNyNi3
We have synthesized a new superconductor ZnNyNi3 with Tc ~3 K. The crystal
structure has the same anti-perovskite-type such as MgCNi3 and CdCNi3. As far
as we know, this is the third superconducting material in Ni-based
anti-perovskite series. For this material, superconducting parameters,
lower-critical field Hc1(0), upper-critical field Hc2(0), coherence length
x(0), penetration depth l(0), and Gintzburg -Landau parameter k(0) have been
experimentally determined.Comment: 13 pages, 3 figures, 1 tabl
Li14Ln5[Si11N19O5]O2F2 with Ln = Ce, Nd-Representatives of a Family of Potential Lithium Ion Conductors
The isotypic layered oxonitridosilicates Li14Ln5[Si11N19O5]O2F2 (Ln = Ce, Nd) have been synthesized using Li as fluxing agent and crystallize in the orthorhombic space group Pmmn (Z = 2, Li14Ce5[Si11N19O5]O2F2: a = 17.178(3), b = 7.6500(15), c = 10.116(2) Å, R1 = 0.0409, wR2 = 0.0896; Li14Nd5 Si11N19O5]O2F2: a = 17.126(2), b = 7.6155 15), c = 10.123(2) Å, R1 = 0.0419, wR2 = 0.0929). The silicate layers consist of dreier and sechser rings interconnected via common corners, yielding an unprecedented silicate substructure. A topostructural analysis indicates possible 1D ion migration pathways between five crystallographic independent Li positions. The specific Li-ionic conductivity and its temperature dependence were determined by impedance spectroscopy as well as DC polarization/depolarization measurements. The ionic conductivity is on the order of 5 ×
10−5 S/cm at 300°C, while the activation energy is 0.69 eV. Further adjustments of the defect chemistry (e.g., through doping)can make these compounds interesting candidates for novel oxonitridosilicate based ion conductors
CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks
We present the first K'-band, long-baseline interferometric observations of
the northern Be stars gamma Cas, phi Per, zeta Tau, and kappa Dra. The
measurements were made with multiple telescope pairs of the CHARA Array
interferometer, and in every case the observations indicate that the
circumstellar disks of the targets are resolved. We fit the interferometric
visibilities with predictions from a simple disk model that assumes an
isothermal gas in Keplerian rotation. We derive fits of the four model
parameters (disk base density, radial density exponent, disk normal
inclination, and position angle) for each of the targets. The resulting
densities are in broad agreement with prior studies of the IR excess flux and
the resulting orientations generally agree with those from interferometric
H-alpha and continuum polarimetric observations. We find that the angular size
of the K' disk emission is smaller than that determined for the H-alpha
emission, and we argue that the difference is the result of a larger H-alpha
opacity and the relatively larger neutral hydrogen fraction with increasing
disk radius. All the targets are known binaries with faint companions, and we
find that companions appear to influence the interferometric visibilities in
the cases of phi Per and kappa Dra. We also present contemporaneous
observations of the H-alpha, H-gamma, and Br-gamma emission lines. Synthetic
model profiles of these lines that are based on the same disk inclination and
radial density exponent as derived from the CHARA Array observations match the
observed emission line strength if the disk base density is reduced by
approximately 1.7 dex.Comment: ApJ in press (2007 Jan 1), 55 pages, 14 figure
Copernicus Marine Service Ocean State Report
This is the final version. Available from Taylor & Francis via the DOI in this record
Challenges for Sustained Observing and Forecasting Systems in the Mediterranean Sea
The Mediterranean community represented in this paper is the result of more than 30 years of EU and nationally funded coordination, which has led to key contributions in science concepts and operational initiatives. Together with the establishment of operational services, the community has coordinated with universities, research centers, research infrastructures and private companies to implement advanced multi-platform and integrated observing and forecasting systems that facilitate the advancement of operational services, scientific achievements and mission-oriented innovation. Thus, the community can respond to societal challenges and stakeholders needs, developing a variety of fit-for-purpose services such as the Copernicus Marine Service. The combination of state-of-the-art observations and forecasting provides new opportunities for downstream services in response to the needs of the heavily populated Mediterranean coastal areas and to climate change. The challenge over the next decade is to sustain ocean observations within the research community, to monitor the variability at small scales, e.g., the mesoscale/submesoscale, to resolve the sub-basin/seasonal and inter-annual variability in the circulation, and thus establish the decadal variability, understand and correct the model-associated biases and to enhance model-data integration and ensemble forecasting for uncertainty estimation. Better knowledge and understanding of the level of Mediterranean variability will enable a subsequent evaluation of the impacts and mitigation of the effect of human activities and climate change on the biodiversity and the ecosystem, which will support environmental assessments and decisions. Further challenges include extending the science-based added-value products into societal relevant downstream services and engaging with communities to build initiatives that will contribute to the 2030 Agenda and more specifically to SDG14 and the UN's Decade of Ocean Science for sustainable development, by this contributing to bridge the science-policy gap. The Mediterranean observing and forecasting capacity was built on the basis of community best practices in monitoring and modeling, and can serve as a basis for the development of an integrated global ocean observing system
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