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Papers by Leandro Althaus

Astronomy & Astrophysics, 2021

Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossomi... more Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossoming area of asteroseismology, particularly of pulsating white dwarfs and pre-white dwarfs, thus continuing the impulse of its predecessor, the Kepler mission. Aims. In this paper, we present the observations from the extended TESS mission in both 120 s short-cadence and 20 s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We identify them as two new GW Vir stars, TIC 333432673 and TIC 095332541. We apply the tools of asteroseismology with the aim of deriving their structural parameters and seismological distances. Methods. We carried out a spectroscopic analysis and a spectral fitting of TIC 333432673 and TIC 095332541. We also processed and analyzed the high-precision TESS photometric light curves of the two target stars, and derived their oscillation frequencies. We performed an asteroseismological analysis of these stars on the basis of PG 1159 evolut...

The Astronomy and Astrophysics Review, 2019

Stars are extremely important astronomical objects that constitute the pillars on which the Unive... more Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components-including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from "extra dimensions", and even crystallization. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has

Astronomy & Astrophysics, 2019

Context. Ultra-massive hydrogen-rich white dwarf stars are expected to harbor oxygen/neon cores r... more Context. Ultra-massive hydrogen-rich white dwarf stars are expected to harbor oxygen/neon cores resulting from the progenitor evolution through the super-asymptotic giant branch phase. As evolution proceeds during the white dwarf cooling phase, a crystallization process resulting from Coulomb interactions in very dense plasmas is expected to occur, leading to the formation of a highly crystallized core. In particular, pulsating ultra-massive white dwarfs offer a unique opportunity to infer and test the occurrence of crystallization in white dwarf interiors as well as physical processes related with dense plasmas. Aims. We aim to assess the adiabatic pulsation properties of ultra-massive hydrogen-rich white dwarfs with oxygen/neon cores. Methods. We studied the pulsation properties of ultra-massive hydrogen-rich white dwarf stars with oxygen/neon cores. We employed a new set of ultra-massive white dwarf evolutionary sequences of models with stellar masses in the range 1.10 ≤ M⋆/M⊙ ≤ ...

The Astrophysical Journal, 2017

We present an asteroseismological analysis of four ZZ Ceti stars observed with Kepler : GD 1212, ... more We present an asteroseismological analysis of four ZZ Ceti stars observed with Kepler : GD 1212, SDSS J113655.17+040952.6, KIC 11911480 and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf stars. We employ a grid of carbon-oxygen core white dwarfs models, characterized by a detailed and consistent chemical inner profile for the core and the envelope. In addition to the observed periods, we take into account other information from the observational data, as amplitudes, rotational splittings and period spacing, as well as photometry and spectroscopy. For each star, we present an asteroseismological model that closely reproduce their observed properties. The asteroseismological stellar mass and effective temperature of the target stars are (0.632 ± 0.027M , 10737 ± 73K) for GD 1212, (0.745 ± 0.007M , 11110 ± 69K) for KIC 4552982, (0.5480 ± 0.01M , 12721 ± 228K) for KIC1191480 and (0.570 ± 0.01M , 12060 ± 300K) for SDSS J113655.17+040952.6. In general, the asteroseismological values are in good agreement with the spectroscopy. For KIC 11911480 and SDSS J113655.17+040952.6 we derive a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6, that is part of a binary system and went through a common envelope phase.

Journal of Cosmology and Astroparticle Physics, 2014

It has been shown that the shape of the luminosity function of white dwarfs

The Astrophysical Journal, 2017

We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmosphe... more We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmospheres. We take into account the evolutionary history of the progenitor stars, all the relevant energy sources involved in the cooling, element diffusion in the very outer layers, and outer boundary conditions provided by new and detailed non-gray white dwarf model atmospheres for pure helium composition. These model atmospheres are based on the most up-to-date physical inputs. Our calculations extend down to very low effective temperatures, of ∼ 2 500 K, provide a homogeneous set of evolutionary cooling tracks that are appropriate for mass and age determinations of old hydrogen-deficient white dwarfs, and represent a clear improvement over previous efforts, which were computed using gray atmospheres.

Astronomy & Astrophysics, 2017

Context. Some globular clusters host multiple stellar populations with different chemical abundan... more Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results. We explored different issues of white dwarf evolution and their helium-rich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M 0.65 M evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.

The Astrophysical Journal, 2016

Because of the large neutron excess of 22 Ne, this isotope rapidly sediments in the interior of t... more Because of the large neutron excess of 22 Ne, this isotope rapidly sediments in the interior of the white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22 Ne in the inner regions of the star, modifies the Brunt-Väisälä frequency, thus altering the pulsational properties of these stars. In this work, we discuss the impact of 22 Ne sedimentation in white dwarfs resulting from Solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs of masses 0.528, 0.576, 0.657 and 0.833 M ⊙ , derived from full evolutionary computations of their progenitor stars, starting at the Zero Age Main Sequence all the way through central hydrogen and helium burning, thermally-pulsing AGB and post-AGB phases. Our computations show that at low luminosities (log(L/L ⊙) −4.25), 22 Ne sedimentation delays the cooling of white dwarfs with Solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22 Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22 Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

The Astrophysical Journal, 2009

We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO ... more We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient 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 asymptotic giant branch 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 eff < 40,000 K, where the NLTE effects are not dominant. These new calculations provide a 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 7000 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.

The Astrophysical Journal, 2009

We present a pulsational stability analysis of hot post-asymptotic giant branch (AGB) H-deficient... more We present a pulsational stability analysis of hot post-asymptotic giant branch (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, through the thermally pulsing AGB phase and born-again episode. 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 eff-log g diagram characterized by short-period g-modes excited by the-mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long-period g-modes destabilized by the classical κ-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. As a natural application of our results, we study the particular case of VV 47, a pulsating planetary nebula nucleus (PG1159 type) that is particularly interesting because it has been reported to exhibit a rich and complex pulsation spectrum including a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical κ-mechanism, while the observed shortperiod branch below ≈300 s could correspond to modes triggered by the He-burning shell through the-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 κ-mechanism and the-mechanism of mode driving are simultaneously operating.

The Astrophysical Journal, 2009

We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white... more 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 for 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 PG 1159 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 J 142625.70+575218.4, an evolutionary link between this star and the massive PG 1159 star H1504+65, is plausible.

The Astrophysical Journal, 2010

We compute new chemical profiles for the core and envelope of white dwarfs appropriate for pulsat... more 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.

The Astrophysical Journal, 2013

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set ... more We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon-oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprises all of the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and 1.05 M known to date. The asteroseismological analysis of a set of 42 stars enables study of the ensemble properties of the massive, pulsating white dwarf stars with carbon-oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass that is high enough to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

Monthly Notices of the Royal Astronomical Society, 2011

We propose a scenario for the formation of DA white dwarfs with very thin helium buffers. For the... more We propose a scenario for the formation of DA white dwarfs with very thin helium buffers. For these stars we explore the possible occurrence of diffusion-induced CNO-flashes during their early cooling stage. In order to obtain very thin helium buffers, we simulate the formation of low-mass remnants through an asymptotic giant branch (AGB) final/late thermal pulse (AFTP/LTP scenario). Then we calculate the consequent white dwarf cooling evolution by means of a consistent treatment of element diffusion and nuclear burning. Based on physically sound white dwarf models, we find that the range of helium buffer masses for these diffusion-induced novae to occur is significantly smaller than that predicted by the only previous study of this scenario. As a matter of fact, we find that these flashes do occur only in some low-mass (M 0.6 M) and low-metallicity (Z ZAMS 0.001) remnants about 10 6-10 7 yr after departing from the AGB. For these objects, we expect the luminosity to increase by about 4 orders of magnitude in less than a decade. We also show that diffusioninduced novae should display a very typical eruption light curve, with an increase of about a few magnitudes per year before reaching a maximum of M V ∼ −5 to −6. Our simulations show that surface abundances after the outburst are characterized by log N H /N He ∼ −0.15. .. 0.6 and N > C O by mass fractions. Contrary to previous speculations we show that these events are not recurrent and do not change substantially the final H-content of the cool (DA) white dwarf. Finally, with the aid of model predictions we discuss the possibility that Nova Vul 1670 (CK Vul) and the recently proposed [WN/WC]-central stars of planetary nebulae could be observational counterparts of this diffusion-induced nova scenario. We conclude that, despite discrepancies with observations, the scenario offers one of the best available explanations for CK Vul and, with minor modifications, explains the observed properties of [WN/WC]-central stars of planetary nebulae.

Monthly Notices of the Royal Astronomical Society, 2012

We employ a state-of-the-art asteroseismological model of G117−B15A, the archetype of the H-rich ... more We employ a state-of-the-art asteroseismological model of G117−B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-baryonic particle considered as a candidate for dark matter of the Universe. Assuming that G117−B15A is truly represented by our asteroseismological model, and, in particular, that the period of the dominant mode is associated with a pulsation g mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m a cos 2 β = 17.4 +2.3 −2.7 meV.

Journal of Cosmology and Astroparticle Physics, 2013

A secular variation of the gravitational constant modifies the structure and evolutionary time sc... more A secular variation of the gravitational constant modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code and an up-to-date pulsational code we compute the effects of a secularly varying G on the pulsational properties of variable white dwarfs. Comparing the the theoretical results obtained taking into account the effects of a running G with the observed periods and measured rates of change of the periods of two well studied pulsating white dwarfs, G117-B15A and R548, we place constraints on the rate of variation of Newton's constant. We derive an upper boundĠ/G ∼ −1.8 × 10 −10 yr −1 using the variable white dwarf G117-B15A, anḋ G/G ∼ −1.3 × 10 −10 yr −1 using R548. Although these upper limits are currently less restrictive than those obtained using other techniques, they can be improved in a future measuring the rate of change of the period of massive white dwarfs.

Journal of Cosmology and Astroparticle Physics, 2012

Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as as... more Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time (Π) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoreticalΠ values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass m a cos 2 β ∼ 17.1 meV at a 2σ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117−B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.

The Astrophysical Journal Supplement Series, 2012

We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital ... more We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital Sky Survey Data Release 7 spectroscopic catalog. We find 20 407 white dwarf spectra, representing 19 712 stars, and provide atmospheric model fits to 14 120 DA and 1011 DB white dwarf spectra from 12 843 and 923 stars, respectively. These numbers represent a more than factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalog based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

Astronomy & Astrophysics, 2009

Aims. We present an asteroseismological study on the two high-gravity pulsating PG 1159 (GW Vir o... more Aims. We present an asteroseismological study on the two high-gravity pulsating PG 1159 (GW Vir or DOV) stars, PG 2131+066 and PG 1707+427, and on the pulsating [WCE] star NGC 1501. All of these stars have been intensively scrutinized through multi-site observations, so they have well resolved pulsation spectra. Methods. We compute adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M. 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. Results. We derive a stellar mass of 0.627 M for PG 2131+066, 0.597 M for PG 1707+427, and 0.571 M for NGC 1501 from a comparison between the observed period spacings and the computed asymptotic period spacings, and a stellar mass of 0.578 M for PG 2131+066, 0.566 M for PG 1707+427, and 0.576 M for NGC 1501 by comparing the observed period spacings with the average of the computed period spacings. We also find, on the basis of a period-fit procedure, asteroseismological models representatives of PG 2131+066 and PG 1707+427. These best-fit models are able to reproduce the observed period patterns of these stars with an average of the period differences of δΠ i = 1.57 s and δΠ i = 1.75 s, respectively. The best-fit model for PG 2131+066 has an effective temperature T eff = 102 100 K, a stellar mass M * = 0.589 M , a surface gravity log g = 7.63, a stellar luminosity and radius of log(L * /L) = 1.57 and log(R * /R) = −1.71, respectively, and a He-rich envelope thickness of M env = 1.6 × 10 −2 M. We derive a seismic distance d ∼ 830 pc and a parallax π ∼ 1.2 mas. The best-fit model for PG 1707+427, on the other hand, has T eff = 89 500 K, M * = 0.542 M , log g = 7.53, log(L * /L) = 1.40, log(R * /R) = −1.68, and M env = 2.5 × 10 −2 M , and the seismic distance and parallax are d ∼ 730 pc and π ∼ 1.4 mas. Finally, we have been unable to find an unambiguous best-fit model for NGC 1501 on the basis of a period-fit procedure. Conclusions. 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 PG 1159 stars with an accuracy comparable or even better than spectroscopy.

Astronomy & Astrophysics, 2006

Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variable... more Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variables). Methods. We performed an extensive g-mode stability analysis on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M , 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 PG 1159 stars have been considered. Results. We found that pulsations in PG 1159 stars are excited by the κ-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.

Astronomy & Astrophysics, 2021

Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossomi... more Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossoming area of asteroseismology, particularly of pulsating white dwarfs and pre-white dwarfs, thus continuing the impulse of its predecessor, the Kepler mission. Aims. In this paper, we present the observations from the extended TESS mission in both 120 s short-cadence and 20 s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We identify them as two new GW Vir stars, TIC 333432673 and TIC 095332541. We apply the tools of asteroseismology with the aim of deriving their structural parameters and seismological distances. Methods. We carried out a spectroscopic analysis and a spectral fitting of TIC 333432673 and TIC 095332541. We also processed and analyzed the high-precision TESS photometric light curves of the two target stars, and derived their oscillation frequencies. We performed an asteroseismological analysis of these stars on the basis of PG 1159 evolut...

The Astronomy and Astrophysics Review, 2019

Stars are extremely important astronomical objects that constitute the pillars on which the Unive... more Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components-including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from "extra dimensions", and even crystallization. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has

Astronomy & Astrophysics, 2019

Context. Ultra-massive hydrogen-rich white dwarf stars are expected to harbor oxygen/neon cores r... more Context. Ultra-massive hydrogen-rich white dwarf stars are expected to harbor oxygen/neon cores resulting from the progenitor evolution through the super-asymptotic giant branch phase. As evolution proceeds during the white dwarf cooling phase, a crystallization process resulting from Coulomb interactions in very dense plasmas is expected to occur, leading to the formation of a highly crystallized core. In particular, pulsating ultra-massive white dwarfs offer a unique opportunity to infer and test the occurrence of crystallization in white dwarf interiors as well as physical processes related with dense plasmas. Aims. We aim to assess the adiabatic pulsation properties of ultra-massive hydrogen-rich white dwarfs with oxygen/neon cores. Methods. We studied the pulsation properties of ultra-massive hydrogen-rich white dwarf stars with oxygen/neon cores. We employed a new set of ultra-massive white dwarf evolutionary sequences of models with stellar masses in the range 1.10 ≤ M⋆/M⊙ ≤ ...

The Astrophysical Journal, 2017

We present an asteroseismological analysis of four ZZ Ceti stars observed with Kepler : GD 1212, ... more We present an asteroseismological analysis of four ZZ Ceti stars observed with Kepler : GD 1212, SDSS J113655.17+040952.6, KIC 11911480 and KIC 4552982, based on a grid of full evolutionary models of DA white dwarf stars. We employ a grid of carbon-oxygen core white dwarfs models, characterized by a detailed and consistent chemical inner profile for the core and the envelope. In addition to the observed periods, we take into account other information from the observational data, as amplitudes, rotational splittings and period spacing, as well as photometry and spectroscopy. For each star, we present an asteroseismological model that closely reproduce their observed properties. The asteroseismological stellar mass and effective temperature of the target stars are (0.632 ± 0.027M , 10737 ± 73K) for GD 1212, (0.745 ± 0.007M , 11110 ± 69K) for KIC 4552982, (0.5480 ± 0.01M , 12721 ± 228K) for KIC1191480 and (0.570 ± 0.01M , 12060 ± 300K) for SDSS J113655.17+040952.6. In general, the asteroseismological values are in good agreement with the spectroscopy. For KIC 11911480 and SDSS J113655.17+040952.6 we derive a similar seismological mass, but the hydrogen envelope is an order of magnitude thinner for SDSS J113655.17+040952.6, that is part of a binary system and went through a common envelope phase.

Journal of Cosmology and Astroparticle Physics, 2014

It has been shown that the shape of the luminosity function of white dwarfs

The Astrophysical Journal, 2017

We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmosphe... more We present a set of full evolutionary sequences for white dwarfs with hydrogen-deficient atmospheres. We take into account the evolutionary history of the progenitor stars, all the relevant energy sources involved in the cooling, element diffusion in the very outer layers, and outer boundary conditions provided by new and detailed non-gray white dwarf model atmospheres for pure helium composition. These model atmospheres are based on the most up-to-date physical inputs. Our calculations extend down to very low effective temperatures, of ∼ 2 500 K, provide a homogeneous set of evolutionary cooling tracks that are appropriate for mass and age determinations of old hydrogen-deficient white dwarfs, and represent a clear improvement over previous efforts, which were computed using gray atmospheres.

Astronomy & Astrophysics, 2017

Context. Some globular clusters host multiple stellar populations with different chemical abundan... more Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ω Centauri, which shows clear evidence of a helium-enriched subpopulation characterized by a helium abundance as high as Y = 0.4 Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of helium-rich progenitors. Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005. Results. We explored different issues of white dwarf evolution and their helium-rich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored. Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the helium-rich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M 0.65 M evolve directly into helium-core white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting low-mass carbon-oxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.

The Astrophysical Journal, 2016

Because of the large neutron excess of 22 Ne, this isotope rapidly sediments in the interior of t... more Because of the large neutron excess of 22 Ne, this isotope rapidly sediments in the interior of the white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22 Ne in the inner regions of the star, modifies the Brunt-Väisälä frequency, thus altering the pulsational properties of these stars. In this work, we discuss the impact of 22 Ne sedimentation in white dwarfs resulting from Solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs of masses 0.528, 0.576, 0.657 and 0.833 M ⊙ , derived from full evolutionary computations of their progenitor stars, starting at the Zero Age Main Sequence all the way through central hydrogen and helium burning, thermally-pulsing AGB and post-AGB phases. Our computations show that at low luminosities (log(L/L ⊙) −4.25), 22 Ne sedimentation delays the cooling of white dwarfs with Solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22 Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22 Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

The Astrophysical Journal, 2009

We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient DO ... more We present full evolutionary calculations appropriate for the study of hot hydrogen-deficient 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 asymptotic giant branch 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 eff < 40,000 K, where the NLTE effects are not dominant. These new calculations provide a 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 7000 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.

The Astrophysical Journal, 2009

We present a pulsational stability analysis of hot post-asymptotic giant branch (AGB) H-deficient... more We present a pulsational stability analysis of hot post-asymptotic giant branch (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, through the thermally pulsing AGB phase and born-again episode. 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 eff-log g diagram characterized by short-period g-modes excited by the-mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long-period g-modes destabilized by the classical κ-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. As a natural application of our results, we study the particular case of VV 47, a pulsating planetary nebula nucleus (PG1159 type) that is particularly interesting because it has been reported to exhibit a rich and complex pulsation spectrum including a series of unusually short pulsation periods. We found that the long periods exhibited by VV 47 can be readily explained by the classical κ-mechanism, while the observed shortperiod branch below ≈300 s could correspond to modes triggered by the He-burning shell through the-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 κ-mechanism and the-mechanism of mode driving are simultaneously operating.

The Astrophysical Journal, 2009

We present the first full evolutionary calculations aimed at exploring the origin of hot DQ white... more 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 for 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 PG 1159 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 J 142625.70+575218.4, an evolutionary link between this star and the massive PG 1159 star H1504+65, is plausible.

The Astrophysical Journal, 2010

We compute new chemical profiles for the core and envelope of white dwarfs appropriate for pulsat... more 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.

The Astrophysical Journal, 2013

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set ... more We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon-oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprises all of the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and 1.05 M known to date. The asteroseismological analysis of a set of 42 stars enables study of the ensemble properties of the massive, pulsating white dwarf stars with carbon-oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass that is high enough to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.

Monthly Notices of the Royal Astronomical Society, 2011

We propose a scenario for the formation of DA white dwarfs with very thin helium buffers. For the... more We propose a scenario for the formation of DA white dwarfs with very thin helium buffers. For these stars we explore the possible occurrence of diffusion-induced CNO-flashes during their early cooling stage. In order to obtain very thin helium buffers, we simulate the formation of low-mass remnants through an asymptotic giant branch (AGB) final/late thermal pulse (AFTP/LTP scenario). Then we calculate the consequent white dwarf cooling evolution by means of a consistent treatment of element diffusion and nuclear burning. Based on physically sound white dwarf models, we find that the range of helium buffer masses for these diffusion-induced novae to occur is significantly smaller than that predicted by the only previous study of this scenario. As a matter of fact, we find that these flashes do occur only in some low-mass (M 0.6 M) and low-metallicity (Z ZAMS 0.001) remnants about 10 6-10 7 yr after departing from the AGB. For these objects, we expect the luminosity to increase by about 4 orders of magnitude in less than a decade. We also show that diffusioninduced novae should display a very typical eruption light curve, with an increase of about a few magnitudes per year before reaching a maximum of M V ∼ −5 to −6. Our simulations show that surface abundances after the outburst are characterized by log N H /N He ∼ −0.15. .. 0.6 and N > C O by mass fractions. Contrary to previous speculations we show that these events are not recurrent and do not change substantially the final H-content of the cool (DA) white dwarf. Finally, with the aid of model predictions we discuss the possibility that Nova Vul 1670 (CK Vul) and the recently proposed [WN/WC]-central stars of planetary nebulae could be observational counterparts of this diffusion-induced nova scenario. We conclude that, despite discrepancies with observations, the scenario offers one of the best available explanations for CK Vul and, with minor modifications, explains the observed properties of [WN/WC]-central stars of planetary nebulae.

Monthly Notices of the Royal Astronomical Society, 2012

We employ a state-of-the-art asteroseismological model of G117−B15A, the archetype of the H-rich ... more We employ a state-of-the-art asteroseismological model of G117−B15A, the archetype of the H-rich atmosphere (DA) white dwarf pulsators (also known as DAV or ZZ Ceti variables), and use the most recently measured value of the rate of period change for the dominant mode of this pulsating star to derive a new constraint on the mass of axion, the still conjectural non-baryonic particle considered as a candidate for dark matter of the Universe. Assuming that G117−B15A is truly represented by our asteroseismological model, and, in particular, that the period of the dominant mode is associated with a pulsation g mode trapped in the H envelope, we find strong indications of the existence of extra cooling in this star, compatible with emission of axions of mass m a cos 2 β = 17.4 +2.3 −2.7 meV.

Journal of Cosmology and Astroparticle Physics, 2013

A secular variation of the gravitational constant modifies the structure and evolutionary time sc... more A secular variation of the gravitational constant modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code and an up-to-date pulsational code we compute the effects of a secularly varying G on the pulsational properties of variable white dwarfs. Comparing the the theoretical results obtained taking into account the effects of a running G with the observed periods and measured rates of change of the periods of two well studied pulsating white dwarfs, G117-B15A and R548, we place constraints on the rate of variation of Newton's constant. We derive an upper boundĠ/G ∼ −1.8 × 10 −10 yr −1 using the variable white dwarf G117-B15A, anḋ G/G ∼ −1.3 × 10 −10 yr −1 using R548. Although these upper limits are currently less restrictive than those obtained using other techniques, they can be improved in a future measuring the rate of change of the period of massive white dwarfs.

Journal of Cosmology and Astroparticle Physics, 2012

Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as as... more Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time (Π) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoreticalΠ values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass m a cos 2 β ∼ 17.1 meV at a 2σ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117−B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.

The Astrophysical Journal Supplement Series, 2012

We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital ... more We present a new catalog of spectroscopically-confirmed white dwarf stars from the Sloan Digital Sky Survey Data Release 7 spectroscopic catalog. We find 20 407 white dwarf spectra, representing 19 712 stars, and provide atmospheric model fits to 14 120 DA and 1011 DB white dwarf spectra from 12 843 and 923 stars, respectively. These numbers represent a more than factor of two increase in the total number of white dwarf stars from the previous SDSS white dwarf catalog based on DR4 data. Our distribution of subtypes varies from previous catalogs due to our more conservative, manual classifications of each star in our catalog, supplementing our automatic fits. In particular, we find a large number of magnetic white dwarf stars whose small Zeeman splittings mimic increased Stark broadening that would otherwise result in an overestimated log g if fit as a non-magnetic white dwarf. We calculate mean DA and DB masses for our clean, non-magnetic sample and find the DB mean mass is statistically larger than that for the DAs.

Astronomy & Astrophysics, 2009

Aims. We present an asteroseismological study on the two high-gravity pulsating PG 1159 (GW Vir o... more Aims. We present an asteroseismological study on the two high-gravity pulsating PG 1159 (GW Vir or DOV) stars, PG 2131+066 and PG 1707+427, and on the pulsating [WCE] star NGC 1501. All of these stars have been intensively scrutinized through multi-site observations, so they have well resolved pulsation spectra. Methods. We compute adiabatic g-mode pulsation periods on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M. 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. Results. We derive a stellar mass of 0.627 M for PG 2131+066, 0.597 M for PG 1707+427, and 0.571 M for NGC 1501 from a comparison between the observed period spacings and the computed asymptotic period spacings, and a stellar mass of 0.578 M for PG 2131+066, 0.566 M for PG 1707+427, and 0.576 M for NGC 1501 by comparing the observed period spacings with the average of the computed period spacings. We also find, on the basis of a period-fit procedure, asteroseismological models representatives of PG 2131+066 and PG 1707+427. These best-fit models are able to reproduce the observed period patterns of these stars with an average of the period differences of δΠ i = 1.57 s and δΠ i = 1.75 s, respectively. The best-fit model for PG 2131+066 has an effective temperature T eff = 102 100 K, a stellar mass M * = 0.589 M , a surface gravity log g = 7.63, a stellar luminosity and radius of log(L * /L) = 1.57 and log(R * /R) = −1.71, respectively, and a He-rich envelope thickness of M env = 1.6 × 10 −2 M. We derive a seismic distance d ∼ 830 pc and a parallax π ∼ 1.2 mas. The best-fit model for PG 1707+427, on the other hand, has T eff = 89 500 K, M * = 0.542 M , log g = 7.53, log(L * /L) = 1.40, log(R * /R) = −1.68, and M env = 2.5 × 10 −2 M , and the seismic distance and parallax are d ∼ 730 pc and π ∼ 1.4 mas. Finally, we have been unable to find an unambiguous best-fit model for NGC 1501 on the basis of a period-fit procedure. Conclusions. 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 PG 1159 stars with an accuracy comparable or even better than spectroscopy.

Astronomy & Astrophysics, 2006

Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variable... more Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variables). Methods. We performed an extensive g-mode stability analysis on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M , 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 PG 1159 stars have been considered. Results. We found that pulsations in PG 1159 stars are excited by the κ-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.