1,206 research outputs found
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models
[abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio
Sivers and Boer-Mulders functions in Light-Cone Quark Models
Results for the naive-time-reversal-odd quark distributions in a light-cone
quark model are presented. The final-state interaction effects are generated
via single-gluon exchange mechanism. The formalism of light-cone wave functions
is used to derive general expressions in terms of overlap of wave-function
amplitudes describing the different orbital angular momentum components of the
nucleon. In particular, the model predictions show a dominant contribution from
S- and P-wave interference in the Sivers function and a significant
contribution also from the interference of P and D waves in the Boer-Mulders
function. The favourable comparison with existing phenomenological
parametrizations motivates further applications to describe azimuthal
asymmetries in hadronic reactions.Comment: references and explanations added; version to appear in Phys. Rev.
The polarizability of the pion: no conflict between dispersion theory and chiral perturbation theory
Recent attempts to determine the pion polarizability by dispersion relations
yield values that disagree with the predictions of chiral perturbation theory.
These dispersion relations are based on specific forms for the absorptive part
of the Compton amplitudes. The analytic properties of these forms are examined,
and the strong enhancement of intermediate-meson contributions is shown to be
connected with spurious singularities. If the basic requirements of dispersion
relations are taken into account, the results of dispersion theory and
effective field theory are not inconsistent.Comment: 30 pages, 8 figures, 6 table
Volatility in Financial Markets: Stochastic Models and Empirical Results
We investigate the historical volatility of the 100 most capitalized stocks
traded in US equity markets. An empirical probability density function (pdf) of
volatility is obtained and compared with the theoretical predictions of a
lognormal model and of the Hull and White model. The lognormal model well
describes the pdf in the region of low values of volatility whereas the Hull
and White model better approximates the empirical pdf for large values of
volatility. Both models fails in describing the empirical pdf over a moderately
large volatility range.Comment: 6 pages, 2 figure
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651
The simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed feature
One-step synthesis of metal/oxide nanocomposites by gas phase condensation
Metallic nanoparticles (NPs), either supported on a porous oxide framework or finely dispersed within an oxide matrix, find applications in catalysis, plasmonics, nanomagnetism and energy conversion, among others. The development of synthetic routes that enable to control the morphology, chemical composition, crystal structure and mutual interaction of metallic and oxide phases is necessary in order to tailor the properties of this class of nanomaterials. With this work, we aim at developing a novel method for the synthesis of metal/oxide nanocomposites based on the assembly of NPs formed by gas phase condensation of metal vapors in a He/O2 atmosphere. This new approach relies on the independent evaporation of two metallic precursors with strongly different oxidation enthalpies. Our goal is to show that the precursor with less negative enthalpy gives birth to metallic NPs, while the other to oxide NPs. The selected case study for this work is the synthesis of a Fe-Co/TiOx nanocomposite, a system of great interest for its catalytic and magnetic properties. By exploiting the new concept, we achieve the desired target, i.e., a nanoscale dispersion of metallic alloy NPs within titanium oxide NPs, the structure of which can be tailored into TiO1-\u3b4 or TiO2 by controlling the synthesis and processing atmosphere. The proposed synthesis technique is versatile and scalable for the production of many NPs-assembled metal/oxide nanocomposites
A variational approach to Ising spin glasses in finite dimensions
We introduce a hierarchical class of approximations of the random Ising spin
glass in dimensions. The attention is focused on finite clusters of spins
where the action of the rest of the system is properly taken into account. At
the lower level (cluster of a single spin) our approximation coincides with the
SK model while at the highest level it coincides with the true -dimensional
system. The method is variational and it uses the replica approach to spin
glasses and the Parisi ansatz for the order parameter. As a result we have
rigorous bounds for the quenched free energy which become more and more precise
when larger and larger clusters are considered.Comment: 16 pages, Plain TeX, uses Harvmac.tex, 4 ps figures, submitted to J.
Phys. A: Math. Ge
Variety and Volatility in Financial Markets
We study the price dynamics of stocks traded in a financial market by
considering the statistical properties both of a single time series and of an
ensemble of stocks traded simultaneously. We use the stocks traded in the
New York Stock Exchange to form a statistical ensemble of daily stock returns.
For each trading day of our database, we study the ensemble return
distribution. We find that a typical ensemble return distribution exists in
most of the trading days with the exception of crash and rally days and of the
days subsequent to these extreme events. We analyze each ensemble return
distribution by extracting its first two central moments. We observe that these
moments are fluctuating in time and are stochastic processes themselves. We
characterize the statistical properties of ensemble return distribution central
moments by investigating their probability density functions and temporal
correlation properties. In general, time-averaged and portfolio-averaged price
returns have different statistical properties. We infer from these differences
information about the relative strength of correlation between stocks and
between different trading days. Lastly, we compare our empirical results with
those predicted by the single-index model and we conclude that this simple
model is unable to explain the statistical properties of the second moment of
the ensemble return distribution.Comment: 10 pages, 11 figure
The Vector Analyzing Power in Elastic Electron-Proton Scattering
We compute the vector analyzing power (VAP) for the elastic scattering of
transversely polarized electrons from protons at low energies using an
effective theory of electrons, protons, and photons. We study all contributions
through second order in , where and are the electron energy and
nucleon mass, respectively. The leading order VAP arises from the imaginary
part of the interference of one- and two-photon exchange amplitudes.
Sub-leading contributions are generated by the nucleon magnetic moment and
charge radius as well as recoil corrections to the leading-order amplitude.
Working to , we obtain a prediction for that is free of
unknown parameters and that agrees with the recent measurement of the VAP in
backward angle scattering.Comment: 24 pages, 11 figures. Typos fixe
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