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

A Laplace transform method for energy multigroup hybrid discrete ordinates slab lattice calculations

In typical lattice cells where a highly absorbing, small fuel element is embedded in the moderator, a large weakly absorbing medium, high-order transport methods become unnecessary. In this work we describe a hybrid discrete ordinates (SN) method for energy multigroup slab lattice calculations. This hybrid SN method combines the convenience of a low-order SN method in the moderator with a high-order SN method in the fuel. The idea is based on the fact that in weakly absorbing media whose physical size is several neutron mean free paths in extent, even the S2 method (P1 approximation), leads to an accurate result. We use special fuel-moderator interface conditions and the Laplace transform (LTSN) analytical numerical method to calculate the two-energy group neutron flux distributions and the thermal disadvantage factor. We present numerical results for a range of typical model problems

A Laplace transform method for energy multigroup hybrid discrete ordinates slab lattice calculations

In typical lattice cells where a highly absorbing, small fuel element is embedded in the moderator, a large weakly absorbing medium, high-order transport methods become unnecessary. In this work we describe a hybrid discrete ordinates (SN) method for energy multigroup slab lattice calculations. This hybrid SN method combines the convenience of a low-order SN method in the moderator with a high-order SN method in the fuel. The idea is based on the fact that in weakly absorbing media whose physical size is several neutron mean free paths in extent, even the S2 method (P1 approximation), leads to an accurate result. We use special fuel-moderator interface conditions and the Laplace transform (LTSN) analytical numerical method to calculate the two-energy group neutron flux distributions and the thermal disadvantage factor. We present numerical results for a range of typical model problems

An integrated approach for analysing and assessing the performance of virtual learning groups

Collaborative distance learning involves a variety of elements and factors that have to be considered and measured in order to analyse and assess group and individual performance more effectively and objectively. This paper presents an approach that integrates qualitative, social network analysis (SNA) and quantitative techniques for evaluating online collaborative learning interactions. Integration of various different data sources, tools and techniques provides a more complete and robust framework for group modelling and guarantees a more efficient evaluation of group effectiveness and individual competence. Our research relies on the analysis of a real, long-term, complex collaborative experience, which is initially evaluated in terms of principled criteria and a basic qualitative process. At the end of the experience, the coded student interactions are further analysed through the SNA technique to assess participatory aspects, identify the most effective groups and the most prominent actors. Finally, the approach is contrasted and completed through a statistical technique which sheds more light on the results obtained that far. The proposal draws a well-founded line toward the development of a principled framework for the monitoring and analysis of group interaction and group scaffolding which can be considered a major issue towards the actual application of the CSCL proposals to real classrooms.Peer ReviewedPostprint (author's final draft

Análise comparativa entre dados TRMM e SPOT-VGT para o território brasileiro.

O presente trabalho teve por objetivo fazer uma análise comparativa entre dados de precipitação mensal obtidos pelo satélite TRMM e dados de índice de vegetação do satélite SPOT-VGT a fim de identificar correspondências entre as imagens. Foram utilizados dados de IVP (índice de vegetação padronizado) com 1 km de resolução espacial e resolução temporal de dez dias, obtidos a partir do sistema operado pela Embrapa Monitoramento por Satélite, e dados de precipitação mensal com resolução espacial de aproximadamente 25 km no período de janeiro de 1998 a março de 2012, do satélite TRMM, produto 3B43. As imagens apresentaram relações mais fortes quando referentes a períodos de estiagem. A resposta do IVP foi condizente quando baixos índices de precipitação foram identificados, porém onde os índices de precipitação foram altos, muitas vezes o IVP foi subestimado. Este resultado foi obtido pois a maior ocorrência de nuvens gera contaminações nas imagens SPOT e prejudica a estimativa dos índices de vegetação. Outro fator que dificultou as comparações foram as diferenças na resolução espacial e temporal entre ambos produtos

Electrodialysis for concentrating cobalt, chromium, manganese, and magnesium from a synthetic solution based on a nickel laterite processing route

[EN] Due to environmental and human health concerns, the need for cleaner techniques able to extract and recover metals from mining process solutions has been increasing. In this work, the use of electrodialysis for recovering cobalt, magnesium, manganese, and chromium ions from an acid multicomponent solution generated in the nickel laterite processing was evaluated. Values of percent extraction above 98% were obtained for Co2+, Mn2+, and Mg(2+ )ions. For Cr3+, the greatest percent extraction obtained was 83%. The results of percent concentration of the species showed the same trend: for Cr3+ ions, it was significantly lower than the others. Such difference in the transport of the metals through the membranes may have occurred due to the lower molar concentration, lower diffusion coefficient, and greater Stokes radius of chromium ions. Thus, the transfer of Cr3+ was hindered by the presence of other cationic species. This was also evidenced by the results of current efficiency and energy consumption associated with each species in solution. Lastly, the results of solution pH throughout the experiments and the final condition of the membranes, which were analyzed by SEM/EDS, indicated that the water dissociation phenomenon occurred at their surfaces.The authors gratefully acknowledge the financial support given by funding agencies CNPq (Process 141346/2016-7; 171241/2017-7; 160320/2019-4) and FAPESP (Process 2012/51871-9) . This study was financed in part by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior-Brasil (CAPES) -Finance Code 001 (Processes 88881.190502/2018-01 and 88887.362657/2019-00) .Feijoo, GC.; Santana-Barros, K.; Scarazzato, T.; Espinosa, D. (2021). Electrodialysis for concentrating cobalt, chromium, manganese, and magnesium from a synthetic solution based on a nickel laterite processing route. Separation and Purification Technology. 275. https://doi.org/10.1016/j.seppur.2021.11919227

Perceptions of secondary school teachers towards school expulsion of pregnant adolescents at Igunga District, Tanzania—a qualitative study

Background:In Tanzania, adolescent pregnancy results in a denial of the girl’s fundamental right to education. School expulsion of pregnant adolescents is a common practice exercised by teachers in this country for decades. This study aimed to explore the perceptions and feelings of secondary school teachers towards this practice. Methods: The study used a qualitative approach comprising focus groups and individual semistructured interviews with a purposive sample of sixteen teachers and nine headmasters (n=25) from nine secondary schools of the rural Igunga district, in Tabora region, Tanzania. Data was collected in March to June 2017 and submitted to thematic analysis. Results: Though most participants were not satisfied with the practice, their role on the expulsion of pregnant students was perceived as mandatory by law and regulations. Main argument in favor was its deterrent effect, yet the review of schools’ registries did not sustain that perception. Stigma and fear of contamination added a relevant contribution to its implementation. Conflicting feelings among teachers were also disclosed. Conclusion: This study was of most value to understand current perceptions and feelings of those who exercise the practice of banning pregnant adolescents from school in Tanzania, while having identified some of the cultural and social believes acting as influential factors in its pervasiveness. International and national human rights organizations should increase their efforts and campaigns in order to strength social awareness of the benefit of females’ education to society as a whole and of adopting policies and practices in support of their equal right to education.info:eu-repo/semantics/publishedVersio