Mostrando recursos 21 - 40 de 194

  1. 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...

  2. 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...

  3. 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...

  4. 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)

  5. 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.

  6. 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...

  7. Underground study of the 17O(p,alpha)18F reaction relevant for explosive hydrogen burning

    Di Leva, A; Scott, D.A.; Caciolli, A.; Formicola, A.; Strieder, F.; Aliotta, M.; Anders, M.; Bemmerer, D.; Broggini, C.; Corvisiero, P.; Elekes, Z.; Fulop, Zs.; Gervino, G.; Guglielmetti, G.; Gustavino, C.; Gyurky, Gy.; Imbriani, G.; José Pont, Jordi; Junker, M.; et al., .
    The 17O(p,γ )18F reaction affects the production of key isotopes (e.g., 18F and 18O) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at Ec.m.= 183 keV and by the combined contributions of direct capture and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae.

  8. High-resolution smoothed particle hydrodynamics simulations of the merger of binary white dwarfs

    Lorén Aguilar, Pablo; Isern Vilaboy, Jordi; García-Berro Montilla, Enrique
    Context. The coalescence of two white dwarfs is the final outcome of a sizeable fraction of binary stellar systems. Moreover, this process has been proposed to explain several interesting astrophysical phenomena. Aims. We present the results of a set of high-resolution simulations of the merging process of two white dwarfs. Methods. We use an up-to-date Smoothed Particle Hydrodynamics code which incorporates very detailed input physics and an improved treatment of the artificial viscosity. Our simulations have been done using a large number of particles (~ 4 × 105 ) and cover the full range of masses and chemical compositions of the coalescing white dwarfs. We also...

  9. The white dwarf population of tri-axial haloes

    Torres Gil, Santiago; Camacho Díaz, Judit; García-Berro Montilla, Enrique
    The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a controversial subject. Substellar objects and stars with masses larger than ~ 1M¿ have been ruled out as major components of a Massive Astrophysical Halo Object (MACHO) Galactic halo. Stars of near half-solar mass, in particular white dwarfs, appear to be the best candidates to explain the observed microlensing events. On the other hand, observational evidence based on the structure of the debris of the Sagittarius dwarf galaxy suggest that the Galactic halo is non-spherical. We use a tri-axial halo model and advanced Monte Carlo techniques to...

  10. Monte Carlo simulations of post-common-envelope white dwarf + main sequence binaries: The effects of including recombination energy

    Zorotovic, Monica; García-Berro Montilla, Enrique; Schreiber, Mathias R.; Camacho Díaz, Judit; Torres Gil, Santiago; Rebassa Mansergas, Alberto; Gänsicke, Boris T.
    Context. Detached white dwarf + main sequence (WD+MS) post-common-envelope binaries (PCEBs) are perhaps the most suitable objects for testing predictions of close-compact binary-star evolution theories, in particular, common-envelope (CE) evolution. Consequently, the population of WD+MS PCEBs has been simulated by several authors in the past and the predictions have been compared with the observations. However, most of those theoretical predictions did not take into account the possible contributions to the envelope ejection from additional sources of energy (mostly recombination energy) stored in the envelope. Aims. Here we update existing binary population models of WD+MS PCEBs by assuming that in addition to...

  11. On the origin of high-field magnetic white dwarfs

    García-Berro Montilla, Enrique; Torres Gil, Santiago; Loren Aguilar, Pablo; Aznar Siguan, Gabriela; Camacho Díaz, Judit; Kulebi, Baybar; Isern Vilaboy, Jordi; Althaus, Leandro Gabriel; Corsico, Alejandro H.
    Every two years, white dwarf researchers and enthusiasts meet to exchange their knowledge and discuss recent developments in white dwarf theory and observations. These proceedings are from the 18th European White Dwarf Workshop held at Pedagogical University of Cracow (Krakow, Poland) on 13th - 17th August, 2012. The scientific topics discussed in this Workshop included: pulsating white dwarfs; luminosity function, mass distribution, and populations; white dwarf structure and evolution; white dwarf catalogs and surveys; central stars of planetary nebulae; supernova progenitors; white dwarfs in novae and cataclysmic variables; white dwarfs in detached binaries; physical processes in white dwarfs and magnetic...

  12. Double degenerate mergers as progenitors of high-field magnetic white dwarfs

    García-Berro Montilla, Enrique; Lorén Aguilar, Pablo; Aznar Siguan, Gabriela; Torres Gil, Santiago; Camacho Díaz, Judit; Althaus, Leandro Gabriel; Córsico, Alejando Hugo; Kulebi, Baybar; Isern Vilaboy, Jordi
    High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mech anism that produces the magnetic field are still unknown. Here we show that the hot, convective, diffe rentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores is able to produce magnetic fields of the required strength that do not decay for long timescales. We also show, using an state-of-the-art Monte Carlo simulator, that the expected number of high-field magn etic white dwarfs produced in this way is consistent with that found in the Solar neighborhood.

  13. Monte Carlo simulations of post-common-envelope white dwarf plus main sequence binaries: comparison with the SDSS DR7 observed sample

    Camacho Díaz, Judit; Torres Gil, Santiago; García-Berro Montilla, Enrique; Zorotovic, Monica; Schreiber, Mathias R.; Rebassa-Mansergas, Alberto; Nebot Gómez-Moran, Ada; Gänsicke, Boris T.
    Context. Detached white dwarf + main sequence (WD+MS) systems represent the simplest population of post-common envelope binaries (PCEBs). Since the ensemble properties of this population carries important information about the characteristics of the common-envelope (CE) phase, it deserves close scrutiny. However, most population synthesis studies do not fully consider the effects of the observational selection biases of the samples used to compare with the theoretical simulations.; Aims. Here we present the results of a set of detailed Monte Carlo simulations of the population of WD+MS binaries in the Sloan Digital Sky Survey (SDSS) Data Release 7.; Methods. We used up-to-date...

  14. White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy

    Torres Gil, Santiago; Camacho Díaz, Judit; Isern Vilaboy, Jordi; García-Berro Montilla, Enrique
    Peer Reviewed

  15. Observational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain

    Soler Duffour, Maria Rosa; Udina, Mireia; Ferreres Soler, Enriqueta
    A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific...

  16. Observational and Numerical Simulation Study of a Sequence of Eight Atmospheric Density Currents in Northern Spain

    Soler Duffour, Maria Rosa; Udina, Mireia; Ferreres Soler, Enriqueta
    A sequence of eight atmospheric density current fronts occurred in consecutive days are identified and analyzed using micrometeorological time series and numerical simulations. Observations were collected in the context of the INTERCLE project, which took place from September 2002 to November 2003 at the CIBA (Research Centre for the Lower Atmosphere) site located over the northern Spanish plateau. Numerical simulations used the Weather Research and Forecast (WRF) model with fine horizontal resolution (1 km). Both observations and simulations agree that the arrival of the density currents are characterized by a sharp change in temperature, wind velocity, wind direction and specific...

  17. Cobalt-56 gamma-ray emission lines from the type Ia supernova 2014J

    Churazov, Eugene; Sunyaev, Rashid; Isern Vilaboy, Jordi; Knodlseder, Jurgen; Jean, Pierre; Lebrun, François; Chugai, Nikolai; Grebenev, Sergei; Bravo Guil, Eduardo; Sazonov, Sergei; Renaud, Matthieu
    A type Ia supernova is thought to be a thermonuclear explosion of either a single carbon–oxygen white dwarf or a pair of merging white dwarfs. The explosion fuses a large amount of radioactive 56Ni. After the explosion, the decay chain from 56Ni to 56Co to 56Fe generates gamma-ray photons, which are reprocessed in the expanding ejecta and give rise to powerful optical emission. Here we report the detection of 56Co lines at energies of 847 and 1,238 kiloelectronvolts and a gamma-ray continuum in the 200–400 kiloelectronvolt band from the type Ia supernova 2014J in the nearby galaxy M82. The line...

  18. Comparison of theoretical white dwarf cooling timescales (Research Note)

    Salaris, Maurizio; Althaus, Leandro Gabriel; García-Berro Montilla, Enrique
    Context. An accurate assessment of white dwarf cooling times is paramount so that white dwarf cosmochronology of Galactic populations can be put on more solid grounds. This issue is particularly relevant in view of the enhanced observational capabilities provided by the next generation of extremely large telescopes, that will offer more avenues to use white dwarfs as probes of Galactic evolution and test-beds of fundamental physics. Aims. We estimate for the first time the consistency of results obtained from independent evolutionary codes for white dwarf models with fixed mass and chemical stratification, when the same input physics is employed in...

  19. The post-merger magnetized evolution of white dwarf binaries: The double-degenerate channel of sub-Chandrasekhar type Ia supernovae and the formation of magnetized white dwarfs

    Ji, Suoqing; Fisher, Robert T.; García-Berro Montilla, Enrique; Tzeferacos, Petros; Jordan, George C.; Lee, Dongwook; Loren Aguilar, Pablo; Cremer, Pascal; Behrends, Jan
    Type Ia supernovae (SNe Ia) play a crucial role as standardizable cosmological candles, though the nature of their progenitors is a subject of active investigation. Recent observational and theoretical work has pointed to merging white dwarf binaries, referred to as the double-degenerate channel, as the possible progenitor systems for some SNe Ia. Additionally, recent theoretical work suggests that mergers which fail to detonate may produce magnetized, rapidly rotating white dwarfs. In this paper, we present the first multidimensional simulations of the post-merger evolution of white dwarf binaries to include the effect of the magnetic field. In these systems, the two...

  20. The effects of metallicity on the Galactic disk population of white dwarfs

    Cojocaru, Elena Ruxandra; Torres Gil, Santiago; Isern Vilaboy, Jordi; García-Berro Montilla, Enrique
    Context. 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. Aims. 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. Methods. 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...

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