Radial variations in the stellar initial mass function of early-type galaxies

The hypothesis of a universal initial mass function (IMF) -- motivated by observations in nearby stellar systems -- has been recently challenged by the discovery of a systematic variation of the IMF with the central velocity dispersion, {\sigma}, of early-type galaxies (ETGs), towards an excess of low-mass stars in high-{\sigma} galaxies. This trend has been derived so far from integrated spectra, and remains unexplained at present. To test whether such trend depends on the local properties within a galaxy, we have obtained new, extremely deep, spectroscopic data, for three nearby ETGs, two galaxies with high {\sigma} (~300 km/s), and one lower mass system, with {\sigma} ~ 100 km/s. From the analysis of IMF-sensitive spectral features, we find that the IMF depends significantly on galactocentric distance in the massive ETGs, with the enhanced fraction of low-mass stars f mostly confined to their central regions. In contrast, the low-{\sigma} galaxy does not show any significant radial gradient in the IMF, well described by a shallower distribution, relative to the innermost regions of massive galaxies, at all radii. Such a result indicates that the IMF should be regarded as a local (rather than global) property, and suggests a significant difference between the formation process of the core and the outer regions of massive ETGs.Comment: 17 pages, 13 figures. Accepted for publication in MNRA

Arithmetic geometry of toric varieties. Metrics, measures and heights

We show that the height of a toric variety with respect to a toric metrized line bundle can be expressed as the integral over a polytope of a certain adelic family of concave functions. To state and prove this result, we study the Arakelov geometry of toric varieties. In particular, we consider models over a discrete valuation ring, metrized line bundles, and their associated measures and heights. We show that these notions can be translated in terms of convex analysis, and are closely related to objects like polyhedral complexes, concave functions, real Monge-Amp\`ere measures, and Legendre-Fenchel duality. We also present a closed formula for the integral over a polytope of a function of one variable composed with a linear form. This allows us to compute the height of toric varieties with respect to some interesting metrics arising from polytopes. We also compute the height of toric projective curves with respect to the Fubini-Study metric, and of some toric bundles.Comment: Revised version, 230 pages, 3 figure

Social security and the search behaviour of workers approaching retirement

This paper explores the links between unemployment, retirement and their associated public insurance programs. It is a contribution to a growing body of literature focused on a better understanding of the labor behavior of advanced—age workers, which has gained importance as the pension crisis looms. The analysis combines the development of a new theoretical model and a detailed exploration of the empirical regularities using the Spanish Muestra Continua de Vidas Laborales (MCVL) dataset. The model is a extension of the standard search model, designed to reproduce the non—stationary environment faced by workers approaching retirement and to explore the interaction of unemployment benefits and retirement pensions. Via calibrated simulations we show that the basic empirical reemployment and retirement patterns can be rationalized as the optimal responses to both the labor market conditions and the institutional incentives. Generous Unemployment Benefits (for durations of up to two years) together with very significant early retirement penalties, make optimal to stay unemployed without searching for large groups of unemployed workers. This moral hazard problem can he substantially alleviated through institutional reform. Setting the early retirement penalties according to the age when the individual withdraws from the labor force (rather than when he/she claims the pension for the first time) seems particularly beneficial. It increases the labor supply, reduces the financial cost for the social security system and generate enough extra resources to compensate for the welfare loss of those unemployed directly hit by the reform.Unemployment search, job benefit, retirement

Retirement incentives, individual heterogeneity and labour transitions of employed and unemployed workers

In this paper we analyze the sensitivity of the labour market decisions of workers close to retirement with respect to the incentives created by public regulations. We improve upon the extensive prior literature on the effect of pension incentives on retirement in two ways. First, by modeling the transitions between employment, unemployment and retirement in a simultaneous manner, paying special attention to the transition from unemployment to retirement (which is particularly important in Spain). Second, by considering the influence of unobserved heterogeneity in the estimation of the effect of our (carefully constructed) incentive variables. Using administrative data, we find that, when properly defined, economic incentives have a strong impact on labour market decisions in Spain. Unemployment regulations are shown to be particularly influential for retirement behaviour, along with the more traditional determinants linked to the pension system. Pension variables also have a major bearing on both workers’ reemployment decisions and on the strategic actions of employers. The quantitative impact of the incentives, however, is greatly affected by the existence of unobserved heterogeneity among workers. Its omission leads to sizable biases in the assessment of the sensitivity to economic incentives, a finding that has clear consequences for the credibility of any model-based policy analysis. We confirm the importance of this potential problem in one especially interesting instance: the reform of early retirement provisions undertaken in Spain in 2002. We use a difference-in-difference approach to measure the behavioural reaction to this change, finding a large overestimation when unobserved heterogeneity is not taken into account.Retirement, unemployment, incentives, Pension system, Unobserved, heterogeneity, Spain.

Determinación teórica de la histéresis eléctrica en materiales compuestos ferroeléctricos

En este trabajo se establecen relaciones entre la histéresis eléctrica macroscópica y el comportamiento de las fases constitutivas en materiales compuestos ferroeléctricos con microestructuras complejas y sometidos a historias de carga arbitrarias. El comportamiento ferroeléctrico de cada fase se describe mediante una densidad de energía almacenada y un potencial de disipación en el marco de los ‘materiales estándar generalizados’. Discretizando en el tiempo las ecuaciones diferenciales correspondientes siguiendo un esquema implícito, se obtiene una representación variacional de la respuesta macroscó- pica del compuesto que involucra un solo potencial incremental. Las estimaciones teóricas se obtienen luego mediante ciertas ‘microgeometrías resolubles’ cuyo potencial incremental macroscópico puede determinarse en forma exacta. Por ser exactas para una clase de materiales, estas estimaciones satisfacen automáticamente todos las características comunmente buscadas en una aproximación teórica: estar de acuerdo con restricciones materiales, satisfacer todas las cotas pertinentes, y poseer todas las propiedades de convexidad correspondientes. La metodología propuesta se aplica a sistemas materiales representativos y los resultados se discuten a la luz de observaciones experimentales existentes.Theoretical estimates are given for the overall hysteretic response of two-phase ferroelectric composites with complex particulate microstructures under arbitrary loading histories. The ferroelectric behavior of the constituent phases is described via a stored energy density and a dissipation potential in accordance with the theory of generalized standard materials. An implicit time-discretization scheme is used to generate a variational representation of the overall response in terms of a single incremental potential. Estimates are then generated by constructing sequentially laminated microgeometries of particulate type whose overall incremental potential can be computed exactly. Because they are realizable, by construction, these estimates are guaranteed to conform with any material constraints, to satisfy all pertinent bounds, and to exhibit the required convexity properties with no duality gap. Predictions for composites and porous ferroelectrics are reported and discussed in the light of existing experimental data.Fil: Idiart, Martín Ignacio. Universidad Nacional de la Plata. Facultad de Ingeniería. Departamento de Aeronautica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Replanteando el futuro de la ciudad americana: ¿hacia una agenda de habitabilidad?

Este texto reflexiona en el esfuerzo producido durante los últimos cuarenta años en los Estados Unidos de re-estructurar la planificación de la ciudad-región y conseguir un modelo de urbanismo basado en indicadores de calidad de vida, indicadores de sostenibilidad y últimamente, intentar crear una aportación social para fomentar un nuevo modelo de habitabilidad (livability). Aunque existen ejemplos bastante variables en diferentes estados del país, es evidente que el urbanismo basado en índices de calidad de vida ha producido una mejora considerable en el carácter físico y social de las ciudades de los Estados Unidos. Pero esto se ha conseguido a base de consumir una cantidad de recursos exorbitantes. Evidentemente, el movimiento de crear un urbanismo basado en índices de sostenibilidad, a pesar de su amplia discusión, ha tenido pocos resultados. La trayectoria del urbanismo sostenible basado en políticas urbanas que impongan leyes integrantes para reducir el consumo de recursos y energía no renovable por ejemplo, no ha podido superar, en la mayoría de los casos, la apatía general de cambiar los hábitos y actitudes inherentes en la estructura social vigente. El nuevo modelo de habitabilidad implica la necesidad de asumir un cambio personal y cultural que pueda producir un alcance similar de calidad de vida u1bana y regional dentro de las realidades que implican una reducción considerable en el índice de consunción de huella ecológica usada actualmente.This article reflects on the efforts made in the United States during the past forty years in creating a system of urban and regional planning based on utilizing quality-of-life indicators as the guiding logic, followed by a second period of planning efforts marked by the new concerns with attaining greater compliance with goals and implementation strategies that center on the idea of sustainable urban and regional planning. While there are significant differences in the level of quality-of-life achieved by different regions throughout the county, it is clear that the physical character and quality-of-life in many North American cities have improved considerably. Yet, this achievement has manifested in a vast increase in the consumption of land and non-renewable resources. It is evident that the movement toward a sustainable urbanism, despite its colloquial acceptability, has produced minimal results in the country al large. The path toward a sustainable urbanism based on implementing sound planning and land use policies has not been able to surpass the general social apathy to change the individual and collective attitudes inherent in our cultural life styles. The new call for the 21st century is based on new measures of achieving a level of livability -a sustainable quality-of-life based on a widespread and well informed public involvement that assume the responsibility to, and the possibility of, changing our existing life styles demanding better urban and regional planning models that are fitting with the necessities of curving significantly our ecological footprint as presently utilized