25,815 research outputs found
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&
A robust motion estimation and segmentation approach to represent moving images with layers
The paper provides a robust representation of moving images based on layers. To that goal, we have designed efficient motion estimation and segmentation techniques by affine model fitting suitable for the construction of layers. Layered representations, originally introduced by Wang and Adelson (see IEEE Transactions on Image Processing, vol.3, no.5, p.625-38, 1994) are important in several applications. In particular they are very appropriate for object tracking, object manipulation and content-based scalability which are among the main functionalities of the future MPEG-4 standard. In addition a variety of examples are provided that give a deep insight into the performance bounds of the representation of moving images using layers.Peer ReviewedPostprint (published version
Endosperm sterol phenotype and germination in wheat
Free and conjugated sterols of endosperm, coats, scutellum, coleoptile and roots have been analysed at different germination stages in two wheat cultivars with different endosperm sterol phenotypes. It seems that sterol metabolism of the developing tissues, namely coleoptile and roots, is not affected by the sterol conjugation profile of the endosperm. Enough sterol is present in the mature embryo to supply the germinating axis during the observation period (144 hr at 16°). The data suggest that sterol is transferred from scutellum to coleoptile and roots during germinatio
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
On equilibrium existence in infinite horizon economies
In sequential economies with finite or infinite-lived real assets in positive net supply, we introduce constraints on the amount of borrowing in terms of the market value of physical endowments. We show that, when utility functions are either unbounded and separable in states of nature or separable in commodities, these borrowing constraints not only preclude Ponzi schemes but also induce endogenous Radner bounds on short-sales. Therefore, we obtain existence of equilibrium. Moreover, equilibrium also exists when both assets are numerarie and utility functions are quasilinear in the commodity used as numerarie.Equilibrium, Infinite horizon incomplete markets, Infinite-lived real assets.
The effects of metallicity on the Galactic disk population of white dwarfs
It has been known for a long time that stellar metallicity plays a
significant role in the determination of the ages of the different Galactic
stellar populations, when main sequence evolutionary tracks are employed. Here
we analyze the role that metallicity plays on the white dwarf luminosity
function of the Galactic disk, which is often used to determine its age. We
employ a Monte Carlo population synthesis code that accounts for the properties
of the population of Galactic disk white dwarfs. Our code incorporates the most
up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich
and hydrogen-deficient atmospheres for both carbon-oxygen and oxygen-neon
cores. We use two different models to assess the evolution of the metallicity,
one in which the adopted metallicity is constant with time, but with a moderate
dispersion, and a second one in which the metallicity increases with time. We
found that our theoretical results are in a very satisfactory agreement with
the observational luminosity functions obtained from the Sloan Digital Sky
Survey (SDSS) and from the SuperCOSMOS Sky Survey (SSS), independently of the
adopted age-metallicity law. In particular, we found that the age-metallicity
law has no noticeable impact in shaping the bright branch of the white dwarf
luminosity function, and that the position of its cut-off is almost insensitive
to the adopoted age-metallicity relationship. Because the shape of the bright
branch of the white dwarf luminosity function is insensitive to the
age-metallicity law, it can be safely employed to test the theoretical
evolutionary sequences, while due to the limited sensitivity of the position of
the drop-off to the distribution of metallicities, its location provides a
robust indicator of the age of the Galactic disk.Comment: 7 pages, 5 figures, accepted for publication in A&
Object-Based Greenhouse Mapping Using Very High Resolution Satellite Data and Landsat 8 Time Series
Greenhouse mapping through remote sensing has received extensive attention over the last decades. In this article, the innovative goal relies on mapping greenhouses through the combined use of very high resolution satellite data (WorldView-2) and Landsat 8 Operational Land Imager (OLI) time series within a context of an object-based image analysis (OBIA) and decision tree classification. Thus, WorldView-2 was mainly used to segment the study area focusing on individual greenhouses. Basic spectral information, spectral and vegetation indices, textural features, seasonal statistics and a spectral metric (Moment Distance Index, MDI) derived from Landsat 8 time series and/or WorldView-2 imagery were computed on previously segmented image objects. In order to test its temporal stability, the same approach was applied for two different years, 2014 and 2015. In both years, MDI was pointed out as the most important feature to detect greenhouses. Moreover, the threshold value of this spectral metric turned to be extremely stable for both Landsat 8 and WorldView-2 imagery. A simple decision tree always using the same threshold values for features from Landsat 8 time series and WorldView-2 was finally proposed. Overall accuracies of 93.0% and 93.3% and kappa coefficients of 0.856 and 0.861 were attained for 2014 and 2015 datasets, respectively
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