41 resultados para stars : chemically peculiar
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Trabalho de Projecto apresentado para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Antropologia dos Direitos Humanos e Movimentos Sociais.
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This work presents the results of the experimental study of proton induced nuclear reactions in lithium, namely the 7Li(p,α) 4He, 6Li(p,α) 3He and 7Li(p,p)7Li reactions. The amount of 7Li and 6Li identified as primordial and observed in very old stars of the Milky Way galactic halo strongly deviates from the predictions of primordial nucleosynthesis and stellar evolution models which depend, among other factors, on the cross sections of reactions like 7Li(p,α) 4He and 6Li(p,α) 3He. These discrepancies have triggered a large amount of research in the fields of stellar evolution, cosmology, pre-galactic evolution and low energy nuclear reactions. Focusing on nuclear reactions, this work has measured the 7Li(p,α) 4He and 6Li(p,α) 3He reactions cross sections (expressed in terms of the astrophysical S -factor) with higher accuracy, and the electron screening effects in these reactions for different environments (insulators and metallic targets). The 7Li(p,α) 4He angular distributions were also measured. These measurementstook place in two laboratory facilities, in the framework of the LUNA (Laboratory for Undergroud Nuclear Astrophysics) international collaboration, namely the Laboratorio ´ de Feixe de Ioes ˜ in ITN (Instituto Tecnologico ´ e Nuclear) Sacavem, ´ Portugal, and the Dynamitron-TandemLaboratorium in Ruhr-Universitat¨ Bochum, Germany. The ITN target chamber was modified to measure these nuclear reactions, with the design and construction of new components, the addition of one turbomolecular pump and a cold finger. The 7Li(p,α) 4He and 6Li(p,α) 3He reactions were measured concurrently with seven and four targets, respectively. These targets were produced in order to obtain adequate and stable lithium depth profiles. In metallic environments, the measured electron screening potential energies are much higher than the predictions of atomic-physics models. The Debye screening model applied to the metallic conduction electrons is able to explain these high values. It is a simple model, but also very robust. Concerning primordial nucleosynthesis and stellar evolution models, these results are very important as they show that laboratory measurements are well controlled, and the model inputs from these cross sections are therefore correct. In this work the 7Li(p,p)7Li differential cross section was also measured, which is useful to describe the 7Li(p,α) 4He entrance channel.
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FEMS Microbiology Ecology, Vol. 57, nº 1
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The igneous rocks of Benavila area consist predominantly of granodiorite and monzonitic granite. Chemically they exhibit typical calco-alkaline differentiation trends and appear to represent a co-magmatic series on the basis of major - and trace - element geochemistry. There is a lot of somewhat more basic igneous enclaves.
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The Moncorvo Ordovician ironstones in northeastern Portugal consist of iron ore sedimentary horizons frequently interbanded with psamites and quartzites. Ore reserves may probably exceed 1 000 million tonnes and this makes Moncorvo the largest iron ore deposit in the European Union. Compact poorly banded massive layers may exceed 90 meters in thickness which is quite an extraordinary feature for a Phanerozoic deposit. If the thickness of Precambrian deposits may reach a few hundred meters, the thickness of Phanerozoic deposits never exceed a maximum of 15 meters generally forming a number of comparatively thin layers confined to a particular member of a sedimentary sequence. A detailed microscopic analysis of the ores revealed that initially a compact magnetite/quartzite layer, detrital in character (the magnetite occasionally showing chromite cores), was deposited by entrapment in near shore lagoons where rivers debouched, rather than in the open sea. This stage was followed by oscilating and transgressive shore lines which gave rise to breaks in sedimentation in combined river delta and shallow water marine environment where detrital material and fine iron oxide and clay suspensions were deposited in fluctuating environments. These events gave rise to layers of both magnetite (martite) and specularite intergrown with quartz, silicates and phosphates. Textural and mineralogical studies show that the deposits consist of ferruginous clastic sediments and are not chemically deposited cherts. Field, geological and palaeontological evidence also supports a detrital origin, the facies being typical of zones rich in oxygen and close to the feeding continent. The uncommon huge development of Moncorvo was due to the fact that the deposits occur in restricted basins on a continental platform were clastic sediments were predominantly deposited. Not only morphologically but also chemically the deposits are more similar to Precambrian iron formations than to Phanerozoic ironstones.
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Journal of Proteome Research (2006)5: 2720-2726
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Thesis dissertation presented to obtain a PhD degree in Biochemistry at Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa
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Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para a obtenção do grau de Mestre em Engenharia Química e Bioquímica
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Dissertação apresentada para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Filosofia Geral
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Dissertação apresentada para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Estudos Portugueses (Área de Especialização em Estudos Literários)
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Dissertação apresentada para cumprimento dos requisitos necessários à obtenção do grau de Mestre em Ciência Política e Relações Internacionais, especialização em Relações Internacionais
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A teoria do conhecimento desde há muito que chama a atenção para dois mundos distintos da observação dos fenómenos- quer eles se refiram a objectos ou a documentos escritos. Isto significa que antes de se filosofar sobre qualquer objecto ele deve ser escrupulosamente examinado e descrito. Assim, é necessário observar com rigor e descrever com exact idão aquilo a que chamamos conhecimento, "esse peculiar fenómeno da consciência" (Hessen, 1987:25). Fazêrno-lo procurando apreender os traços essenciais do objecto, por meio de uma auto-reflexão sobre o que vimos quando falamos de conhecimento. Este método fenomenológico é distinto do psico lógico. Enquanto o último investiga os processos psíquicos concretos no seu curso regular e a conexão com outros processos, o primeiro aspira a apreender a essê ncia geral no fenómeno concreto (/bidem:26). Desta forma no conhecimento encontram-se frente a frente duas entidades: o Objecto e o Sujeito. O conhecimento apresenta-se. assim. como uma relação entre estes dois elementos, que permanecem eternamente separados um do outro.
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Dissertation presented to obtain the Ph.D degree in Biology
Correlating EPR and X-ray structural analysis of arsenite-inhibited forms of aldehyde oxidoreductase
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J Biol Inorg Chem (2007) 12:353–366 DOI 10.1007/s00775-006-0191-9
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Dissertação para obtenção do Grau de Doutor em Ciência e Engenharia dos Materiais