1,124 research outputs found
Vacuum Polarization of a Charged Massless Scalar Field on Cosmic String Spacetime in the Presence of a Magnetic Field
In this paper we consider a charged massless scalar quantum field operator in
the spacetime of an idealized cosmic string, i.e., an infinitely long, straight
and static cosmic string, which presents a magnetic field confined in a
cylindrical tube of finite radius. Three distinct situations are taking into
account in this analysis: {\it{i)}} a homogeneous field inside the tube,
{\it{ii)}} a magnetic field proportional to and {\it{iii)}} a cylindrical
shell with -function. In these three cases the axis of the infinitely
long tube of radius coincides with the cosmic string. In order to study the
vacuum polarization effects outside the tube, we explicitly calculate the
Euclidean Green function associated with this system for the three above
situations, considering points in the region outside the tube.Comment: 26 pages, LaTex format, 3 figure
New Herbig Ae/Be stars confirmed via high-resolution optical spectroscopy
We present FEROS high-resolution (R~45000) optical spectroscopy of 34 Herbig
Ae/Be star candidates with previously unknown or poorly constrained spectral
types. Within the sample, 16 sources are positionally coincident with nearby
(d<250 pc) star-forming regions (SFRs). All the candidates have IR excess. We
determine the spectral type and luminosity class of the sources, derive their
radial and rotational velocities, and constrain their distances employing
spectroscopic parallaxes. We confirm 13 sources as Herbig Ae/Be stars and find
one classical T Tauri star. Three sources are emission line early-type giants
and may be Herbig Ae/Be stars. One source is a main-sequence A-type star.
Fourteen sources are post-main-sequence giant and supergiant stars. Two sources
are extreme emission-line stars. Most of the sources appear to be background
stars at distances over 700 pc. We show that high-resolution optical
spectroscopy is a crucial tool for distinguishing young stars from post-main
sequence stars in samples taken from emission-line star catalogs based on
low-resolution spectroscopy. Within the sample, 3 young stars (CD-38 4380, Hen
3-1145, and HD 145718) and one early-type luminosity class III giant with
emission lines (Hen 3-416) are at distances closer than 300 pc and are
positionally coincident with a nearby SFR. These 4 sources are likely to be
nearby young stars and are interesting for follow-up observations at
high-angular resolution. Furthermore, seven confirmed Herbig Ae/Be stars at
d>700 pc (Hen 2-80, Hen 3-1121 N&S, HD 313571, MWC 953, WRAY 15-1435, and Th
17-35) are inside or close (<5') to regions with extended 8 micron continuum
emission and in their 20' vicinity have astronomical sources characteristic of
SFRs. These 7 sources are likely to be members of SFRs. These regions are
attractive for future studies of their stellar content.Comment: 24 pages, 6 Figures, accepted by Astronomy and Astrophysics, in
press
Dilatonic current-carrying cosmic strings
We investigate the nature of ordinary cosmic vortices in some scalar-tensor
extensions of gravity. We find solutions for which the dilaton field condenses
inside the vortex core. These solutions can be interpreted as raising the
degeneracy between the eigenvalues of the effective stress-energy tensor,
namely the energy per unit length U and the tension T, by picking a privileged
spacelike or timelike coordinate direction; in the latter case, a phase
frequency threshold occurs that is similar to what is found in ordinary neutral
current-carrying cosmic strings. We find that the dilaton contribution for the
equation of state, once averaged along the string worldsheet, vanishes, leading
to an effective Nambu-Goto behavior of such a string network in cosmology, i.e.
on very large scales. It is found also that on small scales, the energy per
unit length and tension depend on the string internal coordinates in such a way
as to permit the existence of centrifugally supported equilibrium
configuration, also known as vortons, whose stability, depending on the very
short distance (unknown) physics, can lead to catastrophic consequences on the
evolution of the Universe.Comment: 10 pages, ReVTeX, 2 figures, minor typos corrected. This version to
appear in Phys. Rev.
Constraints on dark energy from the lookback time versus redshift test
We use lookback time versus redshift data from galaxy clusters (Capozziello
et al., 2004) and passively evolving galaxies (Simon et al., 2005), and apply a
bayesian prior on the total age of the Universe based on WMAP measurements, to
constrain dark energy cosmological model parameters. Current lookback time data
provide interesting and moderately restrictive constraints on cosmological
parameters. When used jointly with current baryon acoustic peak and Type Ia
supernovae apparent magnitude versus redshift data, lookback time data tighten
the constraints on parameters and favor slightly smaller values of the
nonrelativistic matter energy density.Comment: Physics Letters B in press. 15 pages, 6 figures, 1 table. Minor typos
fixe
Electromagnetic Casimir densities induced by a conducting cylindrical shell in the cosmic string spacetime
We investigate the renormalized vacuum expectation values of the field square
and the energy-momentum tensor for the electromagnetic field inside and outside
of a conducting cylindrical shell in the cosmic string spacetime. By using the
generalized Abel-Plana formula, the vacuum expectation values are presented in
the form of the sum of boundary-free and boundary-induced parts. The asymptotic
behavior of the vacuum expectation values of the field square, energy density
and stresses are investigated in various limiting cases.Comment: 14 pages, 2 figures, discussion and references added, accepted for
publication in Phys. Lett.
Reporting interventions in trials evaluating cognitive rehabilitation in people with Multiple Sclerosis: a systematic review
Objective: To determine the quantity and quality of description of cognitive rehabilitation for cognitive deficits in people with Multiple Sclerosis, using a variety of published checklists, and suggest ways of improving the reporting of these interventions.
Data sources: Ten electronic databases were searched, including MEDLINE, EMBASE, CINAHL and PsycINFO, from inception to May 2017. Grey literature databases, trials registers, reference lists and author citations were also searched.
Review methods: Papers were included if participants were people with multiple sclerosis aged 18 years and over, and if the effectiveness of cognitive rehabilitation in improving functional ability for memory, attention or executive dysfunction, with or without a control group, was being evaluated.
Results: Fifty-four studies were included in this review. The reporting of a number of key aspects of cognitive rehabilitation was poor. This was particularly in relation to content of interventions (reported completely in 26 of the 54 studies), intervention procedures (reported completely in 16 of the 54 studies), delivery mode (reported completely in 24 of the 54 studies) and intervention mechanism of action (reported completely in 21 of the 54 studies).
Conclusion: The quality of reporting of cognitive rehabilitation for memory, attention and executive function for multiple sclerosis, across a range of study designs, is poor. Existing reporting checklists do not adequately cover aspects relevant to cognitive rehabilitation, such as the approaches used to address cognitive deficits. Future checklists could consider these aspects we have identified in this review
Non-perturbative electron dynamics in crossed fields
Intense AC electric fields on semiconductor structures have been studied in
photon-assisted tunneling experiments with magnetic field applied either
parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here
the electron dynamics in a double quantum well when intense AC electric fields
F, and tilted magnetic fields are applied simultaneously. The problem is
treated non-perturbatively by a time-dependent Hamiltonian in the effective
mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the
quasi-energy spectra show two types of crossings: those related to different
Landau levels, and those associated to dynamic localization (DL), where the
electron is confined to one of the wells, despite the non-negligible tunneling
between wells. B_par couples parallel and in-plane motions producing
anti-crossings in the spectrum. However, since our approach is
non-perturbative, we are able to explore the entire frequency range. For high
frequencies, we reproduce the well known results of perfect DL given by zeroes
of a Bessel function. We find also that the system exhibits DL at the same
values of the field F, even as B_par non-zero, suggesting a hidden dynamical
symmetry in the system which we identify with different parity operations. The
return times for the electron at various values of field exhibit interesting
and complex behavior which is also studied in detail. We find that smaller
frequencies shifts the DL points to lower field F, and more importantly, yields
poorer localization by the field. We analyze the explicit time evolution of the
system, monitoring the elapsed time to return to a given well for each Landau
level, and find non-monotonic behavior for decreasing frequencies.Comment: REVTEX4 + 11 eps figs, submitted to Phys. Rev.
DIAGNÓSTICO E USO DE GEOPROCESSAMENTO PARA MANEJO DA ARBORIZAÇÃO URBANA DO BAIRRO CENTRO DA CIDADE DO RIO DE JANEIRO - RJ
Este estudo teve por objetivo caracterizar e avaliar parâmetros quali-quantitativos da arborização urbana do bairro Centro da cidade do Rio de Janeiro - RJ com auxílio de ferramentas de geoprocessamento. Foi utilizado o censo obtendo-se dados quali-quantitativos dos indivíduos arbóreos de ruas e praças. Foram levantados 3.874 indivíduos, distribuídos em 64 famílias e 65 espécies. As 10 espécies mais frequentes na arborização das ruas representam 83,1% dos indivíduos e as 10 espécies localizadas nas praças compreende 68,5%, com maior frequência nas ruas e praças de Licania tomentosa (Benth.) Fritsch (43,4% e 22%, nas ruas e praças respectivamente). Em ambos os casos, menos de 1% da população apresentou com a copa desequilibrada e raiz danificada concomitantemente; e a doença do cancro atinge quase 1,4% da população e 6,1% apresentam cavidade igual ou maior a 30% da circunferência a altura do peito. Os conflitos com a rede elétrica, pedestre e fachada tiveram a frequência de 2,2%, 4,9% e 20,3%, respectivamente, com a classe das árvores maiores que 9,4 m a mais representativa desses conflitos. Com as informações sobre as espécies arbóreas coletadas e com o auxílio das técnicas de geoprocessamento foi possível alocar espacialmente os indivíduos com conflitos e com injúrias, a fim de facilitar o manejo e o monitoramento da arborização urbana
Throughflow centrality is a global indicator of the functional importance of species in ecosystems
To better understand and manage complex systems like ecosystems it is
critical to know the relative contribution of system components to system
functioning. Ecologists and social scientists have described many ways that
individuals can be important; This paper makes two key contributions to this
research area. First, it shows that throughflow, the total energy-matter
entering or exiting a system component, is a global indicator of the relative
contribution of the component to the whole system activity. It is global
because it includes the direct and indirect exchanges among community members.
Further, throughflow is a special case of Hubbell status as defined in social
science. This recognition effectively joins the concepts, enabling ecologists
to use and build on the broader centrality research in network science. Second,
I characterize the distribution of throughflow in 45 empirically-based trophic
ecosystem models. Consistent with expectations, this analysis shows that a
small fraction of the system components are responsible for the majority of the
system activity. In 73% of the ecosystem models, 20% or less of the nodes
generate 80% or more of the total system throughflow. Four or fewer dominant
nodes are required to account for 50% of the total system activity. 121 of the
130 dominant nodes in the 45 ecosystem models could be classified as primary
producers, dead organic matter, or bacteria. Thus, throughflow centrality
indicates the rank power of the ecosystems components and shows the power
concentration in the primary production and decomposition cycle. Although these
results are specific to ecosystems, these techniques build on flow analysis
based on economic input-output analysis. Therefore these results should be
useful for ecosystem ecology, industrial ecology, the study of urban
metabolism, as well as other domains using input-output analysis.Comment: 7 figures, 2 table
Gravitational field around a time-like current-carrying screwed cosmic string in scalar-tensor theories
In this paper we obtain the space-time generated by a time-like
current-carrying superconducting screwed cosmic string(TCSCS). This
gravitational field is obtained in a modified scalar-tensor theory in the sense
that torsion is taken into account. We show that this solution is comptible
with a torsion field generated by the scalar field . The analysis of
gravitational effects of a TCSCS shows up that the torsion effects that appear
in the physical frame of Jordan-Fierz can be described in a geometric form
given by contorsion term plus a symmetric part which contains the scalar
gradient. As an important application of this solution, we consider the linear
perturbation method developed by Zel'dovich, investigate the accretion of cold
dark matter due to the formation of wakes when a TCSCS moves with speed and
discuss the role played by torsion. Our results are compared with those
obtained for cosmic strings in the framework of scalar-tensor theories without
taking torsion into account.Comment: 21 pages, no figures, Revised Version, presented at the "XXIV-
Encontro Nacional de Fisica de Particulas e Campos ", Caxambu, MG, Brazil, to
appear in Phys. Rev.
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