Asteroseismic inferences on GW Vir variable stars in the frame of new PG 1159 evolutionary models
This submission has been removed by arXiv administrators because it is a duplicate of astro-ph/0603735.
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arXiv subjects
L. G. Althaus
Publications and source records attributed to L. G. Althaus.
DQ white-dwarf stars with low C abundance: Possible progenitors
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.
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New evolutionary calculations for the Born Again scenario
We present evolutionary calculations aimed at describing the born-again scenario for post-AGB remnant stars of 0.5842 and 0.5885 \msun. Results are based on a detailed treatment of the physical processes responsible for the chemical abundance changes. We considered two theories of convection: the standard mixing length theory (MLT) and the double-diffusive GNA convection developed by Grossman et al. The latter accounts for the effect of the chemical gradient ($\nablaμ$) in the mixing processes and in the transport of energy. We also explore the dependence of the born-again evolution on some physical hypothesis, such as the effect of the existence of non-zero chemical gradients, the prescription for the velocity of the convective elements and the size of the overshooting zones. Attention is given to the behavior of the born-again times and to the chemical evolution during the ingestion of protons. We find that in our calculations born again times are dependent on time resolution. In particular when the minimum allowed time step is below $5 \times 10^{-5}$ yr we obtain, with the standard mixing length theory, born again times of 5-10 yr. This is true without altering the prescription for the efficiency of convective mixing during the proton ingestion. On the other hand we find that the inclusion of the chemical gradients in the calculation of the mixing velocity tend to increase the born again times by about a factor of two. In addition we find that proton ingestion can be seriously altered if the occurrence of overshooting is modified by the $\nablaμ$-barrier at the H-He interface, strongly altering born again times.
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The gravitational wave radiation of pulsating white dwarfs revisited: the case of BPM 37093 and PG 1159-035
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.
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Mass-radius relations for massive white dwarf stars
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.
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The formation of DA white dwarfs with thin hydrogen envelopes
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.
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Can pulsating PG1159 stars place constraints on the occurrence of core overshooting?
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.
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On the excitation of PG1159-type pulsations
Stability properties are presented of dipole and quadrupole nonradial oscillation modes of model stars that experienced a late helium shell flash on their way to the white-dwarf cooling domain. The computed instability domains are compared with the observed hot variable central stars of planetary nebulae and the GW Vir pulsators.
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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
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.
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Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores
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.
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New evolutionary models for massive ZZ Ceti stars. II. The effects of crystallization on their pulsational properties
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.
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The rate of period change in pulsating DB white dwarf stars
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.
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Mass distribution of DA white dwarfs in the First Data Release of the Sloan Digital Sky Survey
We investigate the sample of 1175 new nonmagnetic DA white dwarfs with the effective temperatures T_eff > 12000 K, which were extracted from the Data Release 1 of the Sloan Digital Sky Survey. We determined masses, radii, and bolometric luminosities of stars in the sample. The above parameters were derived from the effective temperatures T_eff and surface gravities log g published in the DR1, and the new theoretical M - R relations for carbon-core and oxygen-core white dwarfs. Mass distribution of white dwarfs in this sample exhibits the peak at M = 0.562 M_sol (carbon core stars), and the tail towards higher masses. Both the shape of the mass distribution function and the empirical mass - radius relation are practically identical for white dwarfs with either pure carbon or pure oxygen cores.
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The double-layered chemical structure in DB white dwarfs
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.
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Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution
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.
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New evolutionary models for massive ZZ Ceti stars: I. First results for their pulsational properties
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.
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Evolution and colours of helium-core white dwarf stars: the case of low metallicity progenitors
The present work is designed to explore the evolution of helium-core white dwarf (HeWD) stars for the case of metallicities much lower than the solar one (Z=0.001 and Z=0.0002). Evolution is followed in a self-consistent way with the predictions of detalied and new non-grey atmospheres, time-dependent element diffusion and the history of the white dwarf progenitor. Reliable initial models for low mass HeWDs are obtained by applying mass loss rates to a 1msun stellar model. The loss of angular momentum caused by gravitational wave emission and magnetic stellar wind braking are considered. Model atmospheres, based on a detailed treatment of the microphysics entering the WD atmosphere enable us to provide accurate colours and magnitudes at both early and advanced evolutionary stages. We find that most of our evolutionary sequences experience several episodes of hydrogen thermonuclear flashes. In particular, the lower the metallicity, the larger the minimum stellar mass for the occurrence fo flashes induced by CNO cycle reactions. The existence of a mass-threshold for the occurrence of diffusion-induced CNO flashes leadss to a marked dichotomy in the age of our models. Another finding of this study is that our HeWD models experience unstable hydrogen burning via PP nuclear reactions at late cooling stages as a result of hydrogen chemically diffusing inwards. Such PP flashes take place in models with very low metal content. We also find that models experiencing CNO flashes exhibit a pronouncede turn-off in most of their colours at M_V=16 approximately. Finally, colour-magnitude diagrams for our models are presented and compared with recent observational data of HeWD candidates in the globular clusters NGC 6397 and 47 Tucanae.
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Improved synthetic spectra of helium-core white dwarf stars
We examine the emergent fluxes from helium-core white dwarfs following their evolution from the end of pre-white dwarf stages down to advanced cooling stages. For this purpose, we include a detailed treatment of the physics of the atmosphere, particularly an improved representation of the state of the gas by taking into account non-ideal effects according to the so-called occupation probability formalism. The present calculations also incorporate hydrogen line opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and new updated induced-dipole absorption from H$_2$-H$_2$, H$_2$-He and H-He pairs. We find that the non-ideal effects and line absorption alter the appearance of the stellar spectrum and have a significant influence upon the photometric colours in the UBVRI-JHKL system. This occurs specially for hot models $T_{\rm eff}\ga 8000$ due to line and pseudo-continuum opacities, and for cool models $T_{\rm eff}\la 4000$ where the perturbation of atoms and molecules by neighbour particles affects the chemical equilibrium of the gas. In the present study, we also include new cooling sequences for helium-core white dwarfs of very low mass (0.160 and 0.148 M$_\odot$) with metallicity $Z=0.02$. These computations provide theoretical support to search for and identify white dwarfs of very low mass, specially useful for recent and future observational studies of globular cluster, where these objects have began to be detected.
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