Revisiting the luminosity function of single halo white dwarfs

White dwarfs are the fossils left by the evolution of low-and intermediate-mass stars, and have very long evolutionary timescales. This allows us to use them to explore the properties of old populations, like the Galactic halo. We present a population synthesis study of the luminosity function of halo white dwarfs, aimed at investigating which information can be derived from the currently available observed data. We employ an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences for metal poor progenitors as well as an accurate modeling of the observational biases. We find that because the observed sample of halo white dwarfs is restricted to the brightest stars only the hot branch of the white dwarf luminosity function can be used for such purposes, and that its shape function is almost insensitive to the most relevant inputs, like the adopted cooling sequences, the initial mass function, the density profile of the stellar spheroid, or the adopted fraction of unresolved binaries. Moreover, since the cut-off of the observed luminosity has not been yet determined only lower limits to the age of the halo population can be placed. We conclude that the current observed sample of the halo white dwarf population is still too small to obtain definite conclusions about the properties of the stellar halo, and the recently computed white dwarf cooling sequences which incorporate residual hydrogen burning should be assessed using metal-poor globular clusters.Comment: 9 pages, 9 figures, accepted for publication in A&

Quasi-static Response of a Timoshenko Beam Loaded by an Elastically Supported Moving Rigid Beam

The present paper is concerned with the quasi-static response of an elastic beam, loaded by a rigid beam, which is slowly transported along the elastic beam. The elastic beam is modelled as a Timoshenko beam. The present paper provides a limiting case of the model with constant distributed load that is often considered in the study of transported masses. The rigid beam is connected to the Timoshenko beam by means of an interface modelled as a Winkler foundation. We present a non-dimensional study on the influence of the interface stiffness upon the displacement, bending moment and shear force of the Timoshenko beam, when the rigid beam is assumed to suffer a prescribed transverse displacement. Special emphasis is laid on the distribution of pressure transmitted by the interface between the Timoshenko beam and the rigid beam. Considerable pressure concentrations are found to take place and the locations of the maximum bending moments in the Timoshenko beam move towards the ends of the rigid beam

Field Localization and Enhancement of Phase Locked Second and Third Harmonic Generation in Absorbing Semiconductor Cavities

We predict and experimentally observe the enhancement by three orders of magnitude of phase mismatched second and third harmonic generation in a GaAs cavity at 650nm and 433nm, respectively, well above the absorption edge. Phase locking between the pump and the harmonics changes the effective dispersion of the medium and inhibits absorption. Despite hostile conditions the harmonics become localized inside the cavity leading to relatively large conversion efficiencies. Field localization plays a pivotal role and ushers in a new class of semiconductor-based devices in the visible and UV ranges

Green's function of a finite chain and the discrete Fourier transform

A new expression for the Green's function of a finite one-dimensional lattice with nearest neighbor interaction is derived via discrete Fourier transform. Solution of the Heisenberg spin chain with periodic and open boundary conditions is considered as an example. Comparison to Bethe ansatz clarifies the relation between the two approaches.Comment: preprint of the paper published in Int. J. Modern Physics B Vol. 20, No. 5 (2006) 593-60

Optimizing omnidirectional reflection by multilayer mirrors

Periodic layered media can reflect strongly for all incident angles and polarizations in a given frequency range. Quarter-wave stacks at normal incidence are commonplace in the design of such omnidirectional reflectors. We discuss alternative design criteria to optimize these systems.Comment: 9 pages, 6 figures. To be published in J. Opt. A: Pure and Applied Optic

Planificación de la producción basada en control predictivo para plantas termosolares

[Resumen] Se presenta una estrategia basada en control predictivo para resolver la planificación de la producción en plantas termosolares con almacenamiento térmico participando en el mercado diario de electricidad. Mediante esta estrategia, la producción de electricidad es actualizada regularmente haciendo uso del estado actual de la planta y de las más recientes predicciones para el precio de la energía y el recurso solar. La estrategia propuesta se aplica, en un contexto de simulación, a una planta basada en colectores cilindro-parabólicos de 50 MW con almacenamiento térmico bajo las hipótesis de predicción perfecta de precios y participación en el mercado diario español. Se ha analizado un caso de estudio basado en un periodo de cuatro meses con el propósito de abarcar una gran variedad de condiciones meteorológicas. Se han empleado valores reales para los precios de la energía, los costes de penalización, el recurso solar y su predicción. Los resultados muestran una mejora económica significativa frente a la tradicional estrategia a un día vista.Ministerio de Economia y Competitividad; DPI2016-76493-C3-2-

Amicable pairs and aliquot cycles for elliptic curves

An amicable pair for an elliptic curve E/Q is a pair of primes (p,q) of good reduction for E satisfying #E(F_p) = q and #E(F_q) = p. In this paper we study elliptic amicable pairs and analogously defined longer elliptic aliquot cycles. We show that there exist elliptic curves with arbitrarily long aliqout cycles, but that CM elliptic curves (with j not 0) have no aliqout cycles of length greater than two. We give conjectural formulas for the frequency of amicable pairs. For CM curves, the derivation of precise conjectural formulas involves a detailed analysis of the values of the Grossencharacter evaluated at a prime ideal P in End(E) having the property that #E(F_P) is prime. This is especially intricate for the family of curves with j = 0.Comment: 53 page

Optimization of organized silicon nanowires growth inside porous anodic alumina template using hot wire chemical vapor deposition process

International audienceA Hot Wire assisted Chemical Vapor Deposition (HWCVD) process has been developed for producing highdensity arrays of parallel, straight and organized silicon nanowires (SiNWs) inside vertical Porous Anodic Alumina (PAA) templates, exploring temperatures ranging from 430 °C to 600 °C, and pressures varying between 2.5 and 7.5 mbar. In order to prevent parasitic amorphous silicon (a-Si) deposit and to promote the crystalline SiNWs growth, we used a tungsten hot wire to partially crack H2 into atomic hydrogen, which acts like a selective etchant regarding a-Si. Here we describe the optimization route we followed to limit the deposit of a-Si onto the surface of the porous membrane and on the walls of the pores, which led to the possibility to grow SiNWs inside the PAA membranes. Such an approach has high potentialities for device realization, like PIN junctions, FETs or electrodes for Li-ion batteries

The age–metallicity relation in the solar neighbourhood from a pilot sample of white dwarf–main sequence binaries

The age–metallicity relation (AMR) is a fundamental observational constraint for understanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR for the solar neighbourhood, primarily due to the difficulty in obtaining accurate stellar ages for individual field stars. We have started an observational campaign for providing the much needed observational input by using wide white-dwarf–main-sequence (WDMS) binaries. White dwarfs are ‘natural’ clocks and can be used to derive accurate ages. Metallicities can be obtained from the main-sequence companions. Since the progenitors of white dwarfs and the main-sequence stars were born at the same time, WDMS binaries provide a unique opportunity to observationally constrain in a robust way the properties of the AMR. In this work we present the AMR derived from analysing a pilot sample of 23 WDMS binaries and provide clear observational evidence for the lack of correlation between age and metallicity at young and intermediate age