483 research outputs found
Regiao amazonica e as mudancas globais : necessidade de um plano de pesquisas
Reunión: Reunión sobre Capacidad de Investigación en Temas de Medio Ambiente en América Latina y el Caribe, 27-29 enero 1992, Montevideo, UYTexto en portugué
Amazonian deforestation: Regional and global issues
How does the deforestation of Amazonia affect global and regional climates? What are the roles of these changes on Amazonian ecosystems? We have concluded that the relation between Regional Climate Changes (RCC) and Global Climate Change (GCC) is directly associated with anthropogenic activities and therefore sensitive to social, economical and political interventions. The RCCs are caused by actions within the realm of the Brazilian sociopolitical scenario, and prone to changes through the implementation of public policies regulating the sustained use of the renewable resources. The GCCs belong to an international arena, and are caused by the high emission rates of green house gases by the developed countries. The effects of the RCCs could be abated if the developed countries would endeavor to reduce the present emission levels as documented in IPCC meetings and collaborate in the implementation of a regulation to curb the carbon emissions, in accordance to the Kyoto protocol
Is the Large Magellanic Cloud a Large Microlensing Cloud?
An expression is provided for the self-lensing optical depth of the thin LMC
disk surrounded by a shroud of stars at larger scale heights. The formula is
written in terms of the vertical velocity dispersion of the thin disk
population. If tidal forcing causes 1-5 % of the disk mass to have a height
larger than 6 kpc and 10-15 % to have a height above 3 kpc, then the
self-lensing optical depth of the LMC is , which is
within the observational uncertainties. The shroud may be composed of bright
stars provided they are not in stellar hydrodynamical equilibrium.
Alternatively, the shroud may be built from low mass stars or compact objects,
though then the self-lensing optical depths are overestimates of the true
optical depth by a factor of roughly 3. The distributions of timescales of the
events and their spatial variation across the face of the LMC disk offer
possibilities of identifying the dominant lens population. In propitious
circumstances, an experiment lifetime of less than 5 years is sufficient to
decide between the competing claims of Milky Way halos and LMC lenses. However,
LMC disks can sometimes mimic the microlensing properties of Galactic halos for
many years and then decades of survey work are needed. In this case
observations of parallax or binary caustic events offer the best hope for
current experiments to deduce the lens population. The difficult models to
distinguish are Milky Way halos in which the lens fraction is low (< 10 %) and
fattened LMC disks composed of lenses with a typical mass of low luminosity
stars or greater. A next-generation wide-area microlensing survey, such as the
proposed ``SuperMACHO'' experiment, will be able to distinguish even these
difficult models with just a year or two of data.Comment: 25 pages, 4 figures, The Astrophysical Journal (in press
Adipose Stem Cells in Modern-Day Ophthalmology
Stem cells (SCs) have evolved as an interesting and viable factor in ophthalmologic patient care in the past decades. SCs have been classified as either embryonic, mesenchymal, tissue-specific, or induced pluripotent cells. Multiple novel management techniques and clinical trials have been established to date. While available publications are predominantly animal-model-based, significant material is derived from human studies and case-selected scenarios. This possibility of explanting cells from viable tissue to regenerate/repair damaged tissue points to an exciting future of therapeutic options in all fields of medicine, and ophthalmology is surely not left out. Adipose tissue obtained from lipo-aspirates has been shown to produce mesenchymal SCs that are potentially useful in different body parts, including the oculo-visual system. An overview of the anatomy, physiology, and extraction process for adipose-tissue-derived stem cells (ADSC) is important for better understanding the potential therapeutic benefits. This review examines published data on ADSCs in immune-modulatory, therapeutic, and regenerative treatments. We also look at the future of ADSC applications for ophthalmic patient care. The adverse effects of this relatively novel therapy are also discussed
Estimativa da evapotranspiração através de fórmulas relacionadas à temperatura média do ar atmosférico
Thornthwaite and Blaney-Criddle's methods were used to estimate the evapotranspiration rate in a potato field-crop under three soil moisture regimes. The evapotranspiration data obtained by the soil-water balance method were well correlated with those obtained by the two empirical methods, manly from the stage of tuber growth to maturity. Blaney-Criddle's consumptive use factors supplied the higher correlation coefficient and presented monthly consumptive use coefficients very approximate to those observed in another sites, whereas Thornthwaite's formula markedly underestimated the measured values during all lenght of the growing season. This fact could not be justified by the thermal lag. The lack of precision of the estimates observed in the present field-test seems to involve the parameters and constants of the Thornthwaite's formula. At plant emergence and in the first phase of stolon formation, the correlation coefficients were lower than for the other stages, mainly when Thornthwaite's formula was used.As fórmulas de Thornthwaite e Blaney-Criddle foram empregadas para estimativa da evapotranspiração de uma cultura de batata (Solanian tuberosum L.) submetida a três regimes de umidade do solo. Os valores de evapotranspiração determinados pelo método do balanço de água mostraram elevada correlação com aqueles obtidos pelos dois métodos climatológicos empíricos, no período de maior crescimento vegetativo. A fórmula de Blaney-Criddle apresentou os coeficientes de correlação mais elevados e forneceu coeficientes de uso consuntivo mensais e estacionais muito próximos daqueles observados em outras regiões. A fórmula de Thornthwaite, por sua vez, subestimou acentuadamente os valores determinados durante todo o ciclo de crescimento, o que não pode ser explicado apenas em termos do atraso térmico. A imprecisão dos resultados observados nas condições do presente trabalho parece envolver a natureza dos parâmetros e constantes que definem a equação. No período inicial de crescimento os coeficientes de correlação foram pouco significativos, principalmente quand era utilizada a fórmula de Thornthwaite
Self-Lensing Models of the LMC
All of the proposed explanations for the microlensing events observed towards
the LMC have difficulties. One of these proposed explanations, LMC
self-lensing, which invokes ordinary LMC stars as the long sought-after lenses,
has recently gained considerable popularity as a possible solution to the
microlensing conundrum. In this paper, we carefully examine the set of LMC
self-lensing models. In particular, we review the pertinent observations made
of the LMC, and show how these observations place limits on such self-lensing
models. We find that, given current observational constraints, no purely LMC
disk models are capable of producing optical depths as large as that reported
in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider
alternate geometries, and present a framework which encompasses the previous
studies of LMC self-lensing. We discuss which model parameters need to be
pushed in order for such models to succeed. For example, like previous workers,
we find that an LMC halo geometry may be able to explain the observed events.
However, since all known LMC tracer stellar populations exhibit disk-like
kinematics, such models will have difficulty being reconciled with
observations. For SMC self-lensing, we find predicted optical depths differing
from previous results, but more than sufficient to explain all observed SMC
microlensing. In contrast, for the LMC we find a self-lensing optical depth
contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the
set of LMC parameters most consistent with current observations.Comment: 20 pages, Latex, 14 figures, submitted to Ap
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