4,239 research outputs found
Dynamics of the Young Binary LMC Cluster NGC 1850
In this paper we have examined the age and internal dynamics of the young
binary LMC cluster NGC 1850 using BV CCD images and echelle spectra of 52
supergiants. Isochrone fits to a BV color-magnitude diagram revealed that the
primary cluster has an age of Myr while the secondary member
has Myr. BV surface brightness profiles were constructed out
to R 40 pc, and single-component King-Michie (KM) models were applied. The
total cluster luminosity varied from L = 2.60 - 2.65
L\sol\ and L = 1.25 - 1.35 as the anisotropy radius
varied from infinity to three times the scale radius with the isotropic models
providing the best agreement with the data. Of the 52 stars with echelle
spectra, a subset of 36 were used to study the cluster dynamics. The KM radial
velocity distributions were fitted to these velocities yielding total cluster
masses of 5.4 - 5.9 M\sol\ corresponding to M/L =
0.02 M\sol/L\sol\ or M/L = 0.05 M\sol/L\sol.
A rotational signal in the radial velocities has been detected at the 93\%
confidence level implying a rotation axis at a position angle of 100\deg. A
variety of rotating models were fit to the velocity data assuming cluster
ellipticities of . These models provided slightly better
agreement with the radial velocity data than the KM models and had masses that
were systematically lower by a few percent. The preferred value for the slope
of a power-law IMF is a relatively shallow, x = 0.29 \pmm{+0.3}{-0.8}
assuming the B-band M/L or x = 0.71 \pmm{+0.2}{-0.4} for the V-band.Comment: 41 pages (figures available via anonymous FTP as described below
Effective mean-field equations for cigar-shaped and disk-shaped Bose-Einstein condensates
By applying the standard adiabatic approximation and using the accurate
analytical expression for the corresponding local chemical potential obtained
in our previous work [Phys. Rev. A \textbf{75}, 063610 (2007)] we derive an
effective 1D equation that governs the axial dynamics of mean-field
cigar-shaped condensates with repulsive interatomic interactions, accounting
accurately for the contribution from the transverse degrees of freedom. This
equation, which is more simple than previous proposals, is also more accurate.
Moreover, it allows treating condensates containing an axisymmetric vortex with
no additional cost. Our effective equation also has the correct limit in both
the quasi-1D mean-field regime and the Thomas-Fermi regime and permits one to
derive fully analytical expressions for ground-state properties such as the
chemical potential, axial length, axial density profile, and local sound
velocity. These analytical expressions remain valid and accurate in between the
above two extreme regimes. Following the same procedure we also derive an
effective 2D equation that governs the transverse dynamics of mean-field
disk-shaped condensates. This equation, which also has the correct limit in
both the quasi-2D and the Thomas-Fermi regime, is again more simple and
accurate than previous proposals. We have checked the validity of our equations
by numerically solving the full 3D Gross-Pitaevskii equation.Comment: 11 pages, 7 figures; Final version published in Phys. Rev. A;
Manuscript put in the archive and submitted to Phys. Rev. A on 17 July 200
Ground-state properties of trapped Bose-Einstein condensates: Extension of the Thomas-Fermi approximation
We derive general approximate formulas that provide with remarkable accuracy
the ground-state properties of any mean-field scalar Bose-Einstein condensate
with short-range repulsive interatomic interactions, confined in arbitrary
cylindrically symmetric harmonic traps. Our formulation is even applicable for
condensates containing a multiply quantized axisymmetric vortex. We have
checked the validity of our formulas by numerically solving the 3D
Gross-Pitaevskii equation.Comment: 9 pages, 6 figures. Final version published in Phys. Rev. A. New
formulas for the local sound velocity of cigar-shaped and disk-shaped
condensates have been obtained. This paper generalizes our previous work
cond-mat/070169
Gap solitons in elongated geometries: the one-dimensional Gross-Pitaevskii equation and beyond
We report results of a systematic analysis of matter-wave gap solitons (GSs)
in three-dimensional self-repulsive Bose-Einstein condensates (BECs) loaded
into a combination of a cigar-shaped trap and axial optical-lattice (OL)
potential. Basic cases of the strong, intermediate, and weak radial
(transverse) confinement are considered, as well as settings with shallow and
deep OL potentials. Only in the case of the shallow lattice combined with tight
radial confinement, which actually has little relevance to realistic
experimental conditions, does the usual one-dimensional (1D) cubic
Gross-Pitaevskii equation (GPE) furnish a sufficiently accurate description of
GSs. However, the effective 1D equation with the nonpolynomial nonlinearity,
derived in Ref. [Phys. Rev. A \textbf{77}, 013617 (2008)], provides for quite
an accurate approximation for the GSs in all cases, including the situation
with weak transverse confinement, when the soliton's shape includes a
considerable contribution from higher-order transverse modes, in addition to
the usual ground-state wave function of the respective harmonic oscillator.
Both fundamental GSs and their multipeak bound states are considered. The
stability is analyzed by means of systematic simulations. It is concluded that
almost all the fundamental GSs are stable, while their bound states may be
stable if the underlying OL potential is deep enough.Comment: 14 pages, 12 figures; v2: matches published versio
Assessing species habitat using Google Street View: A case study of cliff-nesting vultures
The assessment of a species' habitat is a crucial issue in ecology and conservation. While the collection of habitat data has been boosted by the availability of remote sensing technologies, certain habitat types have yet to be collected through costly, on-ground surveys, limiting study over large areas. Cliffs are ecosystems that provide habitat for a rich biodiversity, especially raptors. Because of their principally vertical structure, however, cliffs are not easy to study by remote sensing technologies, posing a challenge for many researches and managers working with cliff-related biodiversity. We explore the feasibility of Google Street View, a freely available on-line tool, to remotely identify and assess the nesting habitat of two cliff-nesting vultures (the griffon vulture and the globally endangered Egyptian vulture) in northwestern Spain. Two main usefulness of Google Street View to ecologists and conservation biologists were evaluated: i) remotely identifying a species' potential habitat and ii) extracting fine-scale habitat information. Google Street View imagery covered 49% (1,907 km) of the roads of our study area (7,000 km2). The potential visibility covered by on-ground surveys was significantly greater (mean: 97.4%) than that of Google Street View (48.1%). However, incorporating Google Street View to the vulture's habitat survey would save, on average, 36% in time and 49.5% in funds with respect to the on-ground survey only. The ability of Google Street View to identify cliffs (overall accuracy = 100%) outperformed the classification maps derived from digital elevation models (DEMs) (62-95%). Nonetheless, high-performance DEM maps may be useful to compensate Google Street View coverage limitations. Through Google Street View we could examine 66% of the vultures' nesting-cliffs existing in the study area (n = 148): 64% from griffon vultures and 65% from Egyptian vultures. It also allowed us the extraction of fine-scale features of cliffs. This World Wide Web-based methodology may be a useful, complementary tool to remotely map and assess the potential habitat of cliff-dependent biodiversity over large geographic areas, saving survey-related costs.PMT was supported by a postdoctoral grant funded by Junta de Comunidades de Castilla-La Mancha and Fondo Social Europeo.Peer Reviewe
Tactical and operational management of wind energy systems with storage using a probabilistic forecast of the energy resource
The storage of energy facilitates the management of renewable energy systems by reducing the mismatch between the supplied energy and the forecasted production due to forecasting errors. The storage increases the reliability of the renewable energy system and enables participation in the electricity market by committing to the sale of electricity for the following day. Nevertheless, the inclusion of the energy storage capacity requires the development of new management policies. In this paper, we propose a management strategy for a renewable energy system with storage capacity that integrates tactical and operational decisions in a single mathematical model that makes use of an updated probabilistic wind speed forecast. Management policies are obtained by solving a sequence of rolling-horizon stochastic optimization problems whose formulation is inspired by the Stochastic Approximation Average technique. The management policies are illustrated by their application to wind-farms using hydrogen as the energy storage medium
Ca impurity in small mixed He-He clusters
The structure of small mixed helium clusters doped with one calcium atom has
been determined within the diffusion Monte Carlo framework. The results show
that the calcium atom sits at the He-He interface. This is in agreement
with previous studies, both experimental and theoretical, performed for large
clusters. A comparison between the results obtained for the largest cluster we
have considered for each isotope shows a clear tendency of the Ca atom to
reside in a deep dimple at the surface of the cluster for He clusters, and
to become fully solvated for He clusters. We have calculated the absorption
spectrum of Ca around the transition and have found that
it is blue-shifted from that of the free-atom transition by an amount that
depends on the size and composition of the cluster.Comment: 24 pages, 11 figures. Accepted on Journal of Chemical Physic
Dynamics of the Globular Cluster NGC 362
In this paper we have examined the internal dynamics of the globular cluster
NGC 362. A V band surface brightness profile (SBP) was constructed from CCD
images, and, after it was determined that the cluster is not post
core-collapse, fit with single- and multi-mass King-Michie (KM) models. The
total cluster luminosity is 1.70 L\sol. A total of
285 stellar spectra were obtained of 215 stars for radial velocity
determinations. Four stars showed strong evidence for radial velocity
variations and are probably members of binary systems. The true cluster binary
fraction was determined from simulations to be 0.15 for circular orbits or 0.27
for orbits with a distribution function ( is eccentricity). This
relatively high binary detection frequency may indicate that NGC 362 is
overabundant in binaries compared to other clusters. The 208 remaining stars
showed no sign of rotation. The best agreement with both the kinematic data and
the SBP were for shallow mass functions and intermediate
amounts of anisotropy in the velocity dispersion tensor. The cluster mass is M
= M\sol\ for a global mass-to-light ratio of M/L =
1.5 - 2.0 M\sol/L\sol. This low value for is in disagreement with the
correlation between and the height above the Galactic disk seen for a
sample of other clusters. The results are also different from the sharp turn-up
in the low mass end of mass functions derived from some deep luminosity
functions of three other globular clusters.Comment: 31 pages plus five figure
µG2-ELM: an upgraded implementation of µ G-ELM
µG-ELM is a multiobjective evolutionary algorithm which looks for the best (in terms of the MSE) and most compact artificial neural network using the ELM methodology. In this work we present the µG2-ELM, an upgraded version of µG-ELM, previously presented by the authors. The upgrading is based on three key elements: a specifically designed approach for the initialization of the weights of the initial artificial neural networks, the introduction of a re-sowing process when selecting the population to be evolved and a change of the process used to modify the weights of the artificial neural networks. To test our proposal we consider several state-of-the-art Extreme Learning Machine (ELM) algorithms and we confront them using a wide and well-known set of continuous, regression and classification problems. From the conducted experiments it is proved that the µG2-ELM shows a better general performance than the previous version and also than other competitors. Therefore, we can guess that the combination of evolutionary algorithms with the ELM methodology is a promising subject of study since both together allow for the design of better training algorithms for artificial neural networks
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