Las Palmas de Gran Canaria, Spain
Las Palmas de Gran Canaria, Spain

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Aznar-Siguan G.,Polytechnic University of Catalonia | Aznar-Siguan G.,Institute for Space Studies of Catalonia | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We compute the possible observational signatures of white dwarf dynamical interactions in dense stellar environments. Specifically, we compute the emission of gravitational waves, and we compare it with the sensitivity curves of planned space-borne gravitational wave detectors. We also compute the light curves for those interactions in which a detonation occurs, and one of the stars is destroyed, as well as the corresponding neutrino luminosities. We find that for the three possible outcomes of these interactions - which are the formation of an eccentric binary system, a lateral collision in which several mass transfer episodes occur, and a direct one in which just a single mass transfer episode takes place - only those in which an eccentric binary are formed are likely to be detected by the planned gravitational wave mission eLISA, while more sensitive detectors would be able to detect the signals emitted in lateral collisions. On the other hand, the light curves (and the thermal neutrino emission) of these interactions are considerably different, producing both very powerful outbursts and low-luminosity events. Finally, we also calculate the X-ray signature produced in the aftermath of those interactions for which a merger occurs. We find that the temporal evolution follows a power law with the same exponent found in the case of the mergers of two neutron stars, although the total energy released is smaller. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Soker N.,Technion - Israel Institute of Technology | Kashi A.,University of Nevada, Las Vegas | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We argue that the multiple shells of circumstellar material (CSM) and the supernovae (SNe) ejecta interaction with the CSM starting 59 d after the explosion of the Type Ia SN PTF 11kx are best described by a violent prompt merger. In this prompt merger scenario, the common envelope (CE) phase is terminated by a merger of a white dwarf (WD) companion with the hot core of a massive asymptotic giant branch star. In most cases, the WD is disrupted and accreted on to the more massive core. However, in the rare cases, where the merger takes place when the WD is denser than the core, the core will be disrupted and accreted on to the cooler WD. In such cases, the explosion might occur with no appreciable delay, i.e. months to years after the termination of the CE phase. This, we propose, might be the evolutionary route that could lead to the explosion of PTF 11kx. This scenario can account for the very massive CSM within ~1000 au of the exploding PTF 11kx star, for the presence of hydrogen, and for the presence of shells in the CSM. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Rohrmann R.D.,CONICET | Althaus L.G.,National University of La Plata | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the outer boundary conditions. Aims. We provide detailed outer boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of outer boundary conditions provided by updated model atmospheres on the cooling times of 0.60 and 0.90 M white dwarf sequences. Results. Our results show that the use of detailed outer boundary conditions becomes relevant for effective temperatures lower than 5800 K for sequences with 0.60 M and 6100 K with 0.90 M. Detailed model atmospheres predict ages that are up to ≈ 10% shorter at log (L/L) =-4 when compared with the ages derived using Eddington-like approximations at τ Ross = 2/3. We also analyze the effects of various assumptions and physical processes that are relevant in the calculation of outer boundary conditions. In particular, we find that the Lyα red wing absorption does not substantially affect the evolution of white dwarfs. Conclusions. White dwarf cooling timescales are sensitive to the surface boundary conditions for T eff ≤ 6000 K. Interestingly enough, nongray effects have few consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have no noticeable effects on their cooling rates, except throughout the Rosseland mean opacity. © 2012 ESO.


Loren-Aguilar P.,Polytechnic University of Catalonia | Loren-Aguilar P.,Institute for Space Studies of Catalonia | Isern J.,Institute for Space Studies of Catalonia | Isern J.,Institute Of Ciencies Of Lespai | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

The collision of two white dwarfs is a quite frequent event in dense stellar systems, like globular clusters and galactic nuclei. In this paper, we present the results of a set of simulations of the close encounters and collisions of two white dwarfs. We use an up-to-date smoothed particle hydrodynamics code that 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 covering a wide range of velocities and initial distances of the colliding white dwarfs. We discuss in detail when the initial eccentric binary white dwarf survives the closest approach, when a lateral collision in which several mass transfer episodes occur is the outcome of the newly formed binary system, and which range of input parameters leads to a direct collision, in which only one mass transfer episode occurs. We also discuss the characteristics of the final configuration and assess the possible observational signatures of the merger, such as the associated gravitational waveforms and the fallback luminosities. We find that the overall evolution of the system and the main characteristics of the final object agree with those found in previous studies. We also find that the fallback luminosities are close to 1048 erg s-1. Finally, we find that in the case of lateral and direct collisions the gravitational waveforms are characterized by large-amplitude peaks which are followed by a ring-down phase, while in the case in which the binary white dwarf survives the closest approach, the gravitational pattern shows a distinctive behaviour, typical of eccentric systems. © 2010 The Authors. Journal compilation © 2010 RAS.


Soker N.,Technion - Israel Institute of Technology | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | Althaus L.G.,National University of La Plata
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

We argue that the properties of the Type Ia supernova (SN Ia) SN 2011fe can be best explained within the frame of the core-degenerate (CD) scenario. In the CD scenario, a white dwarf (WD) merges with the core of an asymptotic giant branch (AGB) star and forms a rapidly rotating WD, with a mass close to and above the critical mass for explosion. Rapid rotation prevents immediate collapse and/or explosion. Spinning down over a time of 0-1010 yr brings theWD to explosion. A very long delayed explosion to post-crystallization phase, which lasts for about 2 × 109 yr, leads to the formation of a highly carbon-enriched outer layer. This can account for the carbon-rich composition of the fastest-moving ejecta of SN 2011fe. In reaching the conclusion that the CD scenario best explains the observed properties of SN 2011fe, we consider both its specific properties, like a very compact exploding object and carbon-rich composition of the fastest-moving ejecta, and the general properties of SNe Ia ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Papish O.,Technion - Israel Institute of Technology | Soker N.,Technion - Israel Institute of Technology | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We conduct numerical simulations of the interacting ejecta from an exploding CO white dwarf (WD) with a He WD donor in the double-detonation scenario for Type Ia supernovae (SNe Ia), and study the possibility of exploding the companion WD. We also study the long time imprint of the collision on the supernova remnant. When the donor He WD has a low mass, MWD = 0.2M⊙, it is at a distance of 0.08 R⊙ from the explosion, and helium is not ignited. The low-mass He WD casts an 'ejecta shadow' behind it. By evolving the ejecta for longer times, we find that the outer parts of the shadowed side are fainter and its boundary with the ambient gas is somewhat flat. More massive He WD donors, MWD ≃ 0.4M⊙, must be closer to the CO WD to transfer mass. At a distance of a ≤ 0.045R⊙ helium is detonated and the He WD explodes, leading to a triple detonation scenario. In the explosion of the donor WD approximately 0.15M⊙ of unburned helium is ejected. This might be observed as a peculiar Type Ib supernova. © 2015 The Authors.


Salaris M.,Liverpool John Moores University | Althaus L.G.,National University of La Plata | Althaus L.G.,CONICET | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia
Astronomy and Astrophysics | Year: 2013

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 the calculations. Methods. We compute and compare cooling times obtained from two independent and widely used stellar evolution codes, BaSTI and LPCODE evolutionary codes, using exactly the same input physics for 0.55 M⊙ white dwarf models with both pure carbon and uniform carbon-oxygen (50/50 mass fractions) cores, and pure hydrogen layers with mass fraction qH = 10-4MWD on top of pure helium buffers of mass qHe = 10-2MWD. Results. Using the same radiative and conductive opacities, photospheric boundary conditions, neutrino energy loss rates, and equation of state, cooling times from the two codes agree within ~2% at all luminosities, except when log (L/L⊙) > -1.5 where differences up to ~8% do appear, because of the different thermal structures of the first white dwarf converged models at the beginning of the cooling sequence. This agreement is true for both pure carbon and uniform carbon-oxygen stratification core models, and also when the release of latent heat and carbon-oxygen phase separation are considered. We have also determined quantitatively and explained the effect of varying equation of state, low-temperature radiative opacities, and electron conduction opacities in our calculations, Conclusions. We have assessed for the first time the maximum possible accuracy in the current estimates of white dwarf cooling times, resulting only from the different implementations of the stellar evolution equations and homogeneous input physics in two independent stellar evolution codes. This accuracy amounts to ~2% at luminosities lower than log (L/L⊙) ~ -1.5. This difference is smaller than the uncertainties in cooling times attributable to the present uncertainties in the white dwarf chemical stratification. Finally, we extend the scope of our work by providing tabulations of our cooling sequences and the required input physics that can be used as a comparison test of cooling times obtained from other white dwarf evolutionary codes. © ESO, 2013.


Aznar-Siguan G.,Polytechnic University of Catalonia | Aznar-Siguan G.,Institute for Space Studies of Catalonia | Garcia-Berro E.,Polytechnic University of Catalonia | Garcia-Berro E.,Institute for Space Studies of Catalonia | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here, we present the results of a comprehensive set of smoothed particle hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the burning. We thus characterize under which circumstances a detonation is likely to occur as a result of the impact of the disrupted star on the surface of the more massive white dwarf. Finally, we also study which interactions result in bound systems, and in which ones the more massive white dwarf is also disrupted as a consequence of the dynamical interaction. The sizeable number of simulations performed in this work allows us to find how the outcome of the interaction depends on the distance at closest approach, and on the masses of the colliding white dwarfs, and which is the chemical pattern of the nuclearly processed material. Finally, we also discuss the influence of the masses and core chemical compositions of the interacting white dwarfs and the different kinds of impact in the properties of the remnants. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Kulebi B.,CSIC - Institute of Marine Sciences | Kulebi B.,Institute for Space Studies of Catalonia | Eksi K. Y.,Technical University of Istanbul | Loren-Aguilar P.,University of Exeter | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of two white dwarfs, as there are viable mechanisms for producing sustainable magnetic fields within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has always been considered a problematic issue of this scenario. Smoothed particle hydrodynamics simulations show that in mergers in which the two white dwarfs have different masses, a disc around the central compact object is formed. If the central object is magnetized, it can interact with the disc through its magnetosphere. The torque applied by the disc changes the spin of the star, whereas the transferred angular momentum from the star to the disc determines the properties of the disc. In this work, we build a model for the disc evolution under the effect of magnetic accretion, and for the angular momentum evolution of the star, which can be compared with the observations. Our model predicts that the magnetospheric interaction of magnetic white dwarfs with their discs results in a significant spin-down, and we show that for magnetic white dwarfs with relatively strong fields (larger than 10 MG) the observed rotation periods of the magnetic white dwarf population can be reproduced. We also investigate whether turbulence can be sustained during the late phases of the evolution of the system. When a critical temperature below which turbulence is not sustained is introduced into the model, the periods of the three fast rotating, strongly magnetic, massive white dwarfs in the solar neighbourhood are recovered. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Kepler S.O.,Federal University of Rio Grande do Sul | Pelisoli I.,Federal University of Rio Grande do Sul | Jordan S.,University of Heidelberg | Kleinman S.J.,Gemini Observatory | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 to 733 MG, which amounts to 4 per cent of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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