750 research outputs found
Online social lending: Borrower-generated content
This article explores online social lending, an innovative venture that represents a reintermediation in financial services. Borrowers and lenders now have access to online financial information services such as Motley Fool, http://www.fool.com/ , and the opportunity to communicate directly with each other online, sharing user-generated content, in the spirit of Web 2.0.
In this environment, new possibilities emerge. Drawing on the literature of community banks, finance, and online banking, we conducted a structurational analysis of ZOPA(2007) a newly founded venture in online social lending whereby borrower/lender interactions take place within an open and transparent environment using discussion boards and blogs. ZOPA offers a service as an intermediary but one that differs from the intermediating role played by a traditional bank.
We analyzed the possible attractions and risks of ZOPA’s service to customers, from the perspective of social lending and social networking, using public data from ZOPA’s website. Our intention is to understand the nature of this reintermediation and explain the development of this process through Giddens’ propositions
Proximal methods for stationary mean field games with local couplings
© 2018 Society for Industrial and Applied Mathematics. We address the numerical approximation of mean field games with local couplings. For power-like Hamiltonians, we consider a stationary system and also a system involving density constraints modeling hard congestion effects. For finite difference discretization of the mean field game system developed in [Y. Achdou and I. Capuzzo-Dolcetta, SIAM J. Numer. Anal., 48 (2010), pp. 1136-1162], we follow a variational approach. We prove that the aforementioned schemes can be obtained as the optimality system of suitably defined optimization problems. In order to prove the existence of solutions of the scheme with a variational argument, monotonicity assumptions on the coupling term are not needed, which allows us to recover general existence results proved by Achdou and Capuzzo-Dolcetta. Next, assuming that the coupling term is nondecreasing, the variational problem is cast as a convex optimization problem, for which we study and compare several proximal-type methods. These algorithms have several interesting features, such as global convergence and stability with respect to the viscosity parameter, which can eventually be zero. We assess the performance of the methods via numerical experiments
Influence of the Pump Control System in the Selection of the Number of Fixed Speed and Variable Speed Drive Pumps in Water Pumping Stations
[EN] Proper design of a pumping system requires the use of a pump curve and set-point curve system. Both have to be as close as possible to optimize energy use. This is achieved by control systems in which the type of control (flow or pressure) and the combination between fixed speed drive (FSD) pumps and variable speed drive (VSD) pumps are involved. The objective of this work is to determine the optimal number of FSD and VSD pumps for each flow rate range in order to discuss the classic design of pumping stations and their control systems. For this, a methodology is applied that defines the parametric form of the pump curve, efficiency curve, and set-point curve in relation to the most efficient point. In this way, dimensionless expressions are obtained and the influence of the set-point parameters on the design of the control system can be analyzed. Additionally, the method includes an expression that estimates the performance of the frequency inverter, which is based on the load and pump speed rotation. The application of the methodology to different case studies allows us to question many classic procedures for pumping stations. In summary, it can be concluded that the appropriate number of variable speed pumps for each control system cannot be established in advance but requires an in-depth study of different available options.Briceño, C.; Iglesias Rey, PL.; MartÃnez-Solano, FJ. (2019). Influence of the Pump Control System in the Selection of the Number of Fixed Speed and Variable Speed Drive Pumps in Water Pumping Stations. Proceedings. 48(1):1-11. https://doi.org/10.3390/ECWS-4-06445S11148
Elemental abundances of low-mass stars in the young clusters 25 Ori and lambda Ori
Aims. We aim to derive the chemical pattern of the young clusters 25 Orionis
and lambda Orionis through homogeneous and accurate measurements of elemental
abundances. Methods. We present FLAMES/UVES observations of a sample of 14
K-type targets in the 25 Ori and lambda Ori clusters; we measure their radial
velocities, in order to confirm cluster membership. We derive stellar
parameters and abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code
MOOG. Results. All the 25 Ori stars are confirmed cluster members without
evidence of binarity; in lambda Ori we identify one non-member and one
candidate single-lined binary star. We find an average metallicity
[Fe/H]=-0.05+/-0.05 for 25 Ori, where the error is the 1sigma standard
deviation from the average. lambda Ori members have a mean iron abundance value
of 0.01+/-0.01. The other elements show close-to-solar ratios and no
star-to-star dispersion. Conclusions. Our results, along with previous
metallicity determinations in the Orion complex, evidence a small but
detectable dispersion in the [Fe/H] distribution of the complex. This appears
to be compatible with large-scale star formation episodes and initial
non-uniformity in the pre-cloud medium. We show that, as expected, the
abundance distribution of star forming regions is consistent with the chemical
pattern of the Galactic thin disk.Comment: Accepted by Astronomy and Astrophysics. 14 pages, 10 figures, 7
table
El aprendizaje de algunos conceptos gramaticales y ortográficos en español, de los alumnos de sexto de primaria del sur del estado de Veracruz
Se presentan los resultados de una investigación cualitativa-interpretativa realizada en el sur del estado de Veracruz, que responde a la pregunta ¿cuáles son los procesos mentales que favorecen en la práctica los docentes de sexto grado de primaria para que los alumnos aprendan los conceptos gramaticales y ortográficos del componente de reflexión sobre la lengua y las competencias lingüÃsticas de la asignatura de español en un contexto sociocultural dado? Los participantes son siete maestras de primaria que trabajaban en seis escuelas diferentes: dos urbanas, dos semiurbanas y dos rurales. El diseño tuvo una duración de dos años, con un carácter descriptivo e interpretativo. Los sujetos fueron alumnos de sexto grado cuyas edades estaban entre los 11 y 14 años. El propósito era sumergirse en la complejidad de los acontecimientos reales del aprendizaje y la enseñanza para indagar sobre la formación de conceptos. Las categorÃas de análisis encontradas fueron: discurso directo e indirecto, los signos de puntuación, tiempos verbales, errores ortográficos, concordancia, irregularidades en la correspondencia sonoro gráfica de las letras, mayúsculas y minúsculas, la conjunción, acentuación, adjetivos y adverbios. Se usó la técnica de observación no participante y la metodologÃa de análisis por categorización nominal y axial. La perspectiva teórica que sustenta este trabajo es la sociocultural de Vygotsky, Wertsch, Cole, Luria, Leontiev y otros. Los resultados encontrados muestran que el proceso de formación conceptual en los alumnos, en cuanto a gramática y ortografÃa, es complicado e incompleto. No logran formar conceptos verdaderos, sino solo complejos asociativos, difusos, tipo colección, pseudoconceptos, inicialmente abstractos y potenciales; ellos fueron formando sus propios conceptos con funciones mentales como la observación, la atención, la discriminación y el análisis. No hubo sÃntesis abstracta, ni conceptualización. Los docentes ignoran cómo se forman los conceptos y predominó en ellos un pensamiento complejo. El modelo pedagógico vigente de competencias incidió en el aprendizaje de los alumnos en cuanto a la no formación de conceptos. Hubo demasiadas incongruencias entre, programas de estudio, pensamiento de las maestras y la Real Academia Española.Consejo Nacional de Ciencia y TecnologÃaUniversidad Iberoamericana Puebl
Pulsating young brown dwarfs
We present the results of a nonadiabatic, linear stability analysis of models
of very low-mass stars (VLMSs) and brown dwarfs (BDs) during the deuterium
burning phase in the center. We find unstable fundamental modes with periods
varying between ~5 hr for a 0.1 Msun star and ~1 hr for a 0.02 Msun BD. The
growth time of the instability decreases with decreasing mass and remains well
below the deuterium burning time scale in the mass range considered (0.1--0.02
Msun). These results are robust against variations of the relevant input
physics in the evolutionary models. We identify possible candidates for
pulsational variability among known VLMSs and BDs in nearby star forming
regions whose location in the HR diagram falls within or close to the boundary
of the instability strip. Finally, we discuss the possibility that the
variability observed in a few objects with periods of ~1 hr can be interpreted
in terms of pulsation.Comment: 5 pages, 3 figures, A&A Letters (in press
An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Accurate measurements of lithium abundances in young low-mass stars provide
an independent and reliable age diagnostics. Previous studies of nearby star
forming regions have identified significant numbers of Li-depleted stars,often
at levels inconsistent with the ages indicated by their luminosity. We aim at a
new and accurate analysis of Li abundances in a sample of ~100 pre-main
sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar
parameters and model atmospheres appropriate for the spectral types of the
sample stars.We compute Li abundances using published values of the equivalent
widths of the Li 6708 A doublet obtained from medium/high resolution spectra.
We find that the number of significantly Li-depleted stars in Taurus-Auriga is
greatly reduced with respect to earlier results. Only 13 stars have abundances
lower than the interstellar value by a factor of 5 or greater. All of them are
weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four
stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars
belong to the class of dispersed low-mass stars, distributed around the main
sites of current star formation. If located at the distance of Taurus-Auriga,
the stellar ages implied by the derived Li abundances are in the range 3-30
Myr, greater than the bulk of the Li-rich population with implication on the
star formation history of the region. In order to derive firm conclusions about
the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the
remaining members of the association should be obtained, in particular of the
group of stars that fall in the Li-burning region of the HR diagram.Comment: Accepted for publication in Astronomy & Astrophysics. 20 pages, 5
figure
The Gould's Belt Distances Survey (GOBELINS). V. Distances and Kinematics of the Perseus molecular cloud
We derive the distance and structure of the Perseus molecular cloud by
combining trigonometric parallaxes from Very Long Baseline Array (VLBA)
observations, taken as part of the GOBELINS survey, and Gaia Data Release 2.
Based on our VLBA astrometry, we obtain a distance of 321+/-10 pc for IC 348.
This is fully consistent with the mean distance of 320+/-26 measured by Gaia.
The VLBA observations toward NGC 1333 are insufficient to claim a successful
distance measurement to this cluster. Gaia parallaxes, on the other hand, yield
a mean distance of 293+/-22 pc. Hence, the distance along the line of sight
between the eastern and western edges of the cloud is ~30 pc, which is
significantly smaller than previously inferred. We use Gaia proper motions and
published radial velocities to derive the spatial velocities of a selected
sample of stars. The average velocity vectors with respect to the LSR are
(u,v,w) = (-6.1+/-1.6, 6.8+/-1.1, -0.9+/-1.2) and (-6.4+/-1.0, 2.1+/-1.4,
-2.4+/-1.0) km/s for IC 348 and NGC 1333, respectively. Finally, our analysis
of the kinematics of the stars has shown that there is no clear evidence of
expansion, contraction, or rotational motions within the clusters.Comment: Accepted for publication in Ap
The Gould's Belt Very Large Array Survey III. The Orion region
We present results from a high-sensitivity (60 Jy), large-scale (2.26
square degree) survey obtained with the Karl G. Jansky Very Large Array as part
of the Gould's Belt Survey program. We detected 374 and 354 sources at 4.5 and
7.5 GHz, respectively. Of these, 148 are associated with previously known Young
Stellar Objects (YSOs). Another 86 sources previously unclassified at either
optical or infrared wavelengths exhibit radio properties that are consistent
with those of young stars. The overall properties of our sources at radio
wavelengths such as their variability and radio to X-ray luminosity relation
are consistent with previous results from the Gould's Belt Survey. Our
detections provide target lists for followup VLBA radio observations to
determine their distances as YSOs are located in regions of high nebulosity and
extinction, making it difficult to measure optical parallaxes.Comment: Accepted for publication in ApJ; 51 pages, 15 figures, 5 table
Unveiling extremely veiled T Tauri stars
Photospheric absorption lines in classical T Tauri stars (CTTS) are weak
compared to normal stars. This so-called veiling is normally identified with an
excess continuous emission formed in shock-heated gas at the stellar surface
below the accretion streams. We have selected four stars (RW Aur A, RU Lup, S
CrA NW and S CrA SE) with unusually strong veiling to make a detailed
investigation of veiling versus stellar brightness and emission line strengths
for comparisons to standard accretion models. We have monitored the stars
photometrically and spectroscopically at several epochs. In standard accretion
models a variable accretion rate will lead to a variable excess emission.
Consequently, the stellar brightness should vary accordingly. We find that the
veiling of absorption lines in these stars is strongly variable and usually so
large that it would require the release of several stellar luminosities of
potential energy. At states of very large line dilution, the correspondingly
large veiling factors derived correlate only weakly with brightness. Moreover,
the emission line strengths violate the expected trend of veiling versus line
strength. The veiling can change dramatically in one night, and is not
correlated with the phase of the rotation periods found for two stars. We show
that in at least three of the stars, when the veiling becomes high, the
photospheric lines become filled-in by line emission, which produces large
veiling factors unrelated to changes in any continuous emission from shocked
regions. We also consider to what extent extinction by dust and electron
scattering in the accretion stream may affect veiling measures in CTTS. We
conclude that the degree of veiling cannot be used as a measure of accretion
rates in CTTS with rich emission line spectra.Comment: Accepted for publication in A&A Letters. New language-edited version.
(4 pages, 3 figures
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