An estimate of the time variation of the O/H radial gradient from planetary nebulae

Radial abundance gradients are a common feature of spiral galaxies, and in the case of the Galaxy both the magnitude of the gradients and their variations are among the most important constraints of chemical evolution models. Planetary nebulae (PN) are particularly interesting objects to study the gradients and their variations. Owing to their bright emission spectra, they can be observed even at large galactocentric distances, and the derived abundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex, particularly for the elements that are not synthesized in their progenitor stars. On the other hand, as the offspring of intermediate mass stars, with main sequence masses in the interval of 1 to 8 solar masses, they are representative of objects with a reasonable age span. In this paper, we present an estimate of the time variation of the O/H radial gradient in a sample containing over 200 nebulae with accurate abundances. Our results are consistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to -0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpc during the last 5 Gyr.Comment: 9 pages, 7 encapsulated postscript figures, LaTeX, uses Astronomy and Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and Astrophysics (2002), Also available at: http://www.astro.iag.usp.br/~macie

The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk using a sample of HII regions that spans Galactocentric distances 0--17 kpc. The electron temperature was calculated using high precision radio recombination line and continuum observations for more than 100 HII regions. Nebular Galactocentric distances were calculated in a consistent manner using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk together with the uniformity of our data provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic Center and rises at a rate of 287 +/- 46 K/kpc. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the HII region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures which are almost certainly due to fluctuations in the nebular heavy element abundances. Comparing the HII region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ

A produção do espaço urbano na Redução Jesuítica São Miguel Arcanjo e seus reflexos no processo de urbanização de São Miguel das Missões (RS-BR)

As Missões Jesuíticas possuem relevante papel no processo de ocupação e de urbanização da região noroeste do Estado do Rio Grande do Sul (RS), Brasil (BR). Busca-se verificar as consequências da produção do espaço na Redução Jesuítica de São Miguel Arcanjo (século XXII e XVIII) no processo de urbanização da atual cidade de São Miguel das Missões (RS-BR). A investigação consiste na análise de material bibliográfico e documental sobre o plano urbano de São Miguel Arcanjo e da concepção da organização urbana de São Miguel das Missões, bem como em entrevistas realizadas com agentes públicos. Como conclusão, evidenciou-se que o modelo de produção do espaço estimulou a segregação espacial dos índios guaranis, a concentração de renda nos grandes produtores rurais, o aumento da desigualdade social e o plano urbano da Redução influenciou a forma da cidade de São Miguel das Missões.The Jesuit Missions have a relevant role in the occupation and urbanization process of the northwestern region of the State of Rio Grande do Sul (RS), Brazil (BR). It seeks to verify the consequences of the production of space in the Jesuit Reduction of São Miguel Arcanjo (XXII and XVIII century) in the process of urbanization of the present city of São Miguel (RS-BR). The investigation consists of the analysis of bibliographical and documentary material about the urban plan of São Miguel Arcanjo and of the conception of the urban organization of São Miguel, as well as in interviews with public agents. As a conclusion, it was evidenced that the space production model stimulated the spatial segregation of the Guarani Indians, the concentration of income in the large rural producers, the increase of social inequality and Reduccion urban plan influenced the shape of the city of São Miguel das Missões

On the correlation between metallicity and the presence of giant planets

The correlation between stellar metallicity and the presence of giant planets is well established. It has been tentatively explained by the possible increase of planet formation probability in stellar disks with enhanced amount of metals. However, there are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed towards metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars. None of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk, a property that has been largely neglected until now. We illustrate that a planet-metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of molecular hydrogen in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this region.Comment: Accepted in ApJ

Earth Observing System/Advanced Microwave Sounding Unit-A (EOS/AMSU-A): Developer derating policy

The derating requirements/factors tabulated in Appendix B of the Goddard Space Flight Center Preferred Parts List (GSFC PPL) and Appendix A of MIL-STD-975 (NASA Standard Electrical, Electronic and Electromechanical (EEE) Parts List) should be used. Where differences occur, the PPL derating factors should have precedence over the derating factors of MIL-STD-975. When a derating factor is not provided in either the PPL or MIL-STD-975, the GSFC EOS Parts Branch Specialist should be consulted. In addition, the Performance Assurance Requirement (PAR) stipulates that all piece parts shall function at or above twice the expected ionizing radiation dose

Exact Effective action for (1+1)-dimensional fermions in an Abelian background at finite temperature and chemical potential

In this paper we study the effects of a nonzero chemical potential in the effective action for massless fermions in (1+1) dimensions in an abelian gauge field background at finite temperature. We calculate the n-point function and show that the structure of the amplitudes corresponds to a generalization of the structure noted earlier in a calculation without a chemical potential (the associated integrals carry the dependence on the chemical potential). Our calculation shows that the chiral anomaly is unaffected by the presence of a chemical potential at finite temperature. However, unlike the earlier calculation (in the absence of a chemical potential) odd point functions do not vanish. We trace this to the fact that in the presence of a chemical potential the generalized charge conjugation symmetry of the theory allows for such amplitudes. In fact, we find that all the even point functions are even functions of the chemical potential while the odd point functions are odd functions of it which is consistent with this generalized charge conjugation symmetry. We show that the origin of the structure of the amplitudes is best seen from a formulation of the theory in terms of left and right handed spinors. The calculations are also much simpler in this formulation and it clarifies many other aspects of the theory

Analise semiotica de um ava (ambiente virtual de ensino aprendizagem) : características cts através da escada semotica de Stamper

Este artigo demonstra um estudo e aplicação da análise semiótica, para a avaliação de interfaces de ambientes virtuais de ensino-aprendizagem (AVA), de acordo com a escada semiótica de Stamper. Com base nas características CTS (Ciência,Tecnologia e Sociedade) encontrada nas camadas “degrais” da escada. Em específico, usamos a metodologia para qualidade de interfaces definida por Schimiguel, que instancia critérios de qualidade para cada uma das camadas da escada semiótica de Stamper. Com base nos resultados dessa investigação foi possível sugerir algumas recomendações nas seis camadas analisadas, sinalizando ainda para cada uma dessas recomendações um enfoque CTS, uma descrição que tem como propósito facilitar a implementação e otimização de AVAS