10,446 research outputs found
Jacobi Structures in
The most general Jacobi brackets in are constructed after
solving the equations imposed by the Jacobi identity. Two classes of Jacobi
brackets were identified, according to the rank of the Jacobi structures. The
associated Hamiltonian vector fields are also constructed
The X-ray luminosity function of galaxies in the Coma cluster
The XMM-Newton survey of the Coma cluster of galaxies covers an area of 1.86
square degrees with a mosaic of 16 pointings and has a total useful integration
time of 400 ksec. Detected X-ray sources with extent less than 10" were
correlated with cataloged galaxies in the Coma cluster region. The redshift
information, which is abundant in this region of the sky, allowed us to
separate cluster members from background and foreground galaxies. For the
background sources, we recover a typical LogN-LogS in the flux range 1.e-15 -
1.e-13 ergs/s/cm^2 in the 0.5-2.0 keV band. The X-ray emission from the cluster
galaxies exhibits X-ray colors typical of thermal emission. The luminosities of
Coma galaxies lie in the 1.e39-1.e41 ergs/s interval in the 0.5-2.0 keV band.
The luminosity function of Coma galaxies reveals that their X-ray activity is
suppressed with respect to the field by a factor of 5.6, indicating a lower
level of X-ray emission for a given stellar mass.Comment: 16 pages, 2004 A&A in pres
Groupoid symmetry and constraints in general relativity
When the vacuum Einstein equations are cast in the form of hamiltonian
evolution equations, the initial data lie in the cotangent bundle of the
manifold M\Sigma\ of riemannian metrics on a Cauchy hypersurface \Sigma. As in
every lagrangian field theory with symmetries, the initial data must satisfy
constraints. But, unlike those of gauge theories, the constraints of general
relativity do not arise as momenta of any hamiltonian group action. In this
paper, we show that the bracket relations among the constraints of general
relativity are identical to the bracket relations in the Lie algebroid of a
groupoid consisting of diffeomorphisms between space-like hypersurfaces in
spacetimes. A direct connection is still missing between the constraints
themselves, whose definition is closely related to the Einstein equations, and
our groupoid, in which the Einstein equations play no role at all. We discuss
some of the difficulties involved in making such a connection.Comment: 22 pages, major revisio
Hybrid gamma Doradus/delta Scuti Stars: Comparison Between Observations and Theory
Gamma Doradus are F-type stars pulsating with high order g-modes. Their
instability strip (IS) overlaps the red edge of the delta Scuti one. This
observation has led to search for objects in this region of the HR diagram
showing p and g-modes simultaneously. The existence of such hybrid pulsators
has recently been confirmed (Handler 2009) and the number of candidates is
increasing (Matthews 2007). From a theoretical point of view, non-adiabatic
computations including a time-dependent treatment of convection (TDC) predict
the existence of gamma Dor/delta Sct hybrid pulsators (Dupret et al. 2004;
Grigahcene et al. 2006). Our aim is to confront the properties of the observed
hybrid candidates with the theoretical predictions from non-adiabatic
computations of non-radial pulsations including the convection-pulsation
interaction.Comment: 3 pages, 3 figures, Poster at "Stellar Pulsation: challenges for
theory and observation", Santa Fe, June 200
Magnetic Field scaling of Relaxation curves in Small Particle Systems
We study the effects of the magnetic field on the relaxation of the
magnetization of small monodomain non-interacting particles with random
orientations and distribution of anisotropy constants. Starting from a master
equation, we build up an expression for the time dependence of the
magnetization which takes into account thermal activation only over barriers
separating energy minima, which, in our model, can be computed exactly from
analytical expressions. Numerical calculations of the relaxation curves for
different distribution widths, and under different magnetic fields H and
temperatures T, have been performed. We show how a \svar scaling of the
curves, at different T and for a given H, can be carried out after proper
normalization of the data to the equilibrium magnetization. The resulting
master curves are shown to be closely related to what we call effective energy
barrier distributions, which, in our model, can be computed exactly from
analytical expressions. The concept of effective distribution serves us as a
basis for finding a scaling variable to scale relaxation curves at different H
and a given T, thus showing that the field dependence of energy barriers can be
also extracted from relaxation measurements.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
Entropy and equilibrium state of free market models
Many recent models of trade dynamics use the simple idea of wealth exchanges
among economic agents in order to obtain a stable or equilibrium distribution
of wealth among the agents. In particular, a plain analogy compares the wealth
in a society with the energy in a physical system, and the trade between agents
to the energy exchange between molecules during collisions. In physical
systems, the energy exchange among molecules leads to a state of equipartition
of the energy and to an equilibrium situation where the entropy is a maximum.
On the other hand, in the majority of exchange models, the system converges to
a very unequal condensed state, where one or a few agents concentrate all the
wealth of the society while the wide majority of agents shares zero or almost
zero fraction of the wealth. So, in those economic systems a minimum entropy
state is attained. We propose here an analytical model where we investigate the
effects of a particular class of economic exchanges that minimize the entropy.
By solving the model we discuss the conditions that can drive the system to a
state of minimum entropy, as well as the mechanisms to recover a kind of
equipartition of wealth
Effect of band filling in the Kondo lattice: A mean-field approach
The usual Kondo-lattice, including an antiferromagnetic exchange interaction
between nearest-neighboring localized spins, is treated here in a mean-field
scheme that introduces two mean-field parameters: one associated with the local
Kondo effect, and the other related to the magnetic correlations between
localized spins. Phases with short-range magnetic correlations or coexistence
between those and the Kondo effect are obtained. By varying the number of
electrons in the conduction band, we notice that the Kondo effect tends to be
suppressed away from half filling, while magnetic correlations can survive if
the Heisenberg coupling is strong enough. An enhanced linear coefficient of the
specific heat is obtained at low temperatures in the metallic state.Comment: 7 pages, ReVTeX two-column, 7 figure
Physical activity trends in spanish adolescents
Este estudio se propone conocer la evolución de la práctica de actividad física moderada-vigorosa y examinar si la tendencia se mantiene en función de factores sociodemográficos, como el sexo y la edad, así como de factores socioeconómicos y su influencia según la comunidad autónoma. La muestra está compuesta de adolescentes españoles de 11 a 18 años de las ediciones 2002 y 2006 del estudio Health Behaviour in School-aged Children (HBSC). Los principales resultados muestran que, a pesar de un cierto aumento en el nivel de actividad física en los cuatro años que distan entre ambas ediciones
(especialmente en los adolescentes más pequeños), estos niveles se encuentran todavía lejos de las recomendaciones. Asimismo, los datos señalan como necesidades más urgentes promover un estilo de vida activo en los adolescentes de 13 años en adelante (especialmente en las chicas) y en los adolescentes de niveles socioeconómicos más bajos de determinadas comunidades autónomasThis study tries to find out the tendency of moderate-to-vigorous physical activity and to assess whether the trends are maintained with regard to socio-demographic factors such as sex and age, and also socio-economic factors and how they vary depending on the region. The sample comprises Spanish adolescents aged between 11 and 18 years, from the 2002 and 2006 surveys of the Health Behaviour in School-aged Children (HBSC) study. The main results reveal that, despite a certain increase in physical activity level in the four years span between both surveys (particularly in younger adolescents), these levels are still a long way from the recommendations. Likewise, data highlight, as immediate needs, the promotion of an active lifestyle among adolescents from 13 years onwards (particularly among the girls) and also among adolescents of lower socio-economic status in some regionsEsta investigación ha sido realizada gracias a los sucesivos convenios de colaboración firmados entre el Ministerio de Sanidad y Consumo y la Universidad de Sevilla para la ejecución del estudio HBSC (Health Behaviour in School-aged Children) en Españ
A review on additive manufacturing and materials for catalytic applications: Milestones, key concepts, advances and perspectives
Catalysis, a driving force of the chemical industry is increasingly being influenced by additive manufacturing. The link between them is based on the need to intensify catalytic processes in order to make them more efficient and sustainable. Additive manufacturing can satisfy such a need, generating devices with an advanced design, easy production, and great adaptation, in addition to their catalytic functionality. The exponential growth of examples reported on the application of 3D-printing in catalysis has led to the need to compile and analyse these cases and thus establish, through this review, the most in-depth analysis done to date. The manuscript includes a brief background of the history of additive manufacturing and the classification of the different printing techniques. Subsequently, it identifies the intensification of processes, among other aspects, as the key for understanding the union of additive manufacturing and catalysis. Then, it explores in detail how such a combination occurs, establishing the most comprehensive classification to date between the two large groups of printable devices with catalytic properties. Finally, a series of perspectives are proposed in which the most probable courses of new advances in this field of research are identified.O.H. Laguna agradece a la Universidad de Jaén por el apoyo el contrato postdoctoral a través de la ‘‘Acción 6 del Plan de Apoyo a la Investigación de la Universidad de Jaén (2017-2019).
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