5,223 research outputs found
A new model for gravitational potential perturbations in disks of spiral galaxies. An application to our Galaxy
We propose a new, more realistic, description of the perturbed gravitational
potential of spiral galaxies, with spiral arms having Gaussian-shaped groove
profiles. We investigate the stable stellar orbits in galactic disks, using the
new perturbed potential. The influence of the bulge mass on the stellar orbits
in the inner regions of a disk is also investigated. The new description offers
the advantage of easy control of the parameters of the Gaussian profile of its
potential. We find a range of values for the perturbation amplitude from 400 to
800 km^2 s^{-2} kpc^{-1} which implies a maximum ratio of the tangential force
to the axisymmetric force between 3% and 6%, approximately. Good
self-consistency of arm shapes is obtained between the Inner Lindblad resonance
(ILR) and the 4:1 resonance. Near the 4:1 resonance the response density starts
to deviate from the imposed logarithmic spiral form. This creates bifurcations
that appear as short arms. Therefore the deviation from a perfect logarithmic
spiral in galaxies can be understood as a natural effect of the 4:1 resonance.
Beyond the 4:1 resonance we find closed orbits which have similarities with the
arms observed in our Galaxy. In regions near the center, in the presence of a
massive bulge, elongated stellar orbits appear naturally, without imposing any
bar-shaped potential, but only extending the spiral perturbation a little
inward of the ILR. This suggests that a bar is formed with a half-size around 3
kpc by a mechanism similar to that of the spiral arms. The potential energy
perturbation that we adopted represents an important step in the direction of
self-consistency, compared to previous sine function descriptions of the
potential. Our model produces a realistic description of the spiral structure,
able to explain several details that were not yet understood.Comment: 12 pag., 11 fig. Accepted for publication in A&A, 2012 December 1
Symplectic Quantization of Open Strings and Noncommutativity in Branes
We show how to translate boundary conditions into constraints in the
symplectic quantization method by an appropriate choice of generalized
variables. This way the symplectic quantization of an open string attached to a
brane in the presence of an antisymmetric background field reproduces the non
commutativity of the brane coordinates.Comment: We included a comparison with previous results obtained from Dirac
quantization, emphasizing the fact that in the symplectic case the boundary
conditions, that lead to the non commutativity, show up from the direct
application of the standard method. Version to appear in Phys. Rev.
Forrageamento de operárias de mandaçaia (Melipona mandacaia): 1. Número de viagens.
Suplemento. Edição dos Resumos do II Seminário Brasileiro de Própolis e Pólen; V Congresso Baiano de Apicultura e Meliponicultura; VII Seminário de Própolis do Nordeste; II Feira da Cadeia Produtiva da Apicultura e Meliponicultura Ilhéus, maio, 2013
Avaliação do peso de ninhos de mandaçaia (Melipona mandacaia) usando dois modelos de colmeias em área de Caatinga, em Petrolina (PE).
A utilização de caixa racional adequada à espécie de abelha sem ferrão facilita o manejo e pode incrementar a produção. No polo Petrolina (PE) ? Juazeiro (BA) a abelha mais criada é a mandaçaia (Melipona mandacaia) e o modelo de colmeia mais usado é o modelo baiano alongado. Entretanto, em outros locais, o modelo preferido é o baiano quadrado. Portanto, o objetivo deste trabalho foi verificar qual formato de caixa é mais eficiente para a produção de mel na região. Foram usados os dois modelos baianos: quadrado e alongado, onde foram colocados ninhos de mandaçaia com caracterÃsticas semelhantes. As caixas foram pesadas a cada 15 ou 30 dias, de 05/2012 a 07/2013. Os resultados mostraram que houve maior variação de peso nas caixas alongadas que nas quadradas. Ao se comparar as perdas de peso de cada modelo, o alongado apresentou maior queda (~23%) quando comparado ao quadrado (~4%) (P0,05, Chi-quadrado). Estes resultados ainda não permitem afirmar que o modelo quadrado seja o mais adequado, pois deve-se levar em consideração que o último ano foi extremamente seco o que causou a morte de muitas colônias, que embora tenham sido substituÃdas, podem ter afetado a análise geral do experimento. Além disso, seria relevante comparar ambos os modelos em boas condições de produção
Mtss1 promotes cell-cell junction assembly and stability through the small GTPase Rac1
Cell-cell junctions are an integral part of epithelia and are often disrupted in cancer cells during epithelial-to-mesenchymal transition (EMT), which is a main driver of metastatic spread. We show here that Metastasis suppressor-1 (Mtss1; Missing in Metastasis, MIM), a member of the IMD-family of proteins, inhibits cell-cell junction disassembly in wound healing or HGF-induced scatter assays by enhancing cell-cell junction strength. Mtss1 not only makes cells more resistant to cell-cell junction disassembly, but also accelerates the kinetics of adherens junction assembly. Mtss1 drives enhanced junction formation specifically by elevating Rac-GTP. Lastly, we show that Mtss1 depletion reduces recruitment of F-actin at cell-cell junctions. We thus propose that Mtss1 promotes Rac1 activation and actin recruitment driving junction maintenance. We suggest that the observed loss of Mtss1 in cancers may compromise junction stability and thus promote EMT and metastasis
Factors controlling spatio-temporal variation in carbon dioxide efflux from surface litter, roots, and soil organic matter at four rain forest sites in the eastern Amazon
[1] This study explored biotic and abiotic causes for spatio-temporal variation in soil respiration from surface litter, roots, and soil organic matter over one year at four rain forest sites with different vegetation structures and soil types in the eastern Amazon, Brazil. Estimated mean annual soil respiration varied between 13-17 t C ha(-1) yr(-1), which was partitioned into 0-2 t C ha(-1) yr(-1) from litter, 6-9 t C ha(-1) yr(-1) from roots, and 5-6 t C ha(-1) yr(-1) from soil organic matter. Litter contribution showed no clear seasonal change, though experimental precipitation exclusion over a one-hectare area was associated with a ten-fold reduction in litter respiration relative to unmodified sites. The estimated mean contribution of soil organic matter respiration fell from 49% during the wet season to 32% in the dry season, while root respiration contribution increased from 42% in the wet season to 61% during the dry season. Spatial variation in respiration from soil, litter, roots, and soil organic matter was not explained by volumetric soil moisture or temperature. Instead, spatial heterogeneity in litter and root mass accounted for 44% of observed spatial variation in soil respiration (p < 0.001). In particular, variation in litter respiration per unit mass and root mass accounted for much of the observed variation in respiration from litter and roots, respectively, and hence total soil respiration. This information about patterns of, and underlying controls on, respiration from different soil components should assist attempts to accurately model soil carbon dioxide fluxes over space and time
A Superspace Formulation for the Master Equation
It is shown that the quantum master equation of the Field Antifield
quantization method at one loop order can be translated into the requirement of
a superfield structure for the action. The Pauli Villars regularization is
implemented in this BRST superspace and the case of anomalous gauge theories is
investigated. The quantum action, including Wess Zumino terms, shows up as one
of the components of a superfield that includes the BRST anomalies in the other
component. The example of W2 quantum gravity is also discussed.Comment: The constrained nature of standard BRST superfields and the
importance of using Alfaro and Damgaard's collective fields in the superspace
approach to avoid undefined superfield derivatives was emphasized. To appear
in Phys. Rev. D. Latex file, 20 page
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