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G 29 − 38 (TIC 422526868) is one of the brightest (V  = 13.1) and closest (d  = 17.51 pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the TESS spacecraft in sectors 42 and 56. The atmosphere of G 29 − 38 is polluted by heavy elements that are expected to sink out of visible layers on short time-scales. The photometric TESS data set spans ∼51 d in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with g -mode pulsations, with periods spanning from ∼ 260 to ∼ 1400 s. We identified rotational frequency triplets with a mean separation δνℓ = 1 of 4.67 μHz and a quintuplet with a mean separation δνℓ = 2 of 6.67 μHz, from which we estimated a rotation period of about 1.35 ± 0.1 d. We determined a constant period spacing of 41.20 s for ℓ = 1 modes and 22.58 s for ℓ = 2 modes. We performed period-to-period fit analyses and found an asteroseismological model with M ⋆/M⊙ = 0.632 ± 0.03, |$T_{\rm eff}=11\, 635\pm 178$| K, and log  g  = 8.048 ± 0.005 (with a hydrogen envelope mass of M H ∼ 5.6 × 10−5 M ⋆), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by Gaia (17.51 pc). [ABSTRACT FROM AUTHOR]

One of the largest uncertainties in stellar evolutionary computations is the accuracy of the considered reaction rates. The 12C(α ,  γ)16O reaction is particularly important for the study of low- and intermediate-mass stars as it determines the final C/O ratio in the core which influences the white dwarf (WD) cooling evolution. Thus, there is a need for a study of how the computations of WDs and their progenitors that are made to date may be affected by the uncertainties of the 12C(α ,  γ)16O reaction rates. In this work, we compute fully evolutionary sequences using the mesa code with initial masses in the range of 0.90 ≤ M i/M⊙ ≤ 3.05. We consider different adopted reaction rates, obtained from the literature, as well as the extreme limits within their uncertainties. As expected, we find that previous to the core helium burning (CHB) stage, there are no changes to the evolution of the stars. However, the subsequent stages are all affected by the uncertainties of the considered reaction rate. In particular, we find differences to the convective core mass during the CHB stage which may affect pulsation properties of subdwarfs, the number of thermal pulses during the asymptotic giant branch and trends between final oxygen abundance in the core and the progenitor masses of the remnant WDs. [ABSTRACT FROM AUTHOR]

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% 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 allowed us to make meaningful comparison of evolutionary models with observations. While some information like surface chemical composition, temperature and gravity of isolated white dwarfs can be inferred from spectroscopy, and the total mass and radius can be derived as well when they are in binaries, the internal structure of these compact stars can be unveiled only by means of asteroseismology, an approach based on the comparison between the observed pulsation periods of variable stars and the periods predicted by appropriate theoretical models. The asteroseismological techniques allow us to infer details of the internal chemical stratification, the total mass, and even the stellar rotation profile. In this review, we first revise the evolutionary channels currently accepted that lead to the formation of white-dwarf stars, and then, we give a detailed account of the different sub-types of pulsating white dwarfs known so far, emphasizing the recent observational and theoretical advancements in the study of these fascinating variable stars. [ABSTRACT FROM AUTHOR]

We explore the evolution of hydrogen-rich and hydrogen-deficient white dwarf stars with masses between 1.012 and 1.307 M⊙, and initial metallicity of Z = 0.02. These sequences are the result of main-sequence stars with masses between 8.8 and 11.8 M⊙. The simulations were performed with mesa (Modules for Experiments in Stellar Astrophysics), starting at the zero-age main sequence, through thermally pulsing and mass-loss phases, ending at the white dwarf cooling sequence. We present reliable chemical profiles for the whole mass range considered, covering the different expected central compositions (i.e. C/O, O/Ne and Ne/O/Mg) and its dependence on the stellar mass. In addition, we present detailed chemical profiles of hybrid C/O–O/Ne core white dwarfs, found in the mass range between 1.024 and 1.15 M⊙. We present the initial-to-final mass relation, the mass–radius relation and cooling times considering the effects of atmosphere and core composition. [ABSTRACT FROM AUTHOR]

The blue large-amplitude pulsators (BLAPs) constitute a new class of pulsating stars. They are hot stars with effective temperatures of ∼30 000 K and surface gravities of log g ∼ 4.9, that pulsate with periods in the range 20−40 min. Until now, their origin and evolutionary state, as well as the nature of their pulsations, were not been unveiled. In this paper, we propose that the BLAPs are the hot counterpart of the already known pulsating pre-extremely low mass (pre-ELM) white dwarf (WD) stars, that are He-core low-mass stars resulting from interacting binary evolution. Using fully evolutionary sequences, we show that the BLAPs are well represented by pre-ELM WD models with high effective temperature and stellar masses ∼0.34 M⊙. From the analysis of their pulsational properties, we find that the observed variabilities can be explained by high-order non-radial g-mode pulsations or, in the case of the shortest periods, also by low-order radial modes, including the fundamental radial mode. The theoretical modes with periods in the observed range are unstable due to the κ mechanism associated with the Z-bump in the opacity at log T ∼ 5.25. [ABSTRACT FROM AUTHOR]

Context. The thermally pulsing phase on the asymptotic giant branch (TP-AGB) is the last nuclear burning phase experienced by most low- and intermediate-mass stars. During this phase, the outer chemical stratification above the C/O core of the emerging white dwarf (WD) is built up. The chemical structure resulting from progenitor evolution strongly impacts the whole pulsation spectrum exhibited by ZZ Ceti stars, which are pulsating C/O core white dwarfs located on a narrow instability strip at Teff ~ 12 000 K. Several physical processes occurring during progenitor evolution strongly affect the chemical structure of these stars; those found during the TP-AGB phase are the most relevant for the pulsational properties of ZZ Ceti stars. Aims. We present a study of the impact of the chemical structure built up during the TP-AGB evolution on the stellar parameters inferred from asteroseismological fits of ZZ Ceti stars. Methods. Our analysis is based on a set of carbon-oxygen core white dwarf models with masses from 0.534 to 0:6463 M☉ derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We computed evolutionary sequences that experience different number of thermal pulses (TP). Results. We find that the occurrence or not of thermal pulses during AGB evolution implies an average deviation in the asteroseimological effective temperature of ZZ Ceti stars of at most 8% and on the order of ≲5% in the stellar mass. For the mass of the hydrogen envelope, however, we find deviations up to 2 orders of magnitude in the case of cool ZZ Ceti stars. Hot and intermediate temperature ZZ Ceti stars show no differences in the hydrogen envelope mass in most cases. Conclusions. Our results show that, in general, the impact of the occurrence or not of thermal pulses in the progenitor stars is not negligible and must be taken into account in asteroseismological studies of ZZ Ceti stars. [ABSTRACT FROM AUTHOR]

The so-called sdA stars are defined by having H-rich spectra and surface gravities similar to hot subdwarf stars, but effective temperature below the zero-age horizontal branch. Their evolutionary history is an enigma: their surface gravity is too high for main-sequence stars, but too low for single evolution white dwarfs. They aremost likely byproducts of binary evolution, including blue-stragglers, extremely-low mass white dwarf stars (ELMs) and their precursors (pre-ELMs). A small number of ELMs with similar properties to sdAs is known. Other possibilities include metal-poor A/F dwarfs, second generation stars, or even stars accreted from dwarf galaxies. In this work, we analyse colours, proper motions, and spacial velocities of a sample of sdAs from the Sloan Digital Sky Survey to assess their nature and evolutionary origin. We define a probability of belonging to the main sequence and a probability of being a (pre-)ELM based on these properties. We find that 7 per cent of the sdAs are more likely to be (pre-)ELMs than main-sequence stars. However, the spacial velocity distribution suggests that over 35 per cent of them cannot be explained as single metal-poor A/F stars. [ABSTRACT FROM AUTHOR]

We present a theoretical study on the metallicity dependence of the initial-to-final mass relation and its influence on white dwarf age determinations. We compute a grid of evolutionary sequences from the main sequence to ~3000 K on the white dwarf cooling curve, passing through all intermediate stages. During the thermally pulsing asymptotic giant branch no third dredge-up episodes are considered and thus the photospheric C/O ratio is below unity for sequences with metallicities larger than Z = 0.0001. We consider initial metallicities from Z = 0.0001 to 0.04, accounting for stellar populations in the galactic disc and halo, with initial masses below ~3 M⊙. We found a clear dependence of the shape of the initial-to-final mass relation with the progenitor metallicity, where metal-rich progenitors result in less massive white dwarf remnants, due to an enhancement of the mass-loss rates associated with high metallicity values. By comparing our theoretical computations with semi-empirical data from globular and old open clusters, we found that the observed intrinsic mass spread can be accounted for by a set of initial-to-final mass relations characterized by different metallicity values. Also, we confirm that the lifetime spent before the white dwarf stage increases with metallicity. Finally, we estimate the mean mass at the top of the white dwarf cooling curve for three globular clusters NGC 6397, M4 and 47 Tuc, around 0.53 M⊙, characteristic of old stellar populations. However, we found different values for the progenitor mass, lower for the metal-poor cluster, NGC 6397, and larger for the younger and metal-rich cluster 47 Tuc, as expected from the metallicity dependence of the initial-to-final mass relation. [ABSTRACT FROM AUTHOR]

We report on the identification of 22 new cool DO-type white dwarfs (WDs) detected in data release 10 (DR10) of the Sloan Digital Sky Survey (SDSS). Among them, we found one more member of the so-called hot-wind DO WDs, which show ultrahigh excitation absorption lines. Our non-LTE model atmosphere analyses of these objects and two not previously analyzed hot-wind DO WDs, revealed effective temperatures and gravities in the ranges Teff =45-80 kK and log g=7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by mass). Two of these are the coolest DO WDs ever discovered that still show a considerable amount of C in their atmospheres. This is in strong contradiction with diffusion calculations, and probably, similar to what is proposed for DBWDs, a weak mass-loss is present in DO WDs. One object is the most massive DO WD discovered so far with a mass of 1.07 M☉ if it is an ONe-WD or 1.09 M☉ if it is a CO-WD. We furthermore present the mass distribution of all known hot non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7 spectroscopic sample. The mass distribution of DO WDs beyond the wind limit strongly deviates from the mass distribution of the objects before the wind limit. We address this phenomenon by applying different evolutionary input channels. We argue that the DO WD channel may be fed by about 13% by post-extreme-horizontal branch stars and that PG1159 stars and O(He) stars may contribute in a similar extent to the non-DA WD channel. [ABSTRACT FROM AUTHOR]

We have detected 21 new, very hot hydrogen-deficient (pre-) white dwarfs (spectral types O(He), hot DO, PG1159) in the Data Release 10 of the Sloan Digital Sky Survey. We present the results of non-local thermodynamic equilibrium (non-LTE) model-atmosphere analyses. The seven PG1159 stars have effective temperatures and surface gravities in the ranges Teff = 100 000-140 000K and log g = 6-7.5. The hottest one is a new member of the rare hybrid-PG1159 subtype, indicating the presence of residual hydrogen. The ten new hot DOs span Teff = 65 000-120 000 K and log g = 7.5-8. Carbon abundances were derived and it is found that the PG1159 star values (C/He = 0.33-0.03 by number) and the DO values (<0.0004-0.01) cover distinct but adjacent ranges. With the discovery of four new O(He) stars (Teff = 80 000-100 000 K, log g = 5-5.5, C/He = 0.002-0.01), we double the number of known members of this rare spectral class. One of them is particularly interesting because it is unusually rich in C, N, O, and Si. Unexpected problems arose with the DOs because many He II lines of every other star are so deep that they cannot be matched by any model. This is most conspicuous in the case of one object which exhibits the occurrence of ultrahigh-ionization metal features, a hitherto unexplained phenomenon that is known from a disquieting large fraction of hot DOs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present the discovery of photometric variability in the DQ white dwarf SDSS J103655.39+652252.2(SDSS J1036+6522). Time-series photometry reveals a coherent monoperiodic modulation at a period of 1115.64751(67) s with an amplitude 0.442% ± 0.024%; no other periodic modulations are observed with amplitudes ≳ 0.13%. The period, amplitude, and phase of this modulation are constant within errors over16 months. The spectrum of SDSS J1036+6522 shows magnetic splitting of carbon lines, and we use Paschen–Back formalism to develop a grid of model atmospheres for mixed carbon and helium atmospheres. Our models,while reliant on several simplistic assumptions, nevertheless match the major spectral and photometric properties of the star with a self-consistent set of parameters: Teff ≈ 15,500 K, log g ≈ 9, log(C/He) = −1.0,and a mean magnetic field strength of 3.0 ± 0.2 MG. The temperature and abundances strongly suggest that SDSS J1036+6522 is a transition object between the hot, carbon-dominated DQs and the cool, helium-dominated DQs. The variability of SDSS J1036+6522 has characteristics similar to those of the variable hotcarbon-atmosphere white dwarfs (DQVs), however, its temperature is significantly cooler. The pulse profile of SDSS J1036+6522 is nearly sinusoidal, in contrast with the significantly asymmetric pulse shapes of the known magnetic DQVs. If the variability in SDSS J1036+6522 is due to the same mechanism as other DQVs,then the pulse shape is not a definitive diagnostic on the absence of a strong magnetic field in DQVs. It remains unclear whether the root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the same. [ABSTRACT FROM AUTHOR]

ABSTRACT ZZ Ceti stars form the most numerous group of degenerate variable stars. They are otherwise normal DA (H-rich atmospheres) white dwarfs that exhibit pulsations. Here, we present an asteroseismological analysis for 44 bright ZZ Ceti stars based on a new set of fully evolutionary DA white dwarf models characterized by detailed chemical profiles from the centre to the surface. One of our targets is the archetypal ZZ Ceti star G117−B15A, for which we obtain an asteroseismological model with an effective temperature and a surface gravity in excellent agreement with the spectroscopy. The asteroseismological analysis of a set of 44 ZZ Ceti stars has the potential to characterize the global properties of the class, in particular the thicknesses of the hydrogen envelope and the stellar masses. Our results support the belief that white dwarfs in the solar neighbourhood harbour a broad range of hydrogen-layer thickness. [ABSTRACT FROM AUTHOR]

We used the detected pulsation modes and adiabatic pulsation models to do seismology of the class of ZZ Ceti stars and measure the H layer mass for 83 stars. We found the surface hydrogen layer to be within the range , with an average of , which is thinner than the predicted value of , indicating that the stars lose more mass during their evolution than previously expected. These results are preliminary and do not include the possible effects of realistic C/O profiles on the fits. [ABSTRACT FROM AUTHOR]

We determined masses for the 7167 DA and 507 DB white dwarf stars classified as single and non-magnetic in Data Release 4 of the Sloan Digital Sky Survey (SDSS). We obtained revised and log g determinations for the most massive stars by fitting the SDSS optical spectra with a synthetic spectra grid derived from model atmospheres extending to . We also calculate radii from evolutionary models and create volume-corrected mass distributions for our DA and DB samples. The mean mass for the DA stars brighter than and hotter than is . For the 150 DBs brighter than and hotter than , we find . It appears the mean mass for DB white dwarf stars may be significantly larger than that for DAs. We also report the highest mass white dwarf stars ever found, up to . [ABSTRACT FROM AUTHOR]