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Garcia-Senz D.,Polytechnic University of Catalonia | Garcia-Senz D.,Institute Destudis Espacials Of Catalonia | Cabezon R.M.,University of Basel | Arcones A.,TU Darmstadt | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

The direct impact of white dwarfs has been suggested as a plausible channel for Type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centres, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curve sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper, we address some of the possible sources of these differences, presenting the results of high-resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D axisymmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 to 1.1M⊙, compatible not only with normal-branch Type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass function of white dwarfs favours collisions among 0.6-0.7M⊙ objects, leading to subluminous events.

Garcia-Senz D.,Polytechnic University of Catalonia | Garcia-Senz D.,Institute Destudis Espacials Of Catalonia | Badenes C.,Tel Aviv University | Badenes C.,Weizmann Institute of Science | And 2 more authors.
Astrophysical Journal | Year: 2012

In this paper, we report on the bulk features of the hole carved by the companion star in the material ejected during a Type Ia supernova (SN Ia) explosion. In particular we are interested in the long-term evolution of the hole as well as in its fingerprint in the geometry of the supernova remnant (SNR) after several centuries of evolution, which is a hot topic in current SN Ia studies. We use an axisymmetric smoothed particle hydrodynamics code to characterize the geometric properties of the SNR resulting from the interaction of this ejected material with the ambient medium. Our aim is to use SNR observations to constrain the single degenerate scenario for SN Ia progenitors. Our simulations show that the hole will remain open during centuries, although its partial or total closure at later times due to hydrodynamic instabilities is not excluded. Close to the edge of the hole, the Rayleigh-Taylor instability grows faster, leading to plumes that approach the edge of the forward shock. We also discuss other geometrical properties of the simulations, like the evolution of the contact discontinuity. © 2012. The American Astronomical Society. All rights reserved.

Downen L.N.,University of North Carolina at Chapel Hill | Downen L.N.,Triangle Universities Nuclear Laboratory | Iliadis C.,University of North Carolina at Chapel Hill | Iliadis C.,Triangle Universities Nuclear Laboratory | And 3 more authors.
Astrophysical Journal | Year: 2013

Classical novae are stellar explosions occurring in binary systems, consisting of a white dwarf and a main-sequence companion. Thermonuclear runaways on the surface of massive white dwarfs, consisting of oxygen and neon, are believed to reach peak temperatures of several hundred million kelvin. These temperatures are strongly correlated with the underlying white dwarf mass. The observational counterparts of such models are likely associated with outbursts that show strong spectral lines of neon in their shells (neon novae). The goals of this work are to investigate how useful elemental abundances are for constraining the peak temperatures achieved during these outbursts and determine how robust "nova thermometers" are with respect to uncertain nuclear physics input. We present updated observed abundances in neon novae and perform a series of hydrodynamic simulations for several white dwarf masses. We find that the most useful thermometers, N/O, N/Al, O/S, S/Al, O/Na, Na/Al, O/P, and P/Al, are those with the steepest monotonic dependence on peak temperature. The sensitivity of these thermometers to thermonuclear reaction rate variations is explored using post-processing nucleosynthesis simulations. The ratios N/O, N/Al, O/Na, and Na/Al are robust, meaning they are minimally affected by uncertain rates. However, their dependence on peak temperature is relatively weak. The ratios O/S, S/Al, O/P, and P/Al reveal strong dependences on temperature and the poorly known 30P(p, γ)31S rate. We compare our model predictions to neon nova observations and obtain the following estimates for the underlying white dwarf masses: 1.34-1.35 M (V838 Her), 1.18-1.21 M (V382 Vel), ≤1.3 M (V693 CrA), ≤1.2 M (LMC 1990#1), and ≤1.2 M (QU Vul). © 2013. The American Astronomical Society. All rights reserved..

Jose J.,Polytechnic University of Catalonia | Jose J.,Institute Destudis Espacials Of Catalonia
Bulletin of the Astronomical Society of India | Year: 2012

Classical nova outbursts are thermonuclear stellar explosions driven by charged-particle reactions. Extensive numerical simulations of nova explosions have shown that the accreted envelopes attain peak temperatures ranging between 0.1 and 0.4 GK, for about several hundred seconds, and therefore, their ejecta is expected to show signatures of nuclear processing. Indeed, it has been claimed that novae play some role in the enrichment of the interstellar medium in a number of intermediatemass elements. This includes 17O, 15N, and 13C, significantly overproduced with respect to solar abundances, and a lower contribution to a number of species with A < 40, such as 7Li, 19F, or 26Al. In this paper, we review the modeling of classical nova outbursts, from spherically-symmetric (1D) hydrodynamic models to recent multidi- mensional simulations. The predicted nucleosynthesis accompanying nova outbursts is briefly compared with the abundances determined in meteoritic presolar grains of putative nova origin. The impact of current nuclear uncertainties on the final yields is also outlined.

Jose J.,Polytechnic University of Catalonia | Jose J.,Institute Destudis Espacials Of Catalonia | Iliadis C.,University of North Carolina at Chapel Hill | Iliadis C.,Triangle Universities Nuclear Laboratory
Reports on Progress in Physics | Year: 2011

Half a century has passed since the foundation of nuclear astrophysics. Since then, this discipline has reached its maturity. Today, nuclear astrophysics constitutes a multidisciplinary crucible of knowledge that combines the achievements in theoretical astrophysics, observational astronomy, cosmochemistry and nuclear physics. New tools and developments have revolutionized our understanding of the origin of the elements: supercomputers have provided astrophysicists with the required computational capabilities to study the evolution of stars in a multidimensional framework; the emergence of high-energy astrophysics with space-borne observatories has opened new windows to observe the Universe, from a novel panchromatic perspective; cosmochemists have isolated tiny pieces of stardust embedded in primitive meteorites, giving clues on the processes operating in stars as well as on the way matter condenses to form solids; and nuclear physicists have measured reactions near stellar energies, through the combined efforts using stable and radioactive-ion beam facilities. This review provides comprehensive insight into the nuclear history of the Universe and related topics: starting from the Big Bang, when the ashes from the primordial explosion were transformed to hydrogen, helium and a few trace elements, to the rich variety of nucleosynthesis mechanisms and sites in the Universe. Particular attention is paid to the hydrostatic processes governing the evolution of low-mass stars, red giants and asymptotic giant-branch stars, as well as to the explosive nucleosynthesis occurring in core-collapse and thermonuclear supernovae, γ-ray bursts, classical novae, x-ray bursts, superbursts and stellar mergers. © 2011 IOP Publishing Ltd.

Dunsby P.K.S.,University of Cape Town | Dunsby P.K.S.,South African Astronomical Observatory | Elizalde E.,Institute Destudis Espacials Of Catalonia | Goswami R.,University of Cape Town | And 3 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

Several different explicit reconstructions of f(R) gravity are obtained from the background Friedmann-Laîmatre-Robertson-Walker expansion history. It is shown that the only theory whose Lagrangian is a simple function of the Ricci scalar R, that admits an exact ΛCDM expansion history, is standard general relativity with a positive cosmological constant and the only way to obtain this behavior of the scale factor for more general functions of R is to add additional degrees of freedom to the matter sector. © 2010 The American Physical Society.

Ruiz X.,Institute Destudis Espacials Of Catalonia | Ruiz X.,Rovira i Virgili University | Pallares J.,Rovira i Virgili University
International Journal of Heat and Mass Transfer | Year: 2012

Three different shear cell arrangements are considered here in order to discuss and compare the accuracy of their measurements at low and moderate Rayleigh number, that is to say, in reduced gravity environments. To do so, specific numerical procedures have been designed in order to quantitatively estimate the corresponding associated errors as a function of time and at the end of the diffusive process. The present work clearly shows that the interdiffusion-couple configuration is, from the point of view of accuracy, the worst one. © 2012 Elsevier Ltd. All rights reserved.

Carreno-Luengo H.,University of Barcelona | Camps A.,University of Barcelona | Ramos-Perez I.,University of Barcelona | Rius A.,Institute Destudis Espacials Of Catalonia
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2014

This work describes a novel dual-band Global Navigation Satellite Systems Reflectometer (GNSS-R) that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements. The results derived from two different ground-based field experiments over a dam and over the sea under different surface's roughness conditions are presented. The analysis of the altimetric performance shows that the results obtained using the P(Y) code improve by a factor between 1.4 and 2.4 as compared to the results obtained using the C/A code, respectively, for high and mid-low satellite's elevation angles. © 2008-2012 IEEE.

Carreno-Luengo H.,University of Barcelona | Park H.,University of Barcelona | Camps A.,University of Barcelona | Fabra F.,Institute Destudis Espacials Of Catalonia | Rius A.,Institute Destudis Espacials Of Catalonia
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing | Year: 2013

The results of two airborne experiments performed to test the precision and the relative accuracy of the conventional Global Navigation Satellite Systems Reflectometry (GNSS-R) technique employing only the C/A code are presented. The first and the second experiments demonstrate, respectively, a 17 cm precision for a 500 m flight altitude with a 8 km along-track spatial resolution, and a 6 cm precision for a 3000 m flight altitude with a 6.6 km along-track spatial resolution. In both, the Relative Mean Dynamic Topography (RMDT) is compared with results derived from traditional radar altimetry provided by Jason-2. The Root Mean Square (RMS) of the RMDT difference between both measurement systems is 48 cm for the first flight, and 198 cm for the second flight. During the second flight, the feasibility of the proposed technique to measure the sea slopes is demonstrated by superposing over the aircraft ground track the measured sea surface height with the geoid undulations, which are about 1 meter. © 2013 IEEE.

Salaris M.,Liverpool John Moores University | Cassisi S.,INAF Osservatorio Astronomico di Collurania | Pietrinferni A.,INAF Osservatorio Astronomico di Collurania | Kowalski P.M.,Helmholtz Center Potsdam | And 2 more authors.
Astrophysical Journal | Year: 2010

We present a new set of cooling models and isochrones for both H- and He-atmosphere white dwarfs (WDs), incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive - a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H- and He-burning phases, the number of thermal pulses experienced by the progenitors, progenitor metallicity, and the 12C(α, γ)16O reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase and the 12C(α, γ)16O reaction rate. We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood and the interpretation of the observed number ratios between H- and He-atmosphere WDs. The new WD sequences and an extensive set of WD isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI Web site. © 2010. The American Astronomical Society.

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