32,026 research outputs found
A comparative study of two 47 Tuc giant stars with different s-process enrichment
Here we aim to understand the origin of 47 Tuc's La-rich star Lee 4710. We
report abundances for O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, Zn, Y, Zr,
Ba, La, Ce, Pr, Nd, and Eu, and present a detailed abundance analysis of two 47
Tuc stars with similar stellar parameters but different slow neutron-capture
(s-)process enrichment. Star Lee 4710 has the highest known La abundance ratio
in this cluster ([La/Fe] = 1.14), and star Lee 4626 is known to have normal
s-process abundances (e.g., [Ba/Eu]). The nucleosynthetic pattern of
elements with Z56 for star Lee 4710 agrees with the predicted yields
of a asymptotic giant branch (AGB) star. Therefore, Lee 4710 may
have been enriched by mass transfer from a more massive AGB companion, which is
compatible with its location far away from the center of this relatively
metal-rich ([Fe/H]) globular cluster. A further analysis comparing
the abundance pattern of Lee 4710 with data available in the literature reveals
that nine out of the 47 Tuc stars previously studied show strong
s-process enhancements that point towards later enrichment by more massive AGB
stars.Comment: ApJL in press. 6 pages, 4 figure
Structure, phase behavior and inhomogeneous fluid properties of binary dendrimer mixtures
The effective pair potentials between different kinds of dendrimers in
solution can be well approximated by appropriate Gaussian functions. We find
that in binary dendrimer mixtures the range and strength of the effective
interactions depend strongly upon the specific dendrimer architecture. We
consider two different types of dendrimer mixtures, employing the Gaussian
effective pair potentials, to determine the bulk fluid structure and phase
behavior. Using a simple mean field density functional theory (DFT) we find
good agreement between theory and simulation results for the bulk fluid
structure. Depending on the mixture, we find bulk fluid-fluid phase separation
(macro-phase separation) or micro-phase separation, i.e., a transition to a
state characterized by undamped periodic concentration fluctuations. We also
determine the inhomogeneous fluid structure for confinement in spherical
cavities. Again, we find good agreement between the DFT and simulation results.
For the dendrimer mixture exhibiting micro-phase separation, we observe rather
striking pattern formation under confinement.Comment: 8 pages, 10 figure
Coarse-graining diblock copolymer solutions: a macromolecular version of the Widom-Rowlinson model
We propose a systematic coarse-grained representation of block copolymers,
whereby each block is reduced to a single ``soft blob'' and effective intra- as
well as intermolecular interactions act between centres of mass of the blocks.
The coarse-graining approach is applied to simple athermal lattice models of
symmetric AB diblock copolymers, in particular to a Widom-Rowlinson-like model
where blocks of the same species behave as ideal polymers (i.e. freely
interpenetrate), while blocks of opposite species are mutually avoiding walks.
This incompatibility drives microphase separation for copolymer solutions in
the semi-dilute regime. An appropriate, consistent inversion procedure is used
to extract effective inter- and intramolecular potentials from Monte Carlo
results for the pair distribution functions of the block centres of mass in the
infinite dilution limit.Comment: To be published in mol.phys(2005
Design considerations for large space electric power systems
As power levels of spacecraft rise to the 50 to 100 kW range, it becomes apparent that low voltage (28 V) dc power distribution and management systems will not operate efficiently at these higher power levels. The concept of transforming a solar array voltage at 150 V dc into a 1000 V ac distribution system operating at 20 kHz is examined. The transformation is accomplished with series-resonant inverter by using a rotary transformer to isolate the solar array from the spacecraft. The power can then be distributed in any desired method such as three phase delta to delta. The distribution voltage can be easily transformed to any desired load voltage and operating frequency. The reasons for the voltage limitations on the solar array due to plasma interactions and the many advantages of a high voltage, high frequency at distribution system are discussed
Mean properties and Free Energy of a few hard spheres confined in a spherical cavity
We use analytical calculations and event-driven molecular dynamics
simulations to study a small number of hard sphere particles in a spherical
cavity. The cavity is taken also as the thermal bath so that the system
thermalizes by collisions with the wall. In that way, these systems of two,
three and four particles, are considered in the canonical ensemble. We
characterize various mean and thermal properties for a wide range of number
densities. We study the density profiles, the components of the local pressure
tensor, the interface tension, and the adsorption at the wall. This spans from
the ideal gas limit at low densities to the high-packing limit in which there
are significant regions of the cavity for which the particles have no access,
due the conjunction of excluded volume and confinement. The contact density and
the pressure on the wall are obtained by simulations and compared to exact
analytical results. We also obtain the excess free energy for N=4, by using a
simulated-assisted approach in which we combine simulation results with the
knowledge of the exact partition function for two and three particles in a
spherical cavity.Comment: 11 pages, 9 figures and two table
A numerical renormalization group study of laser induced freezing
We study the phenomenon of laser induced freezing, within a numerical
renormalization scheme which allows explicit comparison with a recent defect
mediated melting theory. Precise values for the `bare' dislocation fugacities
and elastic moduli of the 2-d hard disk system are obtained from a constrained
Monte Carlo simulation sampling only configurations {\em without} dislocations.
These are used as inputs to appropriate renormalization flow equations to
obtain the equilibrium phase diagram which shows excellent agreement with
earlier simulation results. We show that the flow equations need to be correct
at least up to third order in defect fugacity to reproduce meaningful results.Comment: Minor Corrections; Combined version of Europhys. Lett. 67 (2004) p.
814 and Europhys. Lett. 68 (2004) p. 16
Percolation-to-hopping crossover in conductor-insulator composites
Here, we show that the conductivity of conductor-insulator composites in
which electrons can tunnel from each conducting particle to all others may
display both percolation and tunneling (i.e. hopping) regimes depending on few
characteristics of the composite. Specifically, we find that the relevant
parameters that give rise to one regime or the other are (where is
the size of the conducting particles and is the tunneling length) and the
specific composite microstructure. For large values of , percolation
arises when the composite microstructure can be modeled as a regular lattice
that is fractionally occupied by conducting particle, while the tunneling
regime is always obtained for equilibrium distributions of conducting particles
in a continuum insulating matrix. As decreases the percolating behavior
of the conductivity of lattice-like composites gradually crosses over to the
tunneling-like regime characterizing particle dispersions in the continuum. For
values lower than the conductivity has tunneling-like
behavior independent of the specific microstructure of the composite.Comment: 8 pages, 5 figure
Mechanism for Surface Waves in Vibrated Granular Material
We use molecular dynamics simulations to study the formation of surface waves
in vertically vibrated granular material. We find that horizontal movements of
particles, which are essential for the formation of the waves, consist of two
distinct processes. First, the movements sharply increase while the particles
are colliding with a bottom plate, where the duration of the collisions is very
short compared to the period of the vibration. Next, the movements gradually
decrease between the collisions, during which the particles move through the
material. We also find that the horizontal velocity field after the collisions
is strongly correlated to the surface profile before the collisions.Comment: 6 pages, 3 figures (included
Effective Dielectric Tensor for Electromagnetic Wave Propagation in Random Media
We derive exact strong-contrast expansions for the effective dielectric
tensor \epeff of electromagnetic waves propagating in a two-phase composite
random medium with isotropic components explicitly in terms of certain
integrals over the -point correlation functions of the medium. Our focus is
the long-wavelength regime, i.e., when the wavelength is much larger than the
scale of inhomogeneities in the medium. Lower-order truncations of these
expansions lead to approximations for the effective dielectric constant that
depend upon whether the medium is below or above the percolation threshold. In
particular, we apply two- and three-point approximations for \epeff to a
variety of different three-dimensional model microstructures, including
dispersions of hard spheres, hard oriented spheroids and fully penetrable
spheres as well as Debye random media, the random checkerboard, and
power-law-correlated materials. We demonstrate the importance of employing
-point correlation functions of order higher than two for high
dielectric-phase-contrast ratio. We show that disorder in the microstructure
results in an imaginary component of the effective dielectric tensor that is
directly related to the {\it coarseness} of the composite, i.e., local
volume-fraction fluctuations for infinitely large windows. The source of this
imaginary component is the attenuation of the coherent homogenized wave due to
scattering. We also remark on whether there is such attenuation in the case of
a two-phase medium with a quasiperiodic structure.Comment: 40 pages, 13 figure
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