ALBERT

Description: Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asymptotic giant branch (AGB), type Ia supernova explosions, and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these white dwarfs, there are few evolutionary studies in the literature devoted to them. Here we present new ultra-massive white dwarf evolutionary sequences that constitute an improvement over previous ones. In these new sequences we take into account for the first time the process of phase separation expected during the crystallization stage of these white dwarfs by relying on the most up-to-date phase diagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution during the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditions for our evolving models are provided by detailed non-gray white dwarf model atmospheres for hydrogen and helium composition. We assessed the impact of all these improvements on the evolutionary properties of ultra-massive white dwarfs, providing updated evolutionary sequences for these stars. We conclude that crystallization is expected to affect the majority of the massive white dwarfs observed with effective temperatures below 40 000 K. Moreover, the calculation of the phase separation process induced by crystallization is necessary to accurately determine the cooling age and the mass-radius relation of massive white dwarfs. We also provide colors in the Gaia photometric bands for our H-rich white dwarf evolutionary sequences on the basis of new model atmospheres. Finally, these new white dwarf sequences provide a new theoretical frame to perform asteroseismological studies on the recently detected ultra-massive pulsating white dwarfs.

Description: 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⊙ ≤ 1.29 computed by taking into account the complete evolution of the progenitor stars and the white dwarf stage. During the white dwarf cooling phase, we considered element diffusion. When crystallization set on in our models, we took into account latent heat release and also the expected changes in the core chemical composition that are due to phase separation according to a phase diagram suitable for oxygen and neon plasmas. We computed nonradial pulsation g-modes of our sequences of models at the ZZ Ceti phase by taking into account a solid core. We explored the impact of crystallization on their pulsation properties, in particular, the structure of the period spectrum and the distribution of the period spacings. Results. We find that it would be possible, in principle, to discern whether a white dwarf has a nucleus made of carbon and oxygen or a nucleus of oxygen and neon by studying the spacing between periods. Conclusions. The features found in the period-spacing diagrams could be used as a seismological tool to discern the core composition of ultra-massive ZZ Ceti stars, this should be complemented with detailed asteroseismic analysis using the individual observed periods.

Description: Context. The possible existence of warm (Teff ∼ 19 000 K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. These studies reported the occurrence of g-mode pulsational instabilities due to the κ mechanism acting in the partial ionization zone of He below the H envelope in models of DA WDs with very thin H envelopes (MH/M⋆ ≲ 10−10). However, to date, no pulsating warm DA WD has been discovered, despite the varied theoretical and observational evidence suggesting that a fraction of WDs should be formed with a range of very low H content. Aims. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by the TESS satellite up to Sector 9 in order to search for the possible pulsational signal. Methods. We computed WD evolutionary sequences of masses 0.58 and 0.80 M⊙ with H content in the range −14.5 ≲ log(MH/M⋆)≲ − 10, appropriate for the study of pulsational instability of warm DA WDs. Initial models were extracted from progenitors that were evolved through very late thermal pulses on the early cooling branch. We use LPCODE stellar code into which we have incorporated a new full-implicit treatment of time-dependent element diffusion to precisely model the H–He transition zone in evolving WD models with very low H content. The nonadiabatic pulsations of our warm DA WD models were computed in the effective temperature range of 30 000 − 10 000 K, focusing on ℓ = 1 g modes with periods in the range 50 − 1500 s. Results. We find that traces of H surviving the very late thermal pulse float to the surface, eventually forming thin, growing pure H envelopes and rather extended H–He transition zones. We find that such extended transition zones inhibit the excitation of g modes due to partial ionization of He below the H envelope. Only in the cases where the H–He transition is assumed much more abrupt than predicted by diffusion do models exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of −14.5 ≲ log(MH/M⋆)≲ − 10 and at effective temperatures higher than those typical for ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Conclusions. Our study suggests that the nondetection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H–He transition zone. This could be considered as indirect proof that element diffusion indeed operates in the interior of WDs.

Description: Aims. We analyzed the effect of the sedimentation of 22Ne on the local white dwarf luminosity function by studying scenarios under different Galactic metallicity models. Methods. We use an advanced population synthesis code based on Monte Carlo techniques to derive the synthetic luminosity function. The code incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases under different scenarios. We first analyzed the case for a model with constant solar metallicity and compared the models with and without 22Ne sedimentation with the observed luminosity function for a pure thin-disk population. Then we analyzed the possible effects of a thick-disk contribution. We also studied model scenarios with different metallicities, including 22Ne sedimentation. The analysis was quantified from a statistical χ2-test value for the complete and also for the most significant regions of the white dwarf luminosity function. Finally, a best-fit model along with a disk age estimate was derived. Results. Models with constant solar metallicity cannot simultaneously reproduce the peak and cutoff of the white dwarf luminosity function. The additional release of energy due to 22Ne sedimentation piles up more objects in brighter bins of the faint end of the luminosity function. The contribution of a single-burst thick-disk population increases the number of stars in the magnitude interval centered around Mbol = 15.75. The metallicity model that follows a Twarog profile is disposable. Our best-fit model was obtained when a dispersion in metallicities of about solar metallicity was considered along with a 22Ne sedimentation model, a thick-disk contribution, and an age of the thin disk of 8.8 ± 0.2 Gyr. Conclusions. Our population synthesis model is able to reproduce the local white dwarf luminosity function with a high degree of precision when a dispersion in metallicities around a model with solar values is adopted. Although the effects of 22Ne sedimentation are only marginal and the contribution of a thick-disk population is minor, both of them help in better fitting the peak and the cutoff regions of the white dwarf luminosity function.

Description: Context. Ultra-massive (≳1 M⊙) hydrogen-rich (DA) white dwarfs are expected to have a substantial portion of their cores in a crystalline state at the effective temperatures characterising the ZZ Ceti instability strip (Teff ∼ 12 500 K) as a result of Coulomb interactions in very dense plasmas. Asteroseismological analyses of these white dwarfs can provide valuable information related to the crystallisation process, the core chemical composition, and the evolutionary origin of these stars. Aims. We present a thorough asteroseismological analysis of the ultra-massive ZZ Ceti star BPM 37093, which exhibits a rich period spectrum, on the basis of a complete set of fully evolutionary models that represent ultra-massive oxygen/neon (ONe) core DA white dwarf stars harbouring a range of hydrogen (H) envelope thicknesses. We also carry out preliminary asteroseismological inferences on two other ultra-massive ZZ Ceti stars that exhibit fewer periods, GD 518, and SDSS J0840+5222. Methods. We considered g-mode adiabatic pulsation periods for ultra-massive ONe-core DA white dwarf models with stellar masses in the range 1.10 ≲ M⋆/M⊙ ≲ 1.29, effective temperatures in the range 10 000 ≲ Teff ≲ 15 000 K, and H-envelope thicknesses in the interval −10 ≲ log(MH/M⋆)≲ − 6. We explored the effects of employing different H-envelope thicknesses on the mode-trapping properties of our ultra-massive ONe-core DA white dwarf models and performed period-to-period fits to ultra-massive ZZ Ceti stars with the aim of finding an asteroseismological model for each target star. Results. We find that the trapping cycle and trapping amplitude are larger for thinner H envelopes, and that the asymptotic period spacing is longer for thinner H envelopes. We find a mean period spacing of ΔΠ ∼ 17 s in the data of BPM 37093, which is likely to be associated with ℓ = 2 modes. However, we are not able to put constraints on the stellar mass of BPM 37093 using this mean period spacing due to the simultaneous sensitivity of ΔΠ with M⋆, Teff, and MH, which is an intrinsic property of DAV stars. We find asteroseismological models for the three objects under analysis, two of them (BPM 37093 and GD 518) characterised by canonical (thick) H envelopes, and the third one (SDSS J0840+5222) with a thinner H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. The percentage of crystallised mass for these asteroseismological models is 92%, 97%, and 81% for BPM 37093, GD 518, and SDSS J0840+5222, respectively. We also derive asteroseismological distances which differ somewhat from the astrometric measurements of Gaia for these stars. Conclusions. Asteroseismological analyses like the one presented in this paper could lead to a more complete understanding of the processes occurring during crystallisation inside white dwarfs. Also, such analyses could make it possible to deduce the core chemical composition of ultra-massive white dwarfs and, in this way, to infer their evolutionary origin, such as the correlation between a star’s ONe core and its having originated through single-star evolution or a carbon/oxygen (CO) core indicating the star is the product of a merger of the two components of a binary system. However, in order to achieve these objectives, it is necessary to find a greater number of pulsating ultra-massive WDs and to carry out additional observations of known pulsating stars to detect more pulsation periods. Space missions such as TESS can provide a great boost towards achieving these aims.

Description: Aims. We analyzed the velocity space of the thin- and thick-disk Gaia white dwarf population within 100 pc by searching for signatures of the Hercules stellar stream. We aimed to identify objects belonging to the Hercules stream, and by taking advantage of white dwarf stars as reliable cosmochronometers, to derive a first age distribution. Methods. We applied a kernel density estimation to the UV velocity space of white dwarfs. For the region where a clear overdensity of stars was found, we created a 5D space of dynamic variables. We applied a hierarchichal clustering method, HDBSCAN, to this 5D space, and identified those white dwarfs that share similar kinematic characteristics. Finally, under general assumptions and from their photometric properties, we derived an age estimate for each object. Results. The Hercules stream was first revealed as an overdensity in the UV velocity space of the thick-disk white dwarf population. Three substreams were then found: Hercules a and Hercules b, formed by thick-disk stars with an age distribution that peaked 4 Gyr in the past and extends to very old ages; and Hercules c, with a ratio of 65:35 of thin to thick stars and a more uniform age distribution that is younger than 10 Gyr.