Mostrando recursos 121 - 140 de 497

  1. Spiral instability can drive thermonuclear explosions in binary white dwarf mergers

    kashyap, Rahul; Fisher, Robert T.; García-Berro Montilla, Enrique; Aznar Siguan, Gabriela; Ji, Suoqing; Loren Aguilar, Pablo
    Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon-oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger...

  2. The variation of the fine structure constant: testing the dipole model with thermonuclear supernovae

    Kraiselburd, L; Landau, S. J.; García-Berro Montilla, Enrique; Negrelli, C
    The large-number hypothesis conjectures that fundamental constants may vary. Accordingly, the space-time variation of fundamental constants has been an active subject of research for decades. Recently, using data obtained with large telescopes a phenomenological model in which the fine structure constant might vary spatially has been proposed. We test whether this hypothetical spatial variation of alpha, which follows a dipole law, is compatible with the data of distant thermonuclear supernovae. Unlike previous works, in our calculations we consider not only the variation of the luminosity distance when a varying alpha is adopted, but we also take into account the variation...

  3. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    Corsico, Alejandro H.; Althaus, Leandro G.; Miller Bartolami, Marcelo M.; Kepler, S.O.; García-Berro Montilla, Enrique
    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (mu(nu)) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a...

  4. The white dwarf cooling sequence of 47 Tucanae

    García-Berro Montilla, Enrique; Torres Gil, Santiago; Althaus, Leandro G.; Miller Bartolami, Marcelo M.
    Context. 47 Tucanae is one of the most interesting, well-observed, and theoretically studied globular clusters. This allows us to determine the reliability of our understanding of white dwarf cooling sequences, to compare different methods of determining its age, and to assess other important characteristics, such as its star formation history. Aims. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. Methods. To do this, we used an up-to-date population synthesis...

  5. Population Synthesis of Post-Common Envelope White-Dwarf-Main-Sequence Binaries in the Sloan Digital Sky Survey

    Torres Gil, Santiago; Camacho Díaz, Judit; Rebassa Mansergas, Alberto; Zorotovic, Mónica; Schreiber, Mathias R.; García-Berro Montilla, Enrique
    We present preliminary results of detailed Monte Carlo simulations of the population of white dwarf-main sequence binary systems in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7). We have used the most up-to-date stellar evolutionary models, a complete treatment of the Roche lobe overflow episode, as well as a full implementation of the orbital evolution of the binary system. Moreover, we included the different selection criteria and observational biases within SDSS and examined the role played by the binding energy parameter and by the common envelope efficiency. Our Monte Carlo simulator correctly reproduces the properties of the observed...

  6. Transitions between symmetric and nonsymmetric regimes in binary-mixture convection

    Meca Alvarez, Esteban; Mercader Calvo, María Isabel; Ramírez de la Piscina Millán, Laureano
    We present here a comprehensive picture of the different bifurcations found for small to moderate Rayleigh number in binary-mixture convection with lateral heating and negative separation ratio (S). The present work connects the symmetric regime found for pure fluid (S=0)[Mercader et al 2005] with the fundamentally nonsymmetric regime found for S=-1 [Meca et al 2004a-2004b]. We give a global context as well as an interpretation for the different associations of bifurcations found, and in particular we interpret an association of codimension-two bifurcations in terms of a higher codimension bifurcation never found, to our knowledge, in the study of an extended system.

  7. Novel ionospheric activity indicator specifically tailored for GNSS users

    Sanz Subirana, Jaume; Juan Zornoza, José Miguel; González Casado, Guillermo; Prieto Cerdeira, Roberto; Schlüter, S.; Orús Pérez, Raul
    This work introduces a novel ionospheric activity indicator useful for identifying disturbed periods affecting performance for GNSS users, at regional level. This indicator is based in the “Along Arc TEC Rate (AATR) and can be easily computed from GNSS data. The AATR indicator has been assessed over more than one Solar Cycle (2002-2013) involving 140 receivers distributed world-wide. Results show that it is well correlated with the ionospheric activity and, unlike other global indicators linked to the geomagnetic activity (i.e. DST, Ap), it is sensitive to regional behaviour the ionosphere and identifies specific effects on GNSS users. Moreover from a...

  8. Super- and massive AGB stars - IV. Final fates initial-to-final mass relation

    Doherty, Carolyn L.; Gil Pons, Pilar; Siess, Lionel; Lattanzio, John C.; Lau, Herbert B.
    We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero-age main sequence until near the end of the thermally pulsing phase. These super-asymptotic giant branch (super-AGB) and massive AGB star models are in the mass range between 5.0 and 10.0 M circle dot for metallicities spanning the range Z = 0.02-0.0001. We probe the mass limits M-up, M-n and M-mass, the minimum masses for the onset of carbon burning, the formation of a neutron star and the iron core-collapse supernovae, respectively, to constrain the white dwarf/electron-capture supernova (EC-SN) boundary. We provide a theoretical...

  9. White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up

    Althaus, Leandro G.; Camisassa, Maria E; Miller Bertolami, Marcelo M; Corsico, Alejandro H.; García-Berro Montilla, Enrique
    Context. White dwarfs are nowadays routinely used as reliable cosmochronometers, allowing several stellar populations to be dated.; Aims. We present new white dwarf evolutionary sequences for low-metallicity progenitors. This is motivated by the recent finding that residual H burning in low-mass white dwarfs resulting from Z = 0.0001 progenitors is the main energy source over a significant part of their evolution.; Methods. White dwarf sequences have been derived from full evolutionary calculations that take the entire history of progenitor stars into account, including the thermally pulsing and the post-asymptotic giant branch (AGB) phases.; Results. We show that for progenitor metallicities...

  10. Constraining the double-degenerate scenario for Type Ia supernovae from merger ejected matter

    Levanon, Naveh; Soker, Noam; García-Berro Montilla, Enrique
    We follow the mass expelled during the WD-WD merger process in a particular case of the double-degenerate (DD) scenario for Type Ia supernovae (SNe Ia), and find that the interaction of the SN ejecta with the resulting wind affects the early (first day) light-curve in a way that may be in conflict with some SN Ia observations, if the detonation occurs shortly after the merger, i.e. (10(3) s less than or similar to t(exp) less than or similar to 1 d). The main source of the expelled mass is a disc-wind, or jets that are launched by the accretion disc...

  11. White dwarf dynamical interactions

    Aznar Siguan, Gabriela
    Merging white dwarfs is a promising channel to trigger Type Ia supernovae, known as the double degenerate scenario. Supernovae are stellar explosions that radiate as much energy as any ordinary star is expected to emit over its entire life span, outshining briefly the whole hosting galaxy. They enrich the interstellar medium with higher mass elements and trigger the formation of new stars by the produced expanding shock. Additionally, Type Ia supernovae have been used as standard candles and have allowed the discovery that the universe was expanding at an accelerating rate. Despite the important role that Type Ia supernovae play...

  12. Super and massive AGB stars - III. nucleosynthesis in metal-poor and very metal-poor stars - Z = 0.001 and 0.0001

    Doherty, Carolyn L.; Gil Pons, Pilar; Lau, Herbert B.; Lattanzio, John C.; Siess, Lionel; Campbell, Simon W
    We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z = 0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal-rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of 4He, 13C, 14N and 27Al as well as the heavy magnesium isotopes 25Mg and 26Mg. There is a transition in yield...

  13. The large observatory for x-ray timing

    Feroci, Marco; den Herder, J.W.; Bozzo, Enrico; Barret, Didier; Brandt, S.; Hernanz Carbó, Margarita; van der Klis, M; Pohl, M; Santangelo, A; Stella, Luigi; Watts, ana; Willms, Jörn; Sala Cladellas, Glòria
    Postprint (published version)

  14. Contents

    W. J. Henney
    Este documento describe el uso del “L ATEX document class ” rmaa.cls, lo cuál está diseñado para preparar artículos para publicación en la Revista Mexicana de Astronomía y Astrofísica. This document describes the use of the L ATEX document class rmaa.cls, designed for the preparation of papers to be published in the main journal Revista Mexicana de Astronomía y Astrofísica.

  15. Determinación y parametrización de propiedades radiativas de plasmas para la simulación y análisis de experimentos de Astrofísica de Laboratorio

    Espinosa Vivas, Guadalupe
    Programa de doctorado: Sistemas inteligentes y aplicaciones numéricas en Ingeniería

  16. Comunicaciones presentadas en la IV Asamblea Nacional de Astronomía y Astrofísica, Santiago de Compostela

    Sevilla, Miguel J.; Camacho, Antonio G.; Romero, P.
    - Deformación de una tierra elástica por efecto de marea. - Comparación de series de nutación. - Programa "ARIES 2 ML" para reducción de posiciones de estrellas.

  17. Homothetic motions and Newtonian cosmology

    Jaen Herbera, Javier; Molina Compte, Alfred
    We construct a homothetic covariant Newtonian gravitation theory which unifies inertial homothetic forces and gravitational fields. This is achieved through an equivalence principle based on a local homothetic frame of motion. As a consequence, we can obtain a coherent Newtonian cosmology which admits a cosmological principle and leads to the Friedman equations for a dust universe. Finally we prove that this gravity theory can be obtained as the non-relativistic limit of a class of metrics in General Relativity. The Friedmann–Lemaître–Robertson–Walker metric and its limit are also studied.

  18. The unreasonable effectiveness of experiments in constraining nova nucleosynthesis

    Parikh, Anuj Ramesh
    Classical nova explosions arise from thermonuclear ignition in the envelopes of accreting white dwarfs in close binary star systems. Detailed observations of novae have stimulated numerous studies in theoretical astrophysics and experimental nuclear physics. These phenomena are unusual in nuclear astrophysics because most of the thermonuclear reaction rates thought to be involved are constrained by experimental measurements. This situation allows for rather precise statements to be made about which measurements are still necessary to improve the nuclear physics input to astrophysical models. We briefly discuss desired measurements in these environments with an emphasis on recent experimental progress made to better...

  19. Latest generation white dwarf cooling models : theory and applications

    Renedo Rouco, Isabel
    White dwarfs are the most common stellar evolutionary end-point. Moreover, they can be considered as reliable cosmic clocks to infer the age of a wide variety of stellar populations, including globular and open clusters. Consequently, there is a considerable interest in the study of white dwarf cooling models. In this thesis we used two different approaches. From a theorical perspective, we computed a set of new cooling white dwarfs sequences which incorporates the most up-to-date physical inputs for precision white dwarf cosmochronology and for asteroseismological studies of ZZ Ceti stars. Moreover, we studied the role of 22Ne diffusion in the...

  20. Recuperación de información astronómica : nuevas perspectivas de análisis de la cartografía celeste histórica

    Alonso Lifante, María Pilar
    La mejora de la descripción de contenidos de recursos especializados en general, y de astronómicos en particular, es todavía una asignatura pendiente de las Ciencias de la Documentación. Actualmente existe gran cantidad de información astronómica histórica de interés recogida en diversos tipos de documentos que se conservan en archivos y bibliotecas astronómicas de todo el mundo. Entre estos documentos se encuentran los denominados catálogos y atlas de estrellas, así como imágenes astrofotográficas, todavía compilados en la actualidad aunque en formatos bien diferentes y con datos mucho más precisos. Sin embargo, las medidas procedentes de observaciones varios siglos atrás siguen atrayendo...

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