Wolf-Rayet Stars in Starburst Galaxies

Wolf-Rayet stars have been detected in a large number of galaxies experiencing intense bursts of star formation. All stars initially more massive than a certain, metallicity-dependent, value are believed to experience the Wolf-Rayet phase at the end of their evolution, just before collapsing in supernova explosion. The detection of Wolf-Rayet stars puts therefore important constraints on the evolutionary status of starbursts, the properties of their Initial Mass Functions and their star formation regime. In this contribution we review the properties of galaxies hosting Wolf-Rayet stars, with special emphasis on the factors that determine their presence and evolution, as well as their impact on the surrounding medium.Comment: Contribution to the Proceedings of the JENAM 99 conference "The interplay between massive stars and the ISM", held in Toulouse in September 7-11, 1999. 10 pages, 5 figures. Requires elsart.cls latex macr

Household characteristics and consumption behaviour: a nonparametric approach

In this paper we apply nonparametric methods in order to discuss sorne empirical aspects of household consumption behaviour. First, we study the differences in the consumption behaviour between household types. We find that, except for food, there are no elear significant differences. Secondly, we derive the functional form for the food Engel curve, using specification tests consistent in the direction of nonparametric alternatives. Finally, we use this specification to discuss the misleading conelusions that could be reached from a mechanic interpretation of the rejection of Hausman's test, when applied to test the exogeneity of expenditure. The data is obtained from the Spanish Expenditure Survey 1980-81 and 1990-91

Quantifying Einstein-Podolsky-Rosen steering

Einstein-Podolsky-Rosen (EPR) steering is a form of bipartite quantum correlation that is intermediate between entanglement and Bell nonlocality. It allows for entanglement certification when the measurements performed by one of the parties are not characterised (or are untrusted) and has applications in quantum key distribution. Despite its foundational and applied importance, EPR steering lacks a quantitative assessment. Here we propose a way of quantifying this phenomenon and use it to study the steerability of several quantum states. In particular we show that every pure entangled state is maximally steerable, the projector onto the anti-symmetric subspace is maximally steerable for all dimensions, we provide a new example of one-way steering, and give strong support that states with positive-partial-transposition are not steerable.Comment: 9 pages, 1 figure. v2: One example (ex. (iv)) removed. One appendix (E) and one reference ([28]) added. V3: new example of one-way steering included, typos corrected, new reference

Patterns arising from the interaction between scalar and vectorial instabilities in two-photon resonant Kerr cavities

We study pattern formation associated with the polarization degree of freedom of the electric field amplitude in a mean field model describing a nonlinear Kerr medium close to a two-photon resonance, placed inside a ring cavity with flat mirrors and driven by a coherent $\hat x$-polarized plane-wave field. In the self-focusing case, for negative detunings the pattern arises naturally from a codimension two bifurcation. For a critical value of the field intensity there are two wave numbers that become unstable simultaneously, corresponding to two Turing-like instabilities. Considered alone, one of the instabilities would originate a linearly polarized hexagonal pattern whereas the other instability is of pure vectorial origin and would give rise to an elliptically polarized stripe pattern. We show that the competition between the two wavenumbers can originate different structures, being the detuning a natural selection parameter.Comment: 21 pages, 6 figures. http://www.imedea.uib.es/PhysDep

Mean-field solution of the parity-conserving kinetic phase transition in one dimension

A two-offspring branching annihilating random walk model, with finite reaction rates, is studied in one-dimension. The model exhibits a transition from an active to an absorbing phase, expected to belong to the $DP2$ universality class embracing systems that possess two symmetric absorbing states, which in one-dimensional systems, is in many cases equivalent to parity conservation. The phase transition is studied analytically through a mean-field like modification of the so-called {\it parity interval method}. The original method of parity intervals allows for an exact analysis of the diffusion-controlled limit of infinite reaction rate, where there is no active phase and hence no phase transition. For finite rates, we obtain a surprisingly good description of the transition which compares favorably with the outcome of Monte Carlo simulations. This provides one of the first analytical attempts to deal with the broadly studied DP2 universality class.Comment: 4 Figures. 9 Pages. revtex4. Some comments have been improve

Index Insurance, Production Practices, and Probabilistic Climate Forecasts

The failure of the development of commercially viable traditional crop insurance products and innovations in financial markers has fed a renewed interest in the search for alternatives to help producers in developing countries manage their risk exposure. Salient among these is the proposal of several index insurance schemes against weather events. Among the basic tenets are that the presence of index insurance allows producers to intensify their operations and reduce the risks of default and hence may induce creditors to offer loans at affordable rates. The two factors combined are touted as key to help producers in developing countries escape poverty traps. Improvements in seasonal climate forecasts create challenges for the design and effective functioning of the insurance against climate risks. However, very little is known about potential synergies or conflicting impacts of these two institutions, and the interactions between them and input management decisions by producers. We find that insurance and forecast may have synergistic or conflicting effects on input decisions. In the presence of (state contingent) actuarially fair insurance, producers may prefer the forecast information not to be available, especially if the management options available do not result in sufficient changes in profitability. Perhaps surprisingly, we find that forecast information may induce producers to increase the amount of insurance purchased.Climate forecast, Index insurance, Input Decisions, Risk Management, Weather risks, Risk and Uncertainty,