4,474 research outputs found
Wormholes in String Theory
A wormhole is constructed by cutting and joining two spacetimes satisfying
the low energy string equations with a dilaton field. In spacetimes described
by the "string metric" the dilaton energy-momentum tensor need not satisfy the
weak or dominant energy conditions. In the cases considered here the dilaton
field violates these energy conditions and is the source of the exotic matter
required to maintain the wormhole. There is also a surface stress-energy, that
must be produced by additional matter, where the spacetimes are joined. It is
shown that wormholes can be constructed for which this additional matter
satisfies the weak and dominant energy conditions, so that it could be a form
of "normal" matter. Charged dilaton wormholes with a coupling between the
dilaton and the electromagnetic field that is more general than in string
theory are also briefly discussed.Comment: 9 pages, LaTex, submitted to Phys. Rev.
Conformally dressed black hole in 2+1 dimensions
A three dimensional black hole solution of Einstein equations with negative
cosmological constant coupled to a conformal scalar field is given. The
solution is static, circularly symmetric, asymptotically anti-de Sitter and
nonperturbative in the conformal field. The curvature tensor is singular at the
origin while the scalar field is regular everywhere. The condition that the
Euclidean geometry be regular at the horizon fixes the temperature to be
. Using the Hamiltonian formulation including
boundary terms of the Euclidean action, the entropy is found to be
of the standard value (), and in agreement with
the first law of thermodynamics.Comment: LaTeX ,RevTeX, 13pages, no figure
Nonadiabatic charged spherical gravitational collapse
We present a complete set of the equations and matching conditions required
for the description of physically meaningful charged, dissipative, spherically
symmetric gravitational collapse with shear. Dissipation is described with both
free-streaming and diffusion approximations. The effects of viscosity are also
taken into account. The roles of different terms in the dynamical equation are
analyzed in detail. The dynamical equation is coupled to a causal transport
equation in the context of Israel-Stewart theory. The decrease of the inertial
mass density of the fluid, by a factor which depends on its internal
thermodynamic state, is reobtained, with the viscosity terms included. In
accordance with the equivalence principle, the same decrease factor is obtained
for the gravitational force term. The effect of the electric charge on the
relation between the Weyl tensor and the inhomogeneity of energy density is
discussed.Comment: 23 pages, Latex. To appear in Phys. Rev. D. Some references correcte
Optimal Sizing of a Photovoltaic-Hydrogen Power System for HALE Aircraft by means of Particle Swarm Optimization
Over the last decade there has been a growing interest in the research of feasibility to use high altitude long endurance (HALE) aircrafts in order to provide mobile communications. The use of HALEs for telecommunication networks has the potential to deliver a wide range of communication services (from high-quality voice to high-definition videos, as well as high-data-rate wireless channels) cost effectively. One of the main challenges of this technology is to design its power supply system, which must provide the enough energy for long time flights in a reliable way. In this paper a photovoltaic/hydrogen system is proposed as power system for a HALE aircraft due its high power density characteristic. In order to obtain the optimal sizing for photovoltaic/hydrogen system a particle swarm optimizer (PSO) is used. As a case study, theoretical design of the photovoltaic/hydrogen power system for three different HALE aircrafts located at 18° latitude is presented. At this latitude, the range of solar radiation intensity was from 310 to 450 Wh/sq·m/day. The results obtained show that the photovoltaic/hydrogen systems calculated by PSO can operate during one year with efficacies ranging between 45.82% and 47.81%. The obtained sizing result ensures that the photovoltaic/hydrogen system supplies adequate energy for HALE aircrafts
On the static solutions in gravity with massive scalar field in three dimensions
We investigate circularly symmetric static solutions in three-dimensional
gravity with a minimally coupled massive scalar field. We integrate numerically
the field equations assuming asymptotic flatness, where black holes do not
exist and a naked singularity is present. We also give a brief review on the
massless cases with cosmological constant.Comment: 11 pages, LaTeX, 1 Postscript figure. Some changes were don
Sequential star formation at the periphery of the HII regions Sh 217 and Sh 219
The HII regions Sh 217 and Sh 219 are textbook examples of a Stromgren sphere
surrounded by an annular photodissociation region (PDR). The annular PDR is
observed in both the 21 cm atomic hydrogen emission and the dust (PAH) emission
near 8 micron (MSX Survey). An ultracompact radio continuum source is observed
in the direction of the annular PDR, in both Sh 217 and Sh 219. JHKobservations
show the presence of highly reddened stellar clusters (AV ~ 20 mag) in the
directions of these radio sources. These clusters are also IRAS sources, of
luminosities 22700 Lo for Sh 217 and 5900 Lo for Sh 219. Each cluster contains
at least one luminous star with an IR colour excess; the one in the Sh 219
cluster shows H-alpha emission. The cluster associated with Sh 217 is almost
spherical and contains luminous objects at its centre. The cluster associated
with Sh 219 is elongated along the ionization front of this HII region. We
argue that these are `second-generation clusters', which means that the
physical conditions present in the PDRs, close to the ionization fronts, have
favoured the formation of clusters containing massive objects. We discuss the
physical mechanisms which may be at the origin of the observed triggered star
formation.Comment: 12 pages, Late
Psicologia pediátrica: Do comportamento a saúde infantil
Os autores começam por caracterizar de forma sucinta
a psicologia da saúde, realçando o papel do comportamento
na saúde e na doença. Em seguida, são
apresentados aspectos característicos da intervenção
psicológica no âmbito da saúde da criança, denominada
desde há alguns anos como psicologia pediátrica.
Esta é definida e caracterizada como uma prática psicológica
nos serviços médicos de crianças, cujos
objectivos são contribuir para a promoção da saúde e
prevenção da doença na criança, colaborar no tratamento,
recuperação e apoio a criança doente, apoio a
família, e participar na investigação aplicada dos
factores psicossociais associados a saúde e a doença.
E descrita a orientação predominante e objectivos
da intervenção psicológica realizada pelos autores, no
serviço pediátrico em que estão integrados desde há
alguns anos.
Finalmente é apresentada uma referência crítica e
breve a legislação, recentemente publicada no nosso
país, que enquadra os psicólogos que trabalham na
saúde na carreira dos técnicos superiores de saúde,
regulamentando o papel e funções, e a formação destes.The authors caracterize the pediatric psychology
witch is defined as a psychology applied to children’s
medical context, and conceptualized as a health psychology
and different from the clinic psychology.
Some aspects of the psychological intervention in
this context are discribed such as prevention and
promotion of children’s health, intervention in
children’s illness and family, and research directions
in the field.
Some relevant aspects of the psychological intervention
in a portuguese pediatric service are discribed.
Recente portuguese legislation about health psychologist’s
role definition and trainning is analised.info:eu-repo/semantics/publishedVersio
WormBase: a comprehensive resource for nematode research
WormBase (http://www.wormbase.org) is a central data repository for nematode biology. Initially created as a service to the Caenorhabditis elegans research field, WormBase has evolved into a powerful research tool in its own right. In the past 2 years, we expanded WormBase to include the complete genomic sequence, gene predictions and orthology assignments from a range of related nematodes. This comparative data enrich the C. elegans data with improved gene predictions and a better understanding of gene function. In turn, they bring the wealth of experimental knowledge of C. elegans to other systems of medical and agricultural importance. Here, we describe new species and data types now available at WormBase. In addition, we detail enhancements to our curatorial pipeline and website infrastructure to accommodate new genomes and an extensive user base
Controlling molecular tautomerism through supramolecular selectivity
We have isolated the stable as well as the metastable tautomers of 1-deazapurine in the solid state by exploiting principles of supramolecular selectivity in the context of cocrystal design
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