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Search Results: 1 - 10 of 297508 matches for " J. Isern "
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IDDQ Detectable Bridges in Combinational CMOS Circuits
E. Isern,J. Figueras
VLSI Design , 1997, DOI: 10.1155/1997/93809
Simplified treatment of the radiative transfer problem in expanding envelopes
J. Gomez-Gomar,J. Isern
Physics , 1996, DOI: 10.1086/177927
Abstract: In this paper we study the application of a simplified method to solve the dynamic radiative transfer problem in expanding envelopes. The method, which requires a computational effort similar to that of the diffusion approximation, is based on the use of a generalization of the Eddington closure relationship allowing the inclusion of scattering and relativistic corrections to order (v/c). We apply this method to the calculation of light curves of type Ia supernovae, showing that it gives much more accurate results than the diffusion approximation, and that the latter is seriously in error when applied to determine emergent flux and its spectral distribution.
The role of gravitational supernovae on the galactic evolution of the LiBeB isotopes
C. Abia,J. Isern,U. Lisenfeld
Physics , 1998, DOI: 10.1046/j.1365-8711.1998.01821.x
Abstract: The observed Be and B relationships with metallicity clearly support the idea that both elements have a primary origin and that are produced by the same class of objects. Spallation by particles accelerated during gravitational events (SNII, SNIb/c) seems to be a likely origin. We show, in the context of a model of chemical evolution, that it is possible to solve the Li, Be and B abundance puzzle with the yields recently proposed by Ramaty et al. (1997), provided that SNII are unable to significantly accelerate helium nuclei and that different mechanisms are allowed to act simultaneously.
Predictions of gamma-ray emission from classical novae and their detectability by CGRO
M. Hernanz,J. Gomez-Gomar,J. Jose,J. Isern
Physics , 1997, DOI: 10.1063/1.54178
Abstract: An implicit hydrodynamic code following the explosion of classical novae, from the accretion phase up to the final ejection of the envelope, has been coupled to a MonteCarlo code able to simulate their gamma-ray emission. Carbon-oxygen (CO) and oxygen-neon (ONe) novae have been studied and their gamma-ray spectra have been obtained, as well as the gamma-ray light curves for the important lines (e$^{-}$--e$^{+}$ annihilation line at 511 keV, $^{7}$Be decay-line at 478 keV and $^{22}$Na decay-line at 1275 keV). The detectability of the emission by CGRO instruments has been analyzed. It is worth noticing that the $\gamma$-ray signature of a CO nova is different from that of an ONe one. In the CO case, the 478 keV line is very important, but lasts only for $\sim$2 months. In the ONe case, the 1275 keV line is the dominant one, lasting for $\sim$ 4 years. In both cases, the 511 keV line is the most intense line at the beginning, but its short duration ($\sim$ 2 days) makes it very difficult to be detected. It is shown that the negative results from the observations made by COMPTEL up to now are consistent with the theoretical predictions. Predictions of the future detectability by the INTEGRAL mission are also made.
On the ^7 Li and ^7 Be synthesis in novae
M. Hernanz,J. Jose,A. Coc,J. Isern
Physics , 1996,
Abstract: The production of ^7 Li and ^7 Be during the explosive hydrogen-burning that occurs in nova explosions is computed by means of a hydrodynamic code able to treat both the accretion and the explosion stages. Large overproduction factors with respect to solar abundances are obtained, the exact value depending mainly on the chemical composition of the envelope. Although the final ejected masses are small, these results indicate that novae can contribute to the ^7 Li enrichment of the interstellar medium. Furthermore, since ^7 Be decays emiting a gamma-ray (478 KeV), with a half-life of 53.3 days, the synthesis of ^7 Li could be tested during the INTEGRAL mission.
White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy
S. Torres,J. Camacho,J. Isern,E. Garcia-Berro
Physics , 2010, DOI: 10.1051/0004-6361/200912905
Abstract: The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than ~M_{sun} have been ruled out as major components of a Massive Astrophysical Compact Halo Object (MACHO) Galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. In this paper we evaluate the contribution to the dark matter content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For this purpose we use a Monte Carlo simulator which incorporates up-to-date evolutionary sequences of white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres. We also take into account detailed descriptions of the thick disk and the halo of our Galaxy as well as of a reliable model of the LMC. We find that the contribution of white dwarfs with hydrogen-deficient atmospheres moderately increases the theoretical estimate of the optical depth with respect to the value obtained when only hydrogen-rich white dwarfs are considered. We also find that the contribuiton of the thick disk population of white dwarfs is comparable to the halo contribution. However, the contributions of both the halo and the thick disk white-dwarf populations are still insufficient to explain the number of events observed by the MACHO team. Finally, we find that the contribution to the halo dark matter of the entire population under study is less than 10% at the 95% conficence level.
Axions and the white dwarf luminosity function
Isern, J.;Catalan, S.;Garcia-Berro, E.;Torres, S.
High Energy Physics - Phenomenology , 2008, DOI: 10.1088/1742-6596/172/1/012005
Abstract: The evolution of white dwarfs can be described as a simple cooling process. Recently, it has been possible to determine with an unprecedented precision their luminosity function, that is, the number of stars per unit volume and luminosity interval. Since the shape of the bright branch of this function is only sensitive to the average cooling rate, we use this property to check the possible existence of axions, a proposed but not yet detected weakly interacting particle. We show here that the inclusion of the axion emissivity in the evolutionary models of white dwarfs noticeably improves the agreement between the theoretical calculations and the observational white dwarf luminosity function, thus providing the first positive indication that axions could exist. Our results indicate that the best fit is obtained for m_a cos^2beta ~ 2-6 meV, where m_a is the mass of the axion and cos^2beta is a free parameter, and that values larger than 10 meV are clearly excluded.
White dwarf cooling sequences and cosmochronology
Isern J.,Artigas A.,García-Berro E.
EPJ Web of Conferences , 2013, DOI: 10.1051/epjconf/20134305002
Abstract: The evolution of white dwarfs is a simple gravothermal process. This means that their luminosity function, i.e. the number of white dwarfs per unit bolometric magnitude and unit volume as a function of bolometric magnitude, is a monotonically increasing function that decreases abruptly as a consequence of the finite age of the Galaxy. The precision and the accuracy of the white dwarf luminosity functions obtained with the recent large surveys together with the improved quality of the theoretical models of evolution of white dwarfs allow to feed the hope that in a near future it will be possible to reconstruct the history of the different Galactic populations.
Further constraints on white dwarf galactic halos
R. Canal,J. Isern,P. Ruiz-Lapuente
Physics , 1997, DOI: 10.1086/310915
Abstract: The suggestion that roughly half the mass of the galactic halo might be in the form of white dwarfs, together with the limits on the infrared background light and on the initial metallicity of the galactic disk, would set strong constraints on the initial mass function (IMF) of the halo. Particular IMFs have been proposed to cut off both the numbers of low-mass stars contributing to the infrared background and of high-mass stars which contribute to the growth of metallicity when they promptly explode as gravitational-collapse (Type II and Type Ib/c) supernovae. Here we examine the further contribution to metallicity from the Type Ia (thermonuclear) supernovae which would later be produced from the halo white dwarf population. We find that, for most of the evolutionary scenarios for the Type Ia supernova progenitor systems proposed so far, the constraints on the white dwarf mass fraction in the halo from the predicted production of iron would be extremely severe. When the predicted iron excess is not so large, then the exceedingly high Type Ia supernova rate predicted for the present time would also exclude a major contribution of white dwarfs to the halo mass. The white dwarf contribution, in all cases, should be below 5-10%. Besides, for the IMFs considered, the duration of the halo burst should be shorter than 1 Gyr in order to avoid too large a spread in the iron abundances of Population II halo dwarfs, and the predicted halo $[O/Fe]$ ratio would be at odds with observations.
The energetics of crystallizing white dwarfs revisited again
J. Isern,E. Garcia-Berro,M. Hernanz,G. Chabrier
Physics , 1999, DOI: 10.1086/308153
Abstract: The evolution of white dwarfs is a cooling process that depends on the energy stored in the core and on the way in which it is transferred through the envelope. In this paper we show that despite some (erroneous) claims, the redistribution of chemical elements ensuing the crystallization of C/O white dwarfs provides between the 10% and the 20% of the total energy released during the crystallization process, depending on the internal chemical composition, which is not negligible at all, given the present state of the art of the white dwarf cooling theory.
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