732 results on '"Córsico, A. H."'
Search Results
102. The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
- Author
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García-Berro, E., Torres, S., Renedo, I., Camacho, J., Althaus, L. G., Córsico, A. H., Salaris, M., and Isern, J.
- Subjects
Astrophysics - Solar and Stellar Astrophysics ,Astrophysics - Astrophysics of Galaxies - Abstract
NGC 6791 is a well-studied, metal-rich open cluster that is so close to us that can be imaged down to luminosities fainter than that of the termination of its white dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We use a Monte Carlo simulator that employs up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The cooling sequences for carbon-oxygen cores account for the delays introduced by both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase separation upon crystallization. We do not find evidence for a substantial fraction of helium-core white dwarfs, and hence our results support the suggestion that the origin of the bright peak of the white dwarf luminosity function can only be attributed to a population of unresolved binary white dwarfs. Moreover, our results indicate that the number distribution of secondary masses of the population of unresolved binaries has to increase with increasing mass ratio between the secondary and primary components of the progenitor system. We also find that the observed cooling sequence appears to be able to constrain the presence of progenitor sub-populations with different chemical compositions and the fraction of non-DA white dwarfs. Our simulations place interesting constraints on important characteristics of the stellar populations of NGC 6791. In particular, we find that the fraction of single helium-core white dwarfs must be smaller than 5%, that a sub-population of stars with zero metallicity must be <12%, while if the adopted metallicity of the sub-population is solar the upper limit is ~8%. Finally, we also find that the fraction of non-DA white dwarfs in this particular cluster is surprinsingly small <6%., Comment: 9 pages, 14 figures, accepted for publication in Astronomy & Astrophysics
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- 2011
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103. The evolution of white dwarfs with a varying gravitational constant
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Althaus, L. G., Corsico, A. H., Torres, S., Loren-Aguilar, P., Isern, J., and Garcia-Berro, E.
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Astrophysics - Solar and Stellar Astrophysics ,General Relativity and Quantum Cosmology - Abstract
Within the theoretical framework of some modern unification theories the constants of nature are functions of cosmological time. White dwarfs offer the possibility of testing a possible variation of G and, thus, to place constraints to these theories. We present full white dwarf evolutionary calculations in the case that G decreases with time. White dwarf evolution is computed in a self-consistent way, including the most up-to-date physical inputs, non-gray model atmospheres and a detailed core chemical composition that results from the calculation of the full evolution of progenitor stars. We find that the mechanical structure and the energy balance of white dwarfs are strongly modified by the presence of a varying G. In particular, for certain values of the rate of change of G, the evolution of cool white dwarfs is markedly affected. The impact of a varying G is more notorious in the case of more massive white dwarfs. In view of the recent results reporting that a very accurate white dwarf cooling age can be derived for the old and metal-rich open cluster NGC 6791, our study suggests that this cluster could be a potential target to constrain or detect a ypothetical secular variation of G., Comment: 5 pages, 4 figures, accepted for publication in A&A
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- 2011
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104. Evolutionary and pulsational properties of white dwarf stars
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Althaus, Leandro G., Córsico, Alejandro H., Isern, Jordi, and García-Berro, Enrique
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Astrophysics - Solar and Stellar Astrophysics - Abstract
Abridged. White dwarf stars are the final evolutionary stage of the vast majority of stars, including our Sun. The study of white dwarfs has potential applications to different fields of astrophysics. In particular, they can be used as independent reliable cosmic clocks, and can also provide valuable information about the fundamental parameters of a wide variety of stellar populations, like our Galaxy and open and globular clusters. In addition, the high densities and temperatures characterizing white dwarfs allow to use these stars as cosmic laboratories for studying physical processes under extreme conditions that cannot be achieved in terrestrial laboratories. They can be used to constrain fundamental properties of elementary particles such as axions and neutrinos, and to study problems related to the variation of fundamental constants. In this work, we review the essentials of the physics of white dwarf stars. Special emphasis is placed on the physical processes that lead to the formation of white dwarfs as well as on the different energy sources and processes responsible for chemical abundance changes that occur along their evolution. Moreover, in the course of their lives, white dwarfs cross different pulsational instability strips. The existence of these instability strips provides astronomers with an unique opportunity to peer into their internal structure that would otherwise remain hidden from observers. We will show that this allows to measure with unprecedented precision the stellar masses and to infer their envelope thicknesses, to probe the core chemical stratification, and to detect rotation rates and magnetic fields. Consequently, in this work, we also review the pulsational properties of white dwarfs and the most recent applications of white dwarf asteroseismology., Comment: 85 pages, 28 figures. To be published in The Astronomy and Astrophysics Review
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- 2010
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105. Evolution of white dwarf stars with high-metallicity progenitors: the role of 22Ne diffusion
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Althaus, Leandro G., García-Berro, Enrique, Renedo, Isabel, Isern, Jordi, Córsico, Alejandro H., and Rohrmann, Rene D.
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Astrophysics - Solar and Stellar Astrophysics ,Physics - Plasma Physics - Abstract
Motivated by the strong discrepancy between the main sequence turn-off age and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we compute a grid of white dwarf evolutionary sequences that incorporates for the first time the energy released by the processes of 22Ne sedimentation and of carbon/oxygen phase separation upon crystallization. The grid covers the mass range from 0.52 to 1.0 Msun, and it is appropriate for the study of white dwarfs in metal-rich clusters. The evolutionary calculations are based on a detailed and self-consistent treatment of the energy released from these two processes, as well as on the employment of realistic carbon/oxygen profiles, of relevance for an accurate evaluation of the energy released by carbon/oxygen phase separation. We find that 22Ne sedimentation strongly delays the cooling rate of white dwarfs stemming from progenitors with high metallicities at moderate luminosities, whilst carbon/oxygen phase separation adds considerable delays at low luminosities. Cooling times are sensitive to possible uncertainties in the actual value of the diffusion coefficient of 22Ne. Changing the diffusion coefficient by a factor of 2, leads to maximum age differences of approx. 8-20% depending on the stellar mass. We find that the magnitude of the delays resulting from chemical changes in the core is consistent with the slow down in the white dwarf cooling rate that is required to solve the age discrepancy in NGC 6791., Comment: 10 pages, 6 figures, to be published in The Astrophysical Journal
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- 2010
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106. New chemical profiles for the asteroseismology of ZZ Ceti stars
- Author
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Althaus, L. G., Córsico, A. H., Bischoff-Kim, A., Romero, A. D., Renedo, I., García-Berro, E., and Bertolami, M. M. Miller
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We compute new chemical profiles for the core and envelope of white dwarfs appropriate for pulsational studies of ZZ Ceti stars. These profiles are extracted from the complete evolution of progenitor stars, evolved through the main sequence and the thermally-pulsing asymptotic giant branch (AGB) stages, and from time-dependent element diffusion during white dwarf evolution. We discuss the importance of the initial-final mass relationship for the white dwarf carbon-oxygen composition. In particular, we find that the central oxygen abundance may be underestimated by about 15% if the white dwarf mass is assumed to be the hydrogen-free core mass before the first thermal pulse. We also discuss the importance for the chemical profiles expected in the outermost layers of ZZ Ceti stars of the computation of the thermally-pulsing AGB phase and of the phase in which element diffusion is relevant. We find a strong dependence of the outer layer chemical stratification on the stellar mass. In particular, in the less massive models, the double-layered structure in the helium layer built up during the thermally-pulsing AGB phase is not removed by diffusion by the time the ZZ Ceti stage is reached. Finally, we perform adiabatic pulsation calculations and discuss the implications of our new chemical profiles for the pulsational properties of ZZ Ceti stars. We find that the whole $g-$mode period spectrum and the mode-trapping properties of these pulsating white dwarfs as derived from our new chemical profiles are substantially different from those based on chemical profiles widely used in existing asteroseismological studies. Thus, we expect the asteroseismological models derived from our chemical profiles to be significantly different from those found thus far., Comment: 12 pages, 11 figures, 1 table. To be published in ApJ
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- 2010
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107. A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes
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García-Berro, Enrique, Torres, Santiago, Althaus, Leandro G., Renedo, Isabel, Lorén-Aguilar, Pablo, Córsico, Alejandro H., Rohrmann, René D., Salaris, Maurizio, and Isern, Jordi
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Astrophysics - Solar and Stellar Astrophysics - Abstract
NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791., Comment: 3 pages, 2 figures, published in Nature
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- 2010
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108. New cooling sequences for old white dwarfs
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Renedo, Isabel, Althaus, Leandro G., Bertolami, Marcelo M. Miller, Romero, Alejandra D., Corsico, Alejandro H., Rohrmann, Rene D., and Garcia-Berro, Enrique
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We present full evolutionary calculations appropriate for the study of hydrogen-rich DA white dwarfs. This is done by evolving white dwarf progenitors from the zero age main sequence, through the core hydrogen burning phase, the helium burning phase and the thermally pulsing asymptotic giant branch phase to the white dwarf stage. Complete evolutionary sequences are computed for a wide range of stellar masses and for two different metallicities: Z=0.01, which is representative of the solar neighborhood, and Z=0.001, which is appropriate for the study of old stellar systems, like globular clusters. During the white dwarf cooling stage we compute self-consistently the phase in which nuclear reactions are still important, the diffusive evolution of the elements in the outer layers and, finally, we also take into account all the relevant energy sources in the deep interior of the white dwarf, like the release of latent heat and the release of gravitational energy due to carbon-oxygen phase separation upon crystallization. We also provide colors and magnitudes for these sequences, based on a new set of improved non-gray white dwarf model atmospheres, which include the most up-to-date physical inputs like the Lyman alpha quasi-molecular opacity. The calculations are extended down to an effective temperature of 2,500 K. Our calculations provide a homogeneous set of evolutionary cooling tracks appropriate for mass and age determinations of old DA white dwarfs and for white dwarf cosmochronology of the different Galactic populations., Comment: 36 pages, 9 figures. Accepted for publication in The Astrophysical Journal
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- 2010
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109. Axions and the pulsation periods of variable white dwarfs revisited
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Isern, J., García-Berro, E., Althaus, L. G., and Córsico, A. H.
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Astrophysics - Solar and Stellar Astrophysics ,High Energy Physics - Phenomenology - Abstract
Axions are the natural consequence of the introduction of the Peccei-Quinn symmetry to solve the strong CP problem. All the efforts to detect such elusive particles have failed up to now. Nevertheless, it has been recently shown that the luminosity function of white dwarfs is best fitted if axions with a mass of a few meV are included in the evolutionary calculations. Our aim is to show that variable white dwarfs can provide additional and independent evidence about the existence of axions. The evolution of a white dwarf is a slow cooling process that translates into a secular increase of the pulsation periods of some variable white dwarfs, the so-called DAV and DBV types. Since axions can freely escape from such stars, their existence would increase the cooling rate and, consequently, the rate of change of the periods as compared with the standard ones. The present values of the rate of change of the pulsation period of G117-B15A are compatible with the existence of axions with the masses suggested by the luminosity function of white dwarfs, in contrast with previous estimations. Furthermore, it is shown that if such axions indeed exist, the drift of the periods of pulsation of DBV stars would be noticeably perturbed., Comment: Accepted for publication in Astronomy & Astrophysics
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- 2010
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110. New evolutionary sequences for hot H-deficient white dwarfs on the basis of a full account of progenitor evolution
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Althaus, L. G., Panei, J. A., Bertolami, M. M. Miller, García-Berro, E., Córsico, A. H., Romero, A. D., Kepler, S. O., and Rohrmann, R. D.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We present full evolutionary calculations appropriate for the study of hot hydrogen-deficent DO white dwarfs, PG 1159 stars, and DB white dwarfs. White dwarf sequences are computed for a wide range of stellar masses and helium envelopes on the basis of a complete treatment of the evolutionary history of progenitors stars, including the core hydrogen and helium burning phases, the thermally-pulsing AGB phase, and the born-again episode that is responsible for the hydrogen deficiency. We also provide colors and magnitudes for the new sequences for $T_{\rm eff} < 40 000$ K, where the NLTE effects are not dominant. These new calculations provide an homogeneous set of evolutionary tracks appropriate for mass and age determinations for both PG 1159 stars and DO white dwarfs. The calculations are extended down to an effective temperature of 7 000 K. We applied these new tracks to redetermine stellar masses and ages of all known DO white dwarfs with spectroscopically-determined effective temperatures and gravities, and compare them with previous results. We also compare for the first time consistent mass determinations for both DO and PG 1159 stars, and find a considerably higher mean mass for the DO white dwarfs. We discuss as well the chemical profile expected in the envelope of variable DB white dwarfs from the consideration of the evolutionary history of progenitor stars. Finally, we present tentative evidence for a different evolutionary channel, other than that involving the PG 1159 stars, for the formation of hot, hydrogen-deficient white dwarfs., Comment: To be published in The Astrophysical Journal
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- 2009
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111. Hot C-rich white dwarfs: testing the DB-DQ transition through pulsations
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Córsico, A. H., Romero, A. D., Althaus, L. G., and García-Berro, E.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of the SDSS project. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. Here, we present a nonadiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. One of our main aims is to test the convective-mixing picture for the origin of hot DQs through the pulsational properties. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we present a stability analysis of white dwarf models from stages before the blue edge of the DBV instability strip until the domain of the hot DQ white dwarfs, including the transition DB --> hot DQ white dwarf. We found that at evolutionary phases in which the models are characterized by He-dominated atmospheres, they exhibit unstable $g$-mode pulsations typical of DBV stars, and when the models become DQ white dwarfs with carbon-dominated atmospheres, they continue being pulsationally unstable with similar characteristics than DB models, and in agreement with the periods detected in variable hot DQ white dwarfs. Our calculations provide strong support to the convective-mixing picture for the formation of hot DQs. In particular, our results point to the existence of pulsating DB white dwarfs with very thin He-rich envelopes, which after passing the DBV instability strip become variable hot DQ stars. The existence of these DB stars with very thin envelopes could be investigated through asteroseismology., Comment: 10 pages, 10 figures. To be published in Astronomy & Astrophysics
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- 2009
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112. On the possible existence of short-period g-mode instabilities powered by nuclear burning shells in post-AGB H-deficient (PG1159-type) stars
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Córsico, A. H., Althaus, L. G., Bertolami, M. M. Miller, Pérez, J. M. González, and Kepler, S. O.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We present a pulsational stability analysis of hot post-AGB H-deficient pre-white dwarf stars with active He-burning shells. The stellar models employed are state-of-the-art equilibrium structures representative of PG1159 stars derived from the complete evolution of the progenitor stars. On the basis of fully nonadiabatic pulsation computations, we confirmed theoretical evidence for the existence of a separate PG1159 instability strip in the $\log T_{\rm eff} - \log g$ diagram characterized by short-period $g$-modes excited by the $\epsilon$-mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long period $g$-modes destabilized by the classical $\kappa$-mechanism acting on the partial ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159 stars characterized by thick He-rich envelopes and located inside this overlapping region could exhibit both short and intermediate/long periods simultaneously. we study the particular case of VV 47, a pulsating planetary nebula nucleus that has been reported to exhibit a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical $\kappa$-mechanism, while the observed short-period branch below $\approx 300$ s could correspond to modes triggered by the He-burning shell through the $\epsilon$-mechanism, although more observational work is needed to confirm the reality of these short-period modes. Were the existence of short-period $g$-modes in this star convincingly confirmed by future observations, VV 47 could be the first known pulsating star in which both the $\kappa$-mechanism and the $\epsilon$-mechanism of mode driving are simultaneously operating., Comment: 9 pages, 5 figures, 2 tables. To be published in The Astrophysical Journal
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- 2009
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113. Evolution and colors of helium-core white dwarf stars with high-metallicity progenitors
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Althaus, L. G., Panei, J. A., Romero, A. D., Rohrmann, R. D., Córsico, A. H., García-Berro, E., and Bertolami, M. M. Miller
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Astrophysics - Solar and Stellar Astrophysics - Abstract
Motivated by the recent detection of single and binary He-core white dwarfs in metal-rich clusters, we present a full set of evolutionary calculations and colors appropriate for the study of such white dwarfs. The paper is also aimed at investigating whether stable hydrogen burning may constitute a main source of energy for massive He-core white dwarfs resulting from high-metallicity progenitors. White dwarf sequences are derived by taking into account the evolutionary history of progenitor stars with supersolar metallicities. We also incorporate a self-consistent, time-dependent treatment of gravitational settling and chemical diffusion, as well as of the residual nuclear burning. We find that the influence of residual nuclear burning during the late stages of white dwarf evolution is strongly dependent on the occurrence of chemical diffusion at the base of the hydrogen-rich envelope. When no diffusion is considered, residual hydrogen burning strongly influences the advanced stages of white dwarf cooling, introducing evolutionary delays of several Gyr. By contrast, when diffusion is taken into account the role of residual nuclear burning is strongly mitigated, and the evolution is dictated only by the thermal content stored in the ions. In addition, for all of our sequences, we provide accurate color and magnitudes on the basis of new and improved non gray model atmospheres which explicitly include Ly$\alpha$ quasi-molecular opacity., Comment: 10 pages, 10 figures, 1 table. To be published in Astronomy & Astrophysics
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- 2009
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114. Seismological constraints on the high-gravity DOV stars PG2131+066 and PG 1707+427
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Córsico, A. H., Althaus, L. G., Bertolami, M. M. Miller, and García--Berro, E.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
A seismological study of the pulsating PG1159 stars PG2131+066 and PG 1707+427 is presented. We perform extensive adiabatic computations of g-mode pulsation periods of PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Msun. We constrain the stellar mass of PG2131+066 and PG 1707+427 by comparing the observed period spacing of each star with the theoretical asymptotic period spacings and with the average of the computed period spacings. We also employ the individual observed periods to find representative seismological models for both stars., Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 2008
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- 2009
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115. Asteroseismology of hot pre-white dwarf stars: the case of the DOV stars PG 2131+066 and PG 1707+427, and the PNNV star NGC 1501
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Córsico, A. H., Althaus, L. G., Bertolami, M. M. Miller, and García--Berro, E.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We present an asteroseismological study on the two high-gravity pulsating PG1159 (GW Vir or DOV) stars, PG 2131+066 and PG 1707+427, and on the pulsating [WCE] star NGC 1501. We compute adiabatic $g$-mode pulsation periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Msun. These models take into account the complete evolution of progenitor stars, through the thermally pulsing AGB phase, and born-again episode. We constrain the stellar mass of PG 2131+066, PG 1707+427 and NGC 1501 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employ the individual observed periods in search of representative seismological models for each star. This work closes our short series of asteroseismological studies on pulsating pre-white dwarf stars. Our results demonstrate the usefulness of asteroseismology for probing the internal structure and evolutionary status of pre-white dwarf stars. In particular, asteroseismology is able to determine stellar masses of PG1159 stars with an accuracy comparable or even better than spectroscopy., Comment: 10 pages, 4 figures, 6 tables. To be published in Astronomy & Astrophysics
- Published
- 2009
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116. On the formation of hot DQ white dwarfs
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Althaus, L. G., García-Berro, E., Córsico, A. H., Bertolami, M. M. Miller, and Romero, A. D.
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Astrophysics - Solar and Stellar Astrophysics - Abstract
We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white dwarfs. These calculations consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Our calculations provide strong support to the diffusive/convective-mixing picture for the formation of hot DQs. We find that the hot DQ stage is a short-lived stage and that the range of effective temperatures where hot DQ stars are found can be accounted for by different masses of residual helium and/or different initial stellar masses. In the frame of this scenario, a correlation between the effective temperature and the surface carbon abundance in DQs should be expected, with the largest carbon abundances expected in the hottest DQs. From our calculations, we suggest that most of the hot DQs could be the cooler descendants of some PG1159 stars characterized by He-rich envelopes markedly smaller than those predicted by the standard theory of stellar evolution. At least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an evolutionary link between this star and the massive PG1159 star H1504+65 is plausible., Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal Letters
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- 2009
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117. Asteroseismological analysis of the polluted ZZ Ceti star G 29 − 38 with TESS
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Uzundag, Murat, primary, De Gerónimo, Francisco C, additional, Córsico, Alejandro H, additional, Silvotti, Roberto, additional, Bradley, Paul A, additional, Montgomery, Michael H, additional, Catelan, Márcio, additional, Toloza, Odette, additional, Bell, Keaton J, additional, Kepler, S O, additional, Althaus, Leandro G, additional, Kleinman, Scot J, additional, Kilic, Mukremin, additional, Mullally, Susan E, additional, Gänsicke, Boris T, additional, Bąkowska, Karolina, additional, Barber, Sam, additional, and Nitta, Atsuko, additional
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- 2023
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118. On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65
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Althaus, L. G., Córsico, A. H., Torres, S., and García-Berro, E.
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Astrophysics - Abstract
We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after the born-again and during the subsequent evolution through the [WCL] stage. This possibility is sustained by recent observational evidence of extensive mass-loss events in Sakurai's object and is in line with the recent finding that such mass losses give rise to PG1159 models with thin helium-rich envelopes and large rates of period change, as demanded by the pulsating star PG1159-035. We compute the post born again evolution of massive sequences by taking into account different mass-loss rate histories. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope so as to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages only remove at most half of the envelope surviving the violent hydrogen burning during the born-again phase. In view of our results, the recently suggested evolutionary connection born-again stars --> H1504+65 --> white dwarfs with carbon-rich atmospheres is difficult to sustain unless the whole helium-rich envelope could be ejected by non-stationary mass-loss episodes during the Sakurai stage., Comment: 5 pages, 2 figures. To be published in Astronomy & Astrophysics
- Published
- 2008
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119. Revisiting the theoretical DBV (V777 Her) instability strip: the MLT theory of convection
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Córsico, A. H., Althaus, L. G., Bertolami, M. M. Miller, and García-Berro, E.
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Astrophysics - Abstract
We reexamine the theoretical instability domain of pulsating DB white dwarfs (DBV or V777 Her variables). We performed an extensive $g$-mode nonadiabatic pulsation analysis of DB evolutionary models considering a wide range of stellar masses, for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases, the domain of the PG1159 stars, the hot phase of DO white dwarfs, and then the DB white dwarf stage have been considered. We explicitly account for the evolution of the chemical abundance distribution due to time-dependent chemical diffusion processes. We examine the impact of the different prescriptions of the MLT theory of convection and the effects of small amounts of H in the almost He-pure atmospheres of DB stars on the precise location of the theoretical blue edge of the DBV instability strip., Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 2008
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- 2008
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120. Evidence of thin helium envelopes in PG1159 stars
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Althaus, L. G., Córsico, A. H., Bertolami, M. M. Miller, García--Berro, E., and Kepler, S. O.
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Astrophysics - Abstract
We present evidence that PG1159 stars could harbour He--rich envelopes substantially thinner than those predicted by current evolutionary models with current estimates of mass loss, which may be attributable to an extensive mass--loss episode during the born--again AGB phase. Specifically, we show that the models with thin He--rich envelopes predict remarkably large magnitudes of the rates of period change of the trapped and untrapped modes observed in the pulsating star PG 1159$-$035. This is a consequence of the much shorter evolutionary timescale of the models with thin He--rich envelopes during the low--gravity PG1159 regime. Our findings are particularly interesting in view of the suggestion of an evolutionary link between the helium--deficient PG1159 star H1504+65 and the recently discovered white dwarfs with almost pure carbon atmospheres., Comment: 4 pages, 2 figures. To be published as a Letter in ApJ
- Published
- 2008
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121. Gravitational settling of 22Ne and white dwarf evolution
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García--Berro, E., Althaus, L. G., Córsico, A. H., and Isern, J.
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Astrophysics - Abstract
We study the effects of the sedimentation of the trace element 22Ne in the cooling of white dwarfs. In contrast with previous studies, which adopted a simplified treatment of the effects of 22Ne sedimentation, this is done self-consistently for the first time, using an up-to-date stellar evolutionary code in which the diffusion equation is coupled with the full set of equations of stellar evolution. Due the large neutron excess of 22Ne, this isotope rapidly sediments in the interior of the white dwarf. Although we explore a wide range of parameters, we find that using the most reasonable assumptions concerning the diffusion coefficient and the physical state of the white dwarf interior the delay introduced by the ensuing chemical differentation is minor for a typical 0.6 Msun white dwarf. For more massive white dwarfs, say M_Wd about 1.0 Msun, the delay turns out to be considerably larger. These results are in qualitatively good accord with those obtained in previous studies, but we find that the magnitude of the delay introduced by 22Ne sedimentation was underestimated by a factor of about 2. We also perform a preliminary study of the impact of 22Ne sedimentation on the white dwarf luminosity function. Finally, we hypothesize as well on the possibility of detecting the sedimentation of 22Ne using pulsating white dwarfs in the appropriate effective temperature range with accurately determined rates of change of the observed periods., Comment: To apper in The Astrophysical Journal
- Published
- 2007
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122. Asteroseismological measurements on PG 1159-035, the prototype of the GW Vir variable stars
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Córsico, A. H., Althaus, L. G., Kepler, S. O., Costa, J. E. S., and Bertolami, M. M. Miller
- Subjects
Astrophysics - Abstract
An asteroseismological study of PG 1159-035, the prototype of the GW Vir variable stars, has been performed on the basis of detailed and full PG1159 evolutionary models presented by Miller Bertolami & Althaus (2006). We carried out extensive computations of adiabatic g-mode pulsation periods on PG1159 evolutionary models with stellar masses spanning the range 0.530 to 0.741 Mo. We derive a stellar mass in the range 0.56-0.59 Mo from the period-spacing data alone. We also find, on the basis of a period-fit procedure, a seismic model representative of PG 1159-035 that reproduces the observed period pattern with an average of the period differences of 0.64-1.03 s, consistent with the expected model uncertainties. The results of the period-fit analysis carried out in this work suggest that the surface gravity of PG 1159-035 would be 1 sigma larger than the spectroscopically inferred gravity. For our best-fit model of PG 1159-035, all of the pulsation modes are characterized by positive rates of period changes, at odds with the measurements by Costa & Kepler (2007)., Comment: 14 pages, 11 figures, 5 tables. To be published in Astronomy & Astrophysics
- Published
- 2007
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123. The pulsation modes of the pre-white dwarf PG 1159-035
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Costa, J. E. S., Kepler, S. O., Winget, D. E., O'Brien, M. S., Kawaler, S. D., Costa, A. F. M., Giovannini, O., Kanaan, A., Mukadam, A. S., Mullally, F., Nitta, A., Provençal, J. L., Shipman, H., Wood, M. A., Ahrens, T. J., Grauer, A., Kilic, M., Bradley, P. A., Sekiguchi, K., Crowe, R., Jiang, X. J., Sullivan, D., Sullivan, T., Rosen, R., Clemens, J. C., Janulis, R., O'Donoghue, D., Ogloza, W., Baran, A., Silvotti, R., Marinoni, S., Vauclair, G., Dolez, N., Chevreton, M., Dreizler, S., Schuh, S., Deetjen, J., Nagel, T., Solheim, J. -E., Perez, J. M. Gonzalez, Ulla, A., Barstow, Martin, Burleigh, M., Good, S., Metcalfe, T. S., Kim, S. -L., Lee, H., Sergeev, A., Akan, M. C., Çakirli, Ö., Paparo, M., Viraghalmy, G., Ashoka, B. N., Handler, G., Hürkal, Özlem, Johannessen, F., Kleinman, S. J., Kalytis, R., Krzesinski, J., Klumpe, E., Larrison, J., Lawrence, T., Meištas, E., Martinez, P., Nather, R. E., Fu, J. -N., Pakštienė, E., Romero-Colmenero, E., Riddle, R., Seetha, S., Silvestri, N. M., Vučković, M., Warner, B., Zola, S., Althaus, L. G., Córsico, A. H., and Montgomery, M. H.
- Subjects
Astrophysics - Abstract
PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars., Comment: 25 pages, 11 figures, appendix with 6 long tables. The resolution of some of the figures were changed and some of them can appear with a bit lower quality
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- 2007
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124. On the systematics of asteroseismological mass determinations of PG1159 stars
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Althaus, L. G., Córsico, A. H., Kepler, S. O., and Bertolami, M. M. Miller
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Astrophysics - Abstract
We analyze systematics in the asteroseismological mass determination methods in pulsating PG 1159 stars. We compare the seismic masses resulting from the comparison of the observed mean period spacings with the usually adopted asymptotic period spacings, and the average of the computed period spacings. Computations are based on full PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo that take into account the complete evolution of progenitor stars. We conclude that asteroseismology is a precise and powerful technique that determines the masses to a high internal accuracy, but it depends on the adopted mass determination method. In particular, we find that in the case of pulsating PG 1159 stars characterized by short pulsation periods, like PG 2131+066 and PG 0122+200, the employment of the asymptotic period spacings overestimates the stellar mass by about 0.06 Mo as compared with inferences from the average of the period spacings. In this case, the discrepancy between asteroseismological and spectroscopical masses is markedly reduced when use is made of the mean period spacing instead of the asymptotic period spacing., Comment: 7 pages, 4 figures, 1 table. To be published in Astronomy and Astrophysics
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- 2007
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125. Asteroseismological constraints on the coolest GW Vir variable star (PG 1159-type)PG 0122+200
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Córsico, A. H., Bertolami, M. M. Miller, Althaus, L. G., Vauclair, G., and Werner, K.
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Astrophysics - Abstract
We present an asteroseismological study on PG 0122+200, the coolest known pulsating PG1159 (GW Vir) star. Our results are based on an augmented set of the full PG1159 evolutionary models recently presented by Miller Bertolami & Althaus (2006). We perform extensive computations of adiabatic g-mode pulsation periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Msun. We derive a stellar mass of 0.626 Msun from a comparison between the observed period spacing and the computed asymptotic period spacing, and a stellar mass of 0.567 Msun by comparing the observed period spacing with the average of the computed period spacing. We also find, on the basis of a period-fit procedure, an asteroseismological model representative of PG 0122+200 which is able to reproduce the observed period pattern with an average of the period differences of 0.88 s. The model has an effective temperature of 81500 K, a stellar mass of 0.556 Msun, a surface gravity log g= 7.65, a stellar luminosity and radius of log(L/Lsun)= 1.14 and log(R/Rsun)= -1.73, respectively, and a He-rich envelope thickness of Menv= 0.019 Msun. We derive a seismic distance of about 614 pc and a parallax of about 1.6 mas. The results of the period-fit analysis carried out in this work suggest that the asteroseismological mass of PG 0122+200 could be 6-20 % lower than thought hitherto and in closer agreement (to within 5 %) with the spectroscopic mass. This result suggests that a reasonable consistency between the stellar mass values obtained from spectroscopy and asteroseismology can be expected when detailed PG1159 evolutionary models are considered., Comment: 10 pages, 6 figures. To be published in Astronomy & Astrophysics
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- 2007
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126. Low-mass, helium-enriched PG1159 stars: a possible evolutionary origin and the implications for their pulsational stability properties
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Althaus, L. G., Córsico, A. H., and Bertolami, M. M. Miller
- Subjects
Astrophysics - Abstract
We examine a recently-proposed evolutionary scenario that could explain the existence of the low-mass, helium-enriched PG1159 stars. We assess the overstability of pulsation g-modes of stellar models as evolution proceeds along the PG1159 domain. Stellar models are extracted from the full evolution of a 1 Mo model star that experiences its first thermal pulse as a late thermal pulse (LTP) after leaving the AGB. The evolutionary stages corresponding to the born-again episode and the subsequent helium sub-flashes are taken into account in detail. Under reasonable mass-loss rate assumptions, the evolutionary scenario reproduces the high helium abundances observed in some PG1159 stars. We find that, despite the high helium abundance in the driving layers, there exists a narrow region in the log g-log Teff diagram for which the helium-enriched PG1159 sequence exhibits unstable pulsation modes with periods in the range 500 to 1600 s. In particular, the nonpulsating helium-enriched PG1159 star, MCT 0130-1937, is located outside the theoretical instability domain. Our results suggest that MCT 0130-1937 is a real non-pulsating star and that the lack of pulsations should not be attributed to unfavorable geometry. Our study hints at a consistent picture between the evolutionary scenario that could explain the existence of helium-enriched PG1159 stars and the nonvariable nature of MCT 0130-1937. We also present theoretical support for the unusually high helium abundance observed in the nonpulsating PG1159 star HS 1517+7403. We suggest that HS 1517+7403 could be a transition object linking the low-mass helium-rich O(He) stars with the helium-enriched PG1159 stars via the evolutionary connection K1-27-->HS 1517+7403-->MCT 0130-1937., Comment: 7 pages, 5 figures. To be published in Astronomy and Astrophysics (2007)
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- 2007
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127. The age and colors of massive white dwarf stars
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Althaus, L. G., García-Berro, E., Isern, J., Córsico, A. H., and Rohrmann, R. D.
- Subjects
Astrophysics - Abstract
We present evolutionary calculations and colors for massive white dwarfs with oxygen-neon cores for masses between 1.06 and 1.28 Mo. The evolutionary stages computed cover the luminosity range from log(L/Lo) approx. 0.5 down to -5.2. Our cooling sequences are based on evolutionary calculations that take into account the chemical composition expected from massive white dwarf progenitors that burned carbon in partially degenerate conditions. The use of detailed non-gray model atmospheres provides us with accurate outer boundary conditions for our evolving models at low effective temperatures. We examine the cooling age, colors and magnitudes of our sequences. We find that massive white dwarfs are characterized by very short ages to such an extent that they reach the turn-off in their colors and become blue at ages well below 10 Gyr. Extensive tabulations for massive white dwarfs, accessible from our web site, are also presented., Comment: 10 pages, including 1 table and 10 figures. To be published in Astronomy & Astrophysics
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- 2007
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128. Asteroseismological constraints on the pulsating planetary nebula nucleus (PG1159-type) RX J2117.1+3412
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Córsico, A. H., Althaus, L. G., Bertolami, M. M. Miller, and Werner, K.
- Subjects
Astrophysics - Abstract
We present asteroseismological inferences on RX J2117.1+3412, the hottest known pulsating PG1159 star. Our results are based on full PG1159 evolutionary models recently presented by Miller Bertolami & Althaus (2006). We performed extensive computations of adiabatic g-mode pulsation periods on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo. PG1159 stellar models are extracted from the complete evolution of progenitor stars started from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG 1159 stars. We constrained the stellar mass of RX J2117.1+3412 by comparing the observed period spacing with the asymptotic period spacing and with the average of the computed period spacings. We also employed the individual observed periods to find a representative seismological model. We derive a stellar mass of 0.56-0.57 Mo from the period spacing data alone. In addition, we found a best-fit model representative for RX J2117.1+3412 with an effective temperature of 163,400 K, a stellar mass of 0.565 Mo, and a surface gravity log g= 6.61. The derived stellar luminosity and radius are log(L/Lo)= 3.36 and log(R/Ro)= -1.23, respectively, and the He-rich envelope thickness is Menv= 0.02 Mo. We derive a seismic distance of 452 pc and a linear size of the planetary nebula of 1.72 pc. These inferences seem to solve the discrepancy between the RX J2117.1+3412 evolutionary timescale and the size of the nebula. All of the seismological tools we use concur to the conclusion that RX J2117.1+3412 must have a stellar mass of 0.565 Mo much in agreement with recent asteroseismology studies and in clear conflict with the predictions of spectroscopy plus evolutionary tracks., Comment: 10 pages, 6 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics. Erratum available as a separate file
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- 2006
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129. New nonadiabatic pulsation computations on full PG1159 evolutionary models: the theoretical GW Vir instability strip revisited
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Córsico, A. H., Althaus, L. G., and Bertolami, M. M. Miller
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Astrophysics - Abstract
We reexamine the theoretical instability domain of pulsating PG1159 stars (GW Vir variables). We performed an extensive g-mode stability analysis on PG1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 Mo for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG1159 stars have been considered. We found that pulsations in PG1159 stars are excited by the kappa-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the surface layers and the driving region, much in agreement with previous studies. We show, for the first time, the existence of a red edge of the instability strip at high luminosities. We found that all of the GW Vir stars lay within our theoretical instability strip. Our results suggest a qualitative good agreement between the observed and the predicted ranges of unstable periods of individual stars. Finally, we found that generally the seismic masses (derived from the period spacing) of GW Vir stars are somewhat different from the masses suggested by evolutionary tracks coupled with spectroscopy. Improvements in the evolution during the thermally pulsing AGB phase and/or during the core helium burning stage and early AGB could help to alleviate the persisting discrepancies., Comment: 10 pages, 8 figures. To be published in Astronomy and Astrophysics
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- 2006
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130. Asteroseismic inferences on GW Vir variable stars in the frame of new PG 1159 evolutionary models
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Córsico, A. H. and Althaus, L. G.
- Subjects
Astrophysics - Abstract
An adiabatic, nonradial pulsation study of GW Vir stars is presented. The pulsation calculations are based on PG1159 evolutionary sequences with different stellar massess artificially derived from a full evolutionary sequence of 0.5895 Mo that has been computed taking into account the evolutionary history of the progenitor star. The artificial sequences were constructed by appropriately scaling the stellar mass of the 0.5895 Mo sequence well before the models reach the low-luminosity, high-gravity stage of the GW Vir domain. We compute g-mode pulsation periods appropriate to GW Vir variable stars. The implications for the mode-trapping properties of our PG 1159 models are discussed at length. We found that the mode-trapping features characterizing our PG 1159 models are mostly fixed by the stepped shape of the core chemical profile left by prior convective overshooting. This is particularly true at least for the range of periods observed in GW Vir stars. In addition, we make asteroseismic inferences about the internal structure of the GW Vir stars PG 1159-035, PG 2131+066, PG 1707+427 and PG 0122+200., Comment: 24 pages, 6 tables and 11 figures. Accepted for publication in Astronomy and Astrophysics
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- 2006
131. DQ white-dwarf stars with low C abundance: Possible progenitors
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Scóccola, C. G., Althaus, L. G., Serenelli, A. M., Rohrmann, R. D., and Córsico, A. H.
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Astrophysics - Abstract
The present paper focuses on the evolution of hydrogen-deficient white dwarfs with the aim of exploring the consequences of different initial envelope structures on the carbon abundances expected in helium-rich, carbon-contaminated DQ white dwarfs. In particular, the evolutionary link between the DQs with low detected carbon abundances and the PG1159, extreme horizontal branch, and helium-rich R Coronae Borealis (RCrB) stars is explored. We present full evolutionary calculations that take a self-consistent treatment of element diffusion into account as well as expectations for the outer layer chemical stratification of progenitor stars upon entering the white dwarf regime. We find that PG1159 stars cannot be related to any DQ white dwarfs with low C abundances. Instead, we suggest that the latter could constitute the progeny of the giant, helium-rich RCrB stars., Comment: 10 pages, 10 figures. Accepted for publication in Astronomy and Astrophysics
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- 2006
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132. The gravitational wave radiation of pulsating white dwarfs revisited: the case of BPM 37093 and PG 1159-035
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Garcia-Berro, E., Loren-Aguilar, P., Corsico, A. H., Althaus, L. G., Lobo, J. A., and Isern, J.
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Astrophysics - Abstract
We compute the emission of gravitational radiation from pulsating white dwarfs. This is done by using an up-to-date stellar evolutionary code coupled with a state-of-the-art pulsational code. The emission of gravitational waves is computed for a standard 0.6 solar masses white dwarf with a liquid carbon-oxygen core and a hydrogen-rich envelope, for a massive DA white dwarf with a partially crystallized core for which various l=2 modes have been observed (BPM 37093) and for PG 1159-035, the prototype of the GW Vir class of variable stars, for which several quadrupole modes have been observed as well. We find that these stars do not radiate sizeable amounts of gravitational waves through their observed pulsation g-modes, in line with previous studies. We also explore the possibility of detecting gravitational waves radiated by the f-mode and the p-modes. We find that in this case the gravitational wave signal is very large and, hence, the modes decay very rapidly. We also discuss the possible implications of our calculations for the detection of gravitational waves from pulsating white dwarfs within the framework of future space-borne interferometers like LISA., Comment: 11 pages, 6 figures. Accepted for publication in A&A
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- 2005
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133. Mass-radius relations for massive white dwarf stars
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Althaus, L. G., García--Berro, E., Isern, J., and Córsico, A. H.
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Astrophysics - Abstract
We present detailed theoretical mass-radius relations for massive white dwarf stars with oxygen-neon cores. This work is motivated by recent observational evidence about the existence of white dwarf stars with very high surface gravities. Our results are based on evolutionary calculations that take into account the chemical composition expected from the evolutionary history of massive white dwarf progenitors. We present theoretical mass-radius relations for stellar mass values ranging from 1.06 to 1.30 Mo with a step of 0.02 Mo and effective temperatures from 150000 K to approx. 5,000 K. A novel aspect predicted by our calculations is that the mass-radius relation for the most massive white dwarfs exhibits a marked dependence on the neutrino luminosity. Extensive tabulations for massive white dwarfs, accessible from our web site, are presented as well., Comment: 7 pages, including 8 postscript figures. Accepted for publication in Astronomy and Astrophysics
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- 2005
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134. The formation of DA white dwarfs with thin hydrogen envelopes
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Althaus, L. G., Bertolami, M. M. Miller, Córsico, A. H., García-Berro, E., and Gil-Pons, P.
- Subjects
Astrophysics - Abstract
We study the formation and evolution of DA white dwarfs, the progenitors of which have experienced a late thermal pulse (LTP) shortly after the departure from the thermally pulsing AGB. To this end, we compute the complete evolution of an initially 2.7 Mo star all the way from the zero-age main sequence to the white dwarf stage. We find that most of the original H-rich material of the post-AGB remnant is burnt during the post-LTP evolution, with the result that, at entering its white dwarf cooling track, the remaining H envelope becomes 10^-6 Mo in agreement with asteroseismological inferences for some ZZ Ceti stars., Comment: 4 pages including 3 figures. Accepted for publication in Astronomy and Astrophysics Letters
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- 2005
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135. Can pulsating PG1159 stars place constraints on the occurrence of core overshooting?
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Córsico, A. H. and Althaus, L. G.
- Subjects
Astrophysics - Abstract
The present letter is aimed at exploring the influence of overshooting during the central helium burning in pre-white dwarf progenitors on the pulsational properties of PG1159 stars. To this end we follow the complete evolution an intermediate-mass white dwarf progenitor from the zero age main sequence through the thermally pulsing and born-again phases to the domain of the PG1159 stars. Our results suggest that the presence of mode-trapping features in the period spacings of these hot pulsating stars could result from structure in the carbon-oxygen core. We find in particular that in order to get enough core structure consistent with observational demands, the occurrence of overshoot episodes during the central helium burning is needed. This conclusion is valid for thick helium envelopes like those predicted by our detailed evolutionary calculations. If the envelope thickness were substantially smaller, then the occurrence of core overshooting would be more difficult to disentangle from the effects related to the envelope transition zones., Comment: 4 pages, including 3 postscript figures. Accepted for publication in Astronomy and Astrophysics Letters
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- 2005
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136. The formation and evolution of hydrogen-deficient post-AGB white dwarfs: the emerging chemical profile and the expectations for the PG1159-DB-DQ evolutionary connection
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Althaus, L. G., Serenelli, A. M., Panei, J. A., Corsico, A. H., Garcia-Berro, E., and Scoccola, C. G.
- Subjects
Astrophysics - Abstract
This paper is designed to explore the formation and evolution of hydrogen-deficient post-AGB white dwarfs. To this end, we compute the complete evolution of an initially 2.7 M_sun star from the zero-age main sequence through the thermally pulsing and mass-loss phases to the white dwarf stage. Particular attention is given to the chemical abundance changes during the whole evolution. A time-dependent scheme for the simultaneous treatment of abundance changes caused by nuclear reactions, diffusive overshooting, salt fingers and convection is considered. We employed the double-diffusive mixing-length theory of convection for fluids with composition gradients (Grossman & Taam 1996). The study can therefore be considered as a test of its performance in low-mass stars. Also, time-dependent element diffusion for multicomponent gases is taken into account during the white dwarf evolution. The evolutionary stages corresponding to the last helium thermal pulse on the early white-dwarf cooling branch and the following born-again episode are carefully explored. Relevant aspects for PG1159 stars and DB white dwarf evolution are studied in the frame of these new evolutionary models that take into account the history of the white dwarf progenitor. The scope of the calculations is extended to the domain of the helium-rich, carbon-contaminated DQ white dwarfs with the aim of exploring the plausibility of the evolutionary connection PG1159-DB-DQ. In this regard, the implications for the double-layered chemical structure in pulsating DB white dwarfs is investigated. Another aspect of the investigation concerned the consequences of mass-loss episodes during the PG1159 stage for the chemical stratification of the outer layer of DB and DQ white dwarfs., Comment: 20 pages, 15 figures. Accepted by Astronomy & Astrophysics
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- 2005
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137. New evolutionary models for massive ZZ Ceti stars. II. The effects of crystallization on their pulsational properties
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Corsico, A. H., Althaus, L. G., Montgomery, M. H., Garcia-Berro, E., and Isern, J.
- Subjects
Astrophysics - Abstract
We present in this work new pulsational calculations for improved carbon-oxygen DA white dwarf models suitable for the study of massive ZZ Ceti stars. The background models employed in this study, presented in detail in a recent paper by Althaus et al. (2003), are the result of the complete evolution of massive white dwarf progenitors from the zero-age main sequence through the Asymptotic Giant Branch (AGB) and mass loss phases to the white dwarf regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time-dependent mixing due to convection, salt fingers, and diffusive overshoot. In addition, time-dependent element diffusion for multicomponent gases has been considered during the white dwarf evolution. Crystallization and chemical rehomogenization due to phase separation upon crystallization in the core of our models have been fully considered. The effects of crystallization on the period spectrum of these massive white dwarf models are assessed by means of a detailed pulsational analysis. We find that the theoretical pulsation spectrum is strongly modified when crystallization is considered, in particular concerning the mode trapping properties of the equilibrium models. We also discuss at some length the implications of our study for BPM 37093, the most massive ZZ Ceti star presently known. We find that if BPM 37093 has a stellar mass of $\approx 1.00$ \msun its observed spectrum could bear the signature of overshoot episodes during the helium core burning., Comment: 15 Pages, including 17 Postscript figures. Accepted for publication in Astronomy and Astrophysics
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- 2004
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138. Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores
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Corsico, A. H., Garcia-Berro, E., Althaus, L. G., and Isern, J.
- Subjects
Astrophysics - Abstract
We explore the adiabatic pulsational properties of massive white dwarf stars with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this end, we compute the cooling of massive white dwarf models for both core compositions taking into account the evolutionary history of the progenitor stars and the chemical evolution caused by time-dependent element diffusion. In particular, for the oxygen/neon models, we adopt the chemical profile resulting from repeated carbon-burning shell flashes expected in very massive white dwarf progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles resulting from phase separation upon crystallization. For both compositions we also take into account the effects of crystallization on the oscillation eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs are notably different from those made of carbon/oxygen, thus making asteroseismological techniques a promising way to distinguish between both types of stars and, hence, to obtain valuable information about their progenitors., Comment: 11 pages, including 11 postscript figures. Accepted for publication in Astronomy and Astrophysics
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- 2004
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139. The rate of period change in pulsating DB white dwarf stars
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Corsico, A. H. and Althaus, L. G.
- Subjects
Astrophysics - Abstract
In this work, we present the theoretically expected rates of pulsation period change for V777 Her (DBV) variable stars. To this end we employ new evolutionary models representative of pulsating DB white dwarf stars computed in a self-consistent way with the predictions of time-dependent element diffusion. At the hot edge of the DB instability strip, the envelopes of the models are characterized by a diffusion-induced double-layered chemical structure. We compute the numerical values of rates of period change by solving the equations of linear, adiabatic, nonradial stellar oscillations. We examine the effects of varying the stellar mass, the mass of the helium envelope and the neutrino emission on the expected period changes. We present extensive tabulations of our results which could be useful for comparison with future detections of the rate of period change in pulsating DB white dwarfs., Comment: 13 pages, including 5 postscript figures and 9 tables. Accepted for publication in Astronomy and Astrophysics
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- 2004
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140. The double-layered chemical structure in DB white dwarfs
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Althaus, L. G. and Corsico, A. H.
- Subjects
Astrophysics - Abstract
We study the structure and evolution of white dwarf stars with helium-rich atmospheres (DB) in a self-consistent way with the predictions of time-dependent element diffusion. Our treatment of diffusion includes gravitational settling and chemical and thermal diffusion. OPAL radiative opacities for arbitrary metallicity and carbon-and oxygen-rich compositions are employed. Emphasis is placed on the evolution of the diffusion-modeled double-layered chemical structure. This structure, which is characterized by a pure helium envelope atop an intermediate remnant shell rich in helium, carbon and oxygen, is expected for pulsating DB white dwarfs, assuming that they are descendants of hydrogen-deficient PG1159 post-AGB stars. We find that, depending on the stellar mass, if DB white dwarf progenitors are formed with a helium content smaller than \approx 10^-3 M_*, a single-layered configuration is expected to emerge during the DB pulsation instability strip. We also explore the consequences of diffusively evolving chemical stratifications on the adiabatic pulsational properties of our DB white dwarf models. In this context, we find that the evolving shape of the chemical profile translates into a distinct behaviour of the theoretical period distribution as compared with the case in which the shape of the profile is assumed to be fixed during the evolution across the instability strip. Finally, we extend the scope of the calculations to the domain of the helium-rich carbon-contaminated DQ white dwarfs. In particular, we speculate that DQ white dwarfs with low detected carbon abundances would not be descendants of the PG1159 stars., Comment: 10 Pages, including 11 Postscript figures. Accepted for publication in Astronomy and Astrophysics
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- 2004
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141. Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution
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Althaus, L. G., Corsico, A. H., Gautschy, A., Han, Z., Serenelli, A. M., and Panei, J. A.
- Subjects
Astrophysics - Abstract
The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33 Msun white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-Msun Pop. I star with a 1.25 Msun companion, and period P_i= 3 days. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models., Comment: 13 Pages, including 16 Postscript figures. Accepted for publication in MNRAS
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- 2003
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142. New evolutionary models for massive ZZ Ceti stars: I. First results for their pulsational properties
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Althaus, L. G., Serenelli, A. M., Corsico, A. H., and Montgomery, M. H.
- Subjects
Astrophysics - Abstract
We present new and improved evolutionary calculations for carbon-oxygen white dwarf (WD) stars appropriate for the study of massive ZZ Ceti stars. We follow the complete evolution of massive WD progenitors from the zero-age main sequence through the thermally pulsing and mass loss phases to the WD regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time-dependent mixing due to diffusive overshoot, semiconvection and salt fingers. In addition, time-dependent element diffusion for multicomponent gases has been considered during the WD stage. We find that before the ZZ Ceti stage is reached, element diffusion has strongly smoothed out the chemical profile to such a degree that the resulting internal abundance distribution does not depend on the occurrence of overshoot episodes during the thermally pulsing phase. The mass of the hydrogen envelope left at the ZZ Ceti domain amounts to $M_H \approx 2.3 \times 10^{-6}$ \msun. This is about half as large as for the case when element diffusion is neglected. The implications of our new models for the pulsational properties of massive ZZ Ceti stars are discussed. In this regard, we find that the occurrence of core overshooting during central helium burning leaves strong imprints on the theoretical period spectrum of massive ZZ Ceti stars. Finally, we present a simple new prescription for calculating the He/H profile which goes beyond the trace element approximation., Comment: 18 Pages, including 18 Postscript figures (A&A document class). Accepted for publication in Astronomy and Astrophysics
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- 2003
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143. On mode trapping in pulsating DA white dwarf stars
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Benvenuto, O. G., Corsico, A. H., Althaus, L. G., and Serenelli, A. M.
- Subjects
Astrophysics - Abstract
The present work is designed to explore the effects of the time-dependent element diffusion on the mode trapping properties of DA white dwarf models with various thickness of the hydrogen envelope. Our predictions are compared with the standard assumption of diffusive equilibrium in the trace element approximation. We find that element diffusion markedly weakens the presence of mode trapping originated in the outer layers of the models, even for the case of thin hydrogen envelopes., Comment: 5 pages, 6 figures, to be published in MNRAS
- Published
- 2002
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144. The mode trapping properties of full DA white dwarf evolutionary models
- Author
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Corsico, A. H., Althaus, L. G., Benvenuto, O. G., and Serenelli, A. M.
- Subjects
Astrophysics - Abstract
An adiabatic, non-radial pulsation study of a 0.563 M_sun DA white dwarf model is presented on the basis of new evolutionary calculations performed in a self-consistent way with the predictions of time dependent element diffusion, nuclear burning and the history of the white dwarf progenitor. Emphasis is placed on the role played by the internal chemical stratification of these new models in the behaviour of the eigenmodes, and the expectations for the full g-spectrum of periods. The implications for the mode trapping properties are discussed at length. In this regard, we find that, for high periods, the viability of mode trapping as a mode selection mechanism is markedly weaker for our models, as compared with the situation in which the hydrogen-helium transition region is treated assuming equilibrium diffusion in the trace element approximation., Comment: 10 pages, 14 figures. Accepted for publication in Astronomy & Astrophysics
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- 2002
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145. The Effects of Element Diffusion on the Pulsational Properties of Variable DA White Dwarf Stars
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Corsico, A. H., Benvenuto, O. G., Althaus, L. G., and Serenelli, A. M.
- Subjects
Astrophysics - Abstract
We explore the effects of element diffusion due to gravitational settling and thermal and chemical diffusion on the pulsational properties of DA white dwarfs. To this end, we employ an updated evolutionary code coupled with a pulsational, finite difference code for computing the linear, non-radial g-modes in the adiabatic approximation. We follow the evolution of a 0.55 \msun white dwarf model in a self-consistent way with the evolution of chemical abundance distribution as given by time dependent diffusion processes. Results are compared with the standard treatment of diffusive equilibrium in the trace element approximation. Appreciable differences are found between the two employed treatments. We conclude that time dependent element diffusion plays an important role in determining the whole oscillation pattern and the temporal derivative of the periods in DAV white dwarfs. In addition, we discuss the plausibility of the standard description employed in accounting for diffusion in most of white dwarf asteroseismological studies., Comment: 6 pages, 5 figures, accepted for publication in MNRAS
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- 2002
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146. Time dependent diffusion in pulsating white dwarf stars: Asteroseismology of G117-B15A
- Author
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Benvenuto, O. G., Corsico, A. H., Althaus, L. G., and Serenelli, A. M.
- Subjects
Astrophysics - Abstract
We study the structural characteristic of the variable DA white dwarf G117B-15A by applying the methods of asteroseismology. For such a purpose, we construct white dwarf evolutionary models considering a detailed and up-to-date physical description as well as several processes responsible for the occurrence of element diffusion. We have considered several thickness for the outermost hydrogen layer, whereas for the inner helium-, carbon- and oxygen-rich layers we considered realistic profiles predicted by calculations of the white dwarf progenitor evolution. The evolution of each of the considered model sequences were followed down to very low effective temperature; in particular, from 12500K on we computed the dipolar, linear, adiabatic oscillations with low radial order. We find that asteroseismological results are not univocal regarding mode identification for the case of G117B-15A. However, our asteroseismological results are compatible with spectroscopical data only if the observed periods of 215.2, 271.0 and 304.4 s are due to dipolar modes with $k=2, 3, 4$ respectively. Our calculations indicate that the best fit to the observed period pattern of G117B-15A corresponds to a DA white dwarf structure with a stellar mass of 0.525 \msun, with a hydrogen mass fraction \lmh$\gtrsim$-3.83 at an effective temperature \teff$\approx$11800K., Comment: 8 pages, 8 figures, to be published in MNRAS
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- 2002
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147. Evolution of a 3 \msun star from the main sequence to the ZZ Ceti stage: the role played by element diffusion
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Althaus, L. G., Serenelli, A. M., Corsico, A. H., and Benvenuto, O. G.
- Subjects
Astrophysics - Abstract
The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to the white dwarf formation. In particular, we follow the evolution of a 3 \msun model from the zero-age main sequence (the adopted metallicity is Z=0.02) all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. Emphasis is placed on the evolution of the chemical abundance distribution due to diffusion processes and the role played by hydrogen burning during the white dwarf evolution. Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother as compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt-Vaisala frequency., Comment: 11 pages, 11 figures, accepted for publication in MNRAS
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- 2001
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148. New DA white dwarf evolutionary models and their pulsational properties
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Corsico, A. H., Althaus, L. G., Benvenuto, O. G., and Serenelli, A. M.
- Subjects
Astrophysics - Abstract
In this letter we investigate the pulsational properties of ZZ Ceti stars on the basis of new white dwarf evolutionary models calculated in a self-consistent way with the predictions of time dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to the white dwarf formation. Emphasis is placed on the trapping properties of such models. By means of adiabatic, non-radial pulsation calculations, we find, as a result of time dependent diffusion, a much weaker mode trapping effect, particularly for the high-period regime of the pulsation g-spectrum. This result is valid at least for models with massive hydrogen-rich envelopes. Thus, mode trapping would not be an effective mechanism to explain the fact that all the high periods expected from standard models of stratified white dwarfs are not observed in the ZZ Ceti stars., Comment: 3 pages, 5 figures, accepted for publication in Astronomy & Astrophysics Letters
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- 2001
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149. A New Code for Nonradial Stellar Pulsations and its Application to Low - Mass, Helium White Dwarfs
- Author
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Corsico, A. H. and Benvenuto, O. G.
- Subjects
Astrophysics - Abstract
We present a finite difference code intended for computing linear, adiabatic, nonradial pulsations of spherical stars. This code is based on a general Newton - Raphson technique in order to handle the relaxation of the eigenvalue (square of the eigenfrequency) of the modes and their corresponding eigenfunctions. This code has been tested computing the pulsation spectra of polytropic spheres finding a good agreement with previous work. Then, we have coupled this code to our evolutionary code and applied it to the computation of the pulsation spectrum of a low mass, pure - helium white dwarf of 0.3 M_{sun} for a wide range of effective temperatures. In making this calculation we have taken an evolutionary time step short enough such that eigenmodes corresponding to a given model are used as initial approximation to those of the next one. Specifically, we have computed periods, period spacing, eigenfunctions, weight functions, kinetic energies and variational periods for a wide range of modes. To our notice this is the first effort in studying the pulsation properties of helium white dwarfs. The solution we have found working with these realistic white dwarf models are in good accord with the predictions of the asymptotic theory of Tassoul (1980) for high order modes. This indicates that the code presented here is able to work adequately also with realistic stellar models., Comment: 27 pages, 7 figures Accepted for publication in ApSS
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- 2001
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150. The potential of the variable DA white dwarf G117-B15A as a tool for Fundamental Physics
- Author
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Corsico, A. H., Benvenuto, O. G., Althaus, L. G., Isern, J., and Garcia-Berro, E.
- Subjects
Astrophysics ,High Energy Physics - Phenomenology - Abstract
White dwarfs are well studied objects. The relative simplicity of their physics allows to obtain very detailed models which can be ultimately compared with their observed properties. Among white dwarfs there is a specific class of stars, known as ZZ-Ceti objects, which have a hydrogen-rich envelope and show periodic variations in their light curves. G117-B15A belongs to this particular set of stars. The luminosity variations have been successfully explained as due to g-mode pulsations. G117-B15A has been recently claimed to be the most stable optical clock ever found, being the rate of change of its 215.2 s period very small: \dot{P}= (2.3 +- 1.4)x10^{-15} s s^-1, with a stability comparable to that of the most stable millisecond pulsars. The rate of change of the period is closely related to its cooling timescale, which can be accurately computed. In this paper we study the pulsational properties of G117-B15A and we use the observed rate of change of the period to impose constraints on the axion emissivity and, thus, to obtain a preliminary upper bound to the mass of the axion. This upper bound turns out to be 4cos^{2}{\beta} meV at the 95% confidence level. Although there are still several observational and theoretical uncertainties, we conclude that G117-B15A is a very promising stellar object to set up constraints on particle physics., Comment: 32 pages, 14 figures, accepted for publication in New Astronomy
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- 2001
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