The white dwarf population within 40 pc of the Sun

The white dwarf luminosity function is an important tool to understand the properties of the Solar neighborhood, like its star formation history, and its age. Here we present a population synthesis study of the white dwarf population within 40~pc from the Sun, and compare the results of this study with the properties of the observed sample. We use a state-of-the-art population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the Solar neighborhood employing the position of the observed cut-off of the white dwarf luminosity function, obtaining ~8.9+-0.2 Gyr. We conclude that a detailed description of the ensemble properties of the population of white dwarfs within 40pc of the Sun allows us to obtain interesting constraints on the history of the Solar neighborhood.Comment: 8 pages, 7 figures, accepted for publication in A&

Simulating Gaia performances on white dwarfs

One of the most promising space missions of ESA is the astrometric satellite Gaia, which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to V~20, and radial velocities with accuracies of a few km/s for most stars brighter than V ~ 17. Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by Gaia. From this we assess which would be the white dwarf luminosity functions which Gaia will obtain and discuss in depth the scientific returns of Gaia in the specific field of white dwarf populations. Scientific attainable goals include, among others, a reliable determination of the age of the Galactic disk, a better knowledge of the halo of the Milky Way and the reconstruction of the star formation history of the Galactic disk. Our results also demonstrate the potential impact of a mission like Gaia in the current understanding of the white dwarf cooling theory.Comment: 13 pages, 17 figures, accepted for publication in MNRA

New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

Cool white dwarfs are reliable and independent stellar chronometers. The most common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling ages of very cool white dwarfs sensitively depend on the adopted phase diagram of the carbon-oxygen binary mixture. A new phase diagram of dense carbon-oxygen mixtures appropriate for white dwarf interiors has been recently obtained using direct molecular dynamics simulations. In this paper, we explore the consequences of this phase diagram in the evolution of cool white dwarfs. To do this we employ a detailed stellar evolutionary code and accurate initial white dwarf configurations, derived from the full evolution of progenitor stars. We use two different phase diagrams, that of Horowitz et al. (2010), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993), which is of the spindle form. We computed the evolution of 0.593 and 0.878M_sun white dwarf models during the crystallization phase, and we found that the energy released by carbon-oxygen phase separation is smaller when the new phase diagram of Horowitz et al. (2010) is used. This translates into time delays that are on average a factor about 2 smaller than those obtained when the phase diagram of Segretain & Chabrier (1993) is employed. Our results have important implications for white dwarf cosmochronology, because the cooling ages of very old white dwarfs are different for the two phase diagrams. This may have a noticeable impact on the age determinations of very old globular clusters, for which the white dwarf color-magnitude diagram provides an independent way of estimating their age.Comment: 7 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

La entrega de acciones a los empleados: aspectos controvertidos de su tributación en el IRPF

La entrega de acciones a los empleados como retribución en especie del trabajo ha planteado siempre numerosos aspectos dudosos en el ámbito del IRPF. Algunos de ellos no han sido resueltos de manera pacifica y otros ni siquiera se han abordado por los autores. En este análisis destacamos algunas de las cuestiones que más interés revisten en este momento, como premisa para plantear un cambio radical de la regulación vigente. A tal efecto, se examina el encaje de la entrega de acciones como supuesto libre de tributación, considerando seguidamente las posibles consecuencias de la norma en periodos impositivos posteriores, al transmitirse las acciones recibidas. Este aspecto enlaza con el análisis de alguna sentencia reciente del Tribunal Supremo, donde se establece el cómputo del ingreso a cuenta no repercutido al trabajador (al serle entregados los títulos) como mayor valor de adquisición en el momento de ser transmitidos, conclusión que no compartimos en absoluto. Para concluir hemos expuesto algunas consideraciones críticas a propósito de la reducción prevista en las normas vigentes para los rendimientos irregulares, teniendo en cuenta que con frecuencia dicha reducción se aplica a los rendimientos generados por la entrega de acciones, en particular cuando derivan del ejercicio de opciones sobre acciones (stock options). El trabajo se cierra con una síntesis de las conclusiones alcanzadas.Stocks giving to employees as a payment in kind for their work, has always created doubtful aspects in the field of personal income tax. Some of them haven't been solved in a peaceful way and others have not even been handled by people concerned. Some of the most interesting current matters are remarked in this study, as a premise to implant a radical change in the present regulation. To this end, we look into the stocks giving as an assumption free of taxation, considering then, the possible rule consequences in post-taxation periods when the received stocks are transferred. This aspect, connects with the analysis of some recent sentence of the High Court, where it is set up the calculation of the receipts on account with no repercusion to the worker (as they are given the titles) as a bigger value of acquisition at the moment of their transfer, a conclusion we don't share at all. To conclude, we have explained some critical considerations about the expected reduction in the current rules, for irregular profits, considering that this reduction, is often applied to the profits generated by stocks giving, particularly when they derive from stocks options. This study finishes with a synthesis of the obtained conclusions

The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution

NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that can be imaged down to luminosities fainter than that of the termination of its white dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that the number distribution of secondary masses of the population of unresolved binaries has to increase with increasing mass ratio between the secondary and primary components of the progenitor system. We also find that the observed cooling sequence appears to be able to constrain the presence of progenitor sub-populations with different chemical compositions and the fraction of non-DA white dwarfs. Our simulations place interesting constraints on important characteristics of the stellar populations of NGC 6791. In particular, we find that the fraction of single helium-core white dwarfs must be smaller than 5%, that a sub-population of stars with zero metallicity must be <12%, while if the adopted metallicity of the sub-population is solar the upper limit is ~8%. Finally, we also find that the fraction of non-DA white dwarfs in this particular cluster is surprinsingly small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy & Astrophysic