78,231 research outputs found
Revisited fluorine abundances in the globular cluster M22 (NGC 6656)
Fluorine is a fairly good tracer of formation histories of multiple stellar
populations in globular clusters as already revealed by several studies. Large
variations in fluorine abundance in red giant stars of the globular cluster M22
have been recently reported by two different groups. Futhermore, one of these
studies claims that the abundance of fluorine is anti-correlated with sodium
abundances in this cluster, leading to strong conclusions on the chemical
history of M22. To validate this important finding, we re-examine the F
abundance determinations of some of the previously studied stars. We have thus
reanalysed some high-resolution VLT/CRIRES spectra of RGB stars found in M22 in
order to re-estimate their fluorine abundance from the spectral synthesis of
the HF line at 2.336microns. Unlike what has been previously estimated, we show
that only upper limits or doubtful fluorine abundances with large uncertainties
in M22 RGB stars can be derived. This is probably caused by an incorrect
identification of continuum fluctuations as the HF signature combined with a
wrong correction of the stellar radial velocity. Such continuum fluctuations
could be the consequences of telluric residuals that are still present in the
analysed spectra. Therefore, no definitive conclusions on the chemical
pollution caused by the M22 first stellar generation can presently be drawn
from the fluorine content of this cluster.Comment: A&A, in pres
Fluorine abundances and the puzzle of globular cluster chemical history
The abundance of fluorine in a few Galactic globular clusters is known to
strongly vary from star-to-star. These unexpected chemical properties are an
additional confirmation of the chemical inhomogeneities already found in
several GC, and probably caused by the first generations of stars formed in
these systems. The aim of this article is to complement our understanding of
the F-behaviour in GC stars and to look for new constraints on the formation
histories of their multiple stellar populations. We have collected near-IR
spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity:
47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by
spectral synthesis. No anticorrelation between F and Na abundances are found
for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars
indeed exhibit rather small differences in [F/Fe] unlike the larger ones found
for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and
a complex chemical evolution history for 47 Tuc . In M4, one star of our study
confirms the previous Na-F distribution reported by another group in 2005. For
the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances
have been derived. We show that F abundances could be estimated in such
metal-poor GC with current telescopes and spectrographs only if unexpected
F-rich giants are found and/or exceptional observational conditions are met.
The distribution of the F and Na abundances in GC reveal that their RGB members
seem to belong to two well-separated regions. All the RGB stars analysed so far
in the different GC are indeed found to be either F-rich Na-poor or F-poor
Na-rich. Such well-separated bimodal regimes are consistent with the separate
formation episodes suspected in most galactic GC.Comment: Astronomy & Astrophysics, in pres
A review of sample preparation and its influence on pH determination in concrete samples
If we are to monitor the chemical processes in cementitious materials, then pH assays in the pore solutions of cement pastes, mortars, and concretes are of key importance. However, there is no standard method that regulates the sample-preparation method for pH determination. The state-of-the-art of different methods for pH determination in cementitious materials is presented in this paper and the influence of sample preparation in each case. Moreover, an experimental campaign compares three different techniques for pH determination. Its results contribute to establishing a basic criterion to help researchers select the most suitable method, depending on the purpose of the research. A simple tool is described for selecting the easiest and the most economic pH determination method, depending on the objective; especially for researchers and those with limited experience in this field.Peer ReviewedPostprint (published version
Automated derivation of stellar atmospheric parameters and chemical abundances: the MATISSE algorithm
We present an automated procedure for the derivation of atmospheric
parameters (Teff, log g, [M/H]) and individual chemical abundances from stellar
spectra. The MATrix Inversion for Spectral SythEsis (MATISSE) algorithm
determines a basis, B_\theta(\lambda), allowing to derive a particular stellar
parameter \theta by projection of an observed spectrum. The B_\theta(\lambda)
function is determined from an optimal linear combination of theoretical
spectra and it relates, in a quantitative way, the variations in the spectrum
flux with variations in \theta. An application of this method to the GAIA/RVS
spectral range is described, together with its performances for different types
of stars of various metallicities. Blind tests with synthetic spectra of
randomly selected parameters and observed input spectra are also presented. The
method gives rapid, accurate and stable results and it can be efficiently
applied to the study of stellar populations through the analysis of large
spectral data sets, including moderate to low signal to noise spectra
Structural safety analysis of the aqueducts 'Coll de foix' and 'Capdevila' of the Canal of Aragon and Catalonia
The Canal of Aragon and Catalonia (CAC) is 134 km long and irrigates 105,000 ha (131 irrigation user communities) and it is owned by the River Ebro’s Water Agency. The aqueducts are located between km 67 and 71 of the canal and were designed by the civil engineer Félix de los Ríos Martín in 1907. The cross-section of both aqueducts, Coll de Foix and Capdevila, was extended within the framework of the project by Fernando Hué Herrero in 1962 in order to reach design flows of 26.1 m3/s and 25.7 m3/s, respectively. The structural performance of the aqueducts has been satisfactory; nevertheless, the hydraulic capacity has reduced over the years. As a result, the irrigation user communities have expressed the need to extend the cross-section of the aqueducts to meet the irrigation demands. Given the age of the structure and the different design considerations at the time, it is paramount to verify the structural reliability of the aqueducts in the new load configuration. Therefore, the objective of this contribution is to present the structural safety analysis conducted and to describe the new extended cross-section for both aqueducts (maintaining the original structural typology).Peer ReviewedPostprint (published version
A Review of the Role and Impact of Export Processing Zones in World Trade : the Case of Mexico
While the number of export processing zones has risen to about 850 in 2000, their success in expanding employment and trade is mixed. The aim of this paper is to make a review of the role and the impact of EPZ in world trade and its likely impact on host countries' economies, especially in terms of foreign exchange earnings potential, FDI, technology transfer, and employment effects on the local and national economies. To this end we will be looking at Mexico where export processing zones are very developed. Mexico seems an appropriate case study to assess to which extent the predictions of economic theory are realised both because of the abundance and quality of available data and because of its proximity to the US, which makes it a very good example of the international division of labour. We will analyse how the parameters mentioned above (employment, foreign exchange earnings potential, FDI and technology) have evolved over the last 10-15 years and how much of the change can be traced back to the maquila industry. The main source of data is INEGI, the Instituto Nacional de Estadistica Geografica e Informatica of the Mexican Government.export processing zones; foreign exchange earnings potential; Mexico; technology spillovers
Sustainability based-approach to determine the concrete type and reinforcement configuration of TBM tunnels linings. Case study: Extension line to Barcelona Airport T1
Fibre-reinforced concrete (FRC) is a suitable alternative to the traditional reinforced concrete used in the manufacture of precast segments used to line tunnels excavated with a tunnel boring machine (TBM). Moreover, its use as a structural material has been approved by several national codes and by the current fib Model Code (2010). The use of FRC in segmental linings confers several technical and economic advantages, evidenced by the fact that structural fibres have been used to partially or entirely replace reinforcing bars in many TBM tunnels built over the past 20 years or currently under construction. FRC could also have been used in other tunnels, which are currently in the planning stage or under construction. However, despite its technical suitability and approval in current codes, the use of FRC was not possible in some cases. The impediment has sometimes been an incomplete understanding of the structural behaviour of the material, but a more general motive has been that comparisons of materials have taken into account only direct material costs and have not considered indirect costs or social and environmental factors. The aim of the present research is to develop a method for analysing the sustainability of different concrete and reinforcement configurations for segmental linings of TBM tunnels using the MIVES method (a multi-criteria decision making approach for assessing sustainability). This MCDM method allows minimising subjectivity in decision making while integrating economic, environmental and social factors. The model has been used to assess the sustainability of different alternatives proposed for manufacturing the segmental tunnel lining for the extension of the rail line of Ferrocarrils de la Generalitat de Catalunya (FGC) to Terminal 1 of El Prat Airport in Barcelona.Peer ReviewedPostprint (author's final draft
What works?: the culture of evidence in university teaching
This article analyses the culture of
evidence in university teaching and its
implications in the professional
training of teachers in higher educa
tion. The new culture of organisati
on and assessment introduced into
university teaching has brought about the configuration of a management model geared towards results and
accountability based on solid
evidence. Its implementation means that
both administrators and teachers are
asking themselves:
what works?
This study shows that the implementation of a culture of evidence requires the
adoption of a pluralist vision of evidence, as well as cl
ear criteria for determining the validity of evidence. In
addition, teachers should be trained to
mobilise systematic pedagogic know
ledge and transform their practice,
using available institutional support, the systematic analysis of their own experience, and the promotion of best
practic
Parameter Estimation from an Optimal Projection in a Local Environment
The parameter fit from a model grid is limited by our capability to reduce
the number of models, taking into account the number of parameters and the non
linear variation of the models with the parameters. The Local MultiLinear
Regression (LMLR) algorithms allow one to fit linearly the data in a local
environment. The MATISSE algorithm, developed in the context of the estimation
of stellar parameters from the Gaia RVS spectra, is connected to this class of
estimators. A two-steps procedure was introduced. A raw parameter estimation is
first done in order to localize the parameter environment. The parameters are
then estimated by projection on specific vectors computed for an optimal
estimation. The MATISSE method is compared to the estimation using the
objective analysis. In this framework, the kernel choice plays an important
role. The environment needed for the parameter estimation can result from it.
The determination of a first parameter set can be also avoided for this
analysis. These procedures based on a local projection can be fruitfully
applied to non linear parameter estimation if the number of data sets to be
fitted is greater than the number of models
Diffusion of Hydrogen in Pd Assisted by Inelastic Ballistic Hot Electrons
Sykes {\it et al.} [Proc. Natl. Acad. Sci. {\bf 102}, 17907 (2005)] have
reported how electrons injected from a scanning tunneling microscope modify the
diffusion rates of H buried beneath Pd(111). A key point in that experiment is
the symmetry between positive and negative voltages for H extraction, which is
difficult to explain in view of the large asymmetry in Pd between the electron
and hole densities of states. Combining concepts from the theory of ballistic
electron microscopy and electron-phonon scattering we show that H diffusion is
driven by the -band electrons only, which explains the observed symmetry.Comment: 5 pages and 4 figure
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