5,763 research outputs found
Unveiling the nature of the "Green Pea" galaxies
We review recent results on the oxygen and nitrogen chemical abundances in
extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3
recently named to as "Green Pea" galaxies. These galaxies are genuine
metal-poor galaxies ( one fifth solar) with N/O ratios unusually high for
galaxies of the same metallicity. In combination with their known general
properties, i.e., size, stellar mass and star-formation rate, these findings
suggest that these objects could be experiencing a short and extreme phase in
their evolution. The possible action of both recent and massive inflow of gas,
as well as stellar feedback mechanisms are discussed here as main drivers of
the starburst activity and their oxygen and nitrogen abundances.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
On Global Flipped SU(5) GUTs in F-theory
We construct an SU(4) spectral divisor and its factorization of types (3,1)
and (2,2) based on the construction proposed in [1]. We calculate the chiral
spectra of flipped SU(5) GUTs by using the spectral divisor construction. The
results agree with those from the analysis of semi-local spectral covers. Our
computations provide an example for the validity of the spectral divisor
construction and suggest that the standard heterotic formulae are applicable to
the case of F-theory on an elliptically fibered Calabi-Yau fourfold with no
heterotic dual.Comment: 45 pages, 12 tables, 1 figure; typos corrected, footnotes added, and
a reference adde
Flipped SU(5) GUTs from E_8 Singularities in F-theory
In this paper we construct supersymmetric flipped SU(5) GUTs from E_8
singularities in F-theory. We start from an SO(10) singularity unfolded from an
E_8 singularity by using an SU(4) spectral cover. To obtain realistic models,
we consider (3,1) and (2,2) factorizations of the SU(4) cover. After turning on
the massless U(1)_X gauge flux, we obtain the SU(5) X U(1)_X gauge group. Based
on the well-studied geometric backgrounds in the literature, we demonstrate
several models and discuss their phenomenology.Comment: 46 pages, 23 tables, 1 figure, typos corrected, references added, and
new examples presente
Outbreak of Fatal Childhood Lead Poisoning Related to Artisanal Gold Mining in Northwestern Nigeria, 2010.
Background: In May 2010, a team of national and international organizations was assembled to investigate children's deaths due to lead poisoning in villages in northwestern Nigeria. Objectives: To determine the cause of the childhood lead poisoning outbreak, investigate risk factors for child mortality, and identify children aged <5 years in need of emergency chelation therapy for lead poisoning. Methods: We administered a cross-sectional, door-to-door questionnaire in two affected villages, collected blood from children aged 2-59 months, and soil samples from family compounds. Descriptive and bivariate analyses were performed with survey, blood-lead, and environmental data. Multivariate logistic regression techniques were used to determine risk factors for childhood mortality. Results: We surveyed 119 family compounds. One hundred eighteen of 463 (25%) children aged <5 years had died in the last year. We tested 59% (204/345) of children, aged <5 years, and all were lead poisoned (≥10 µg/dL); 97% (198/204) of children had blood-lead levels ≥45 µg/dL, the threshold for initiating chelation therapy. Gold ore was processed inside two-thirds of the family compounds surveyed. In multivariate modeling significant risk factors for death in the previous year from suspected lead poisoning included: the child's age, the mother performing ore-processing activities, community well as primary water source, and the soil-lead concentration in the compound. Conclusion: The high levels of environmental contamination, percentage of children aged <5 years with elevated blood-lead levels (97%, >45 µg/dL), and incidence of convulsions among children prior to death (82%) suggest that most of the recent childhood deaths in the two surveyed villages were caused by acute lead poisoning from gold ore-processing activities. Control measures included environmental remediation, chelation therapy, public health education, and control of mining activities
In situ electrochemical cells to study the oxygen evolution reaction by near ambient pressure x-ray photoelectron spectroscopy
In this contribution, we report the development of in situ electrochemical cells based on proton exchange membranes suitable for studying interfacial structural dynamics of energy materials under operation by near ambient pressure X-ray photoelectron spectroscopy. We will present both the first design of a batch-type two-electrode cell prototype and the improvements attained with a continuous flow three-electrode cell. Examples of both sputtered metal films and carbon-supported metal nanostructures are included demonstrating the high flexibility of the cells to study energy materials. Our immediate focus was on the study of the oxygen evolution reaction, however, the methods described herein can be broadly applied to reactions relevant in energy conversion and storage devices
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