We present a detailed spectroscopic analysis of 115 helium-line (DB) and 28 cool, He-rich hydrogen-line (DA) white dwarfs based on atmosphere fits to optical spectroscopy and photometry. We find that ...63% of our DB population show hydrogen lines, making them DBA stars. We also demonstrate the persistence of pure DB white dwarfs with no detectable hydrogen feature at low effective temperatures. Using state-of-the art envelope models, we next compute the total quantity of hydrogen, MH, that is contained in the outer convection zone as a function of effective temperature and atmospheric H/He ratio. We find that some (Teff, MH) pairs cannot physically exist as a homogeneously mixed structure; such a combination can only occur as stratified objects of the DA spectral type. On that basis, we show that the values of MH inferred for the bulk of the DBA stars are too large and incompatible with the convective dilution scenario. We also present evidence that the hydrogen abundances measured in DBA and cool, helium-rich white dwarfs cannot be globally accounted for by any kind of accretion mechanism onto a pure DB star. We suggest that cool, He-rich DA white dwarfs are most likely created by the convective mixing of a DA star with a thin hydrogen envelope; they are not cooled down DBAs. We finally explore several scenarios that could account for the presence of hydrogen in DBA stars.
As they evolve, white dwarfs undergo major changes in surface composition, a phenomenon known as spectral evolution. In particular, some stars enter the cooling sequence with helium atmospheres (type ...DO) but eventually develop hydrogen atmospheres (type DA), most likely through the upward diffusion of residual hydrogen. Our empirical knowledge of this process remains scarce: the fractions of white dwarfs that are born helium rich and that experience the DO-to-DA transformation are poorly constrained. We tackle this issue by performing a detailed model-atmosphere investigation of 1806 hot ( ≥ 30,000 K) white dwarfs observed spectroscopically by the Sloan Digital Sky Survey. We first introduce our new generations of model atmospheres and theoretical cooling tracks, both appropriate for hot white dwarfs. We then present our spectroscopic analysis, from which we determine the atmospheric and stellar parameters of our sample objects. We find that ∼24% of white dwarfs begin their degenerate life as DO stars, among which ∼2/3 later become DA stars. We also infer that the DO-to-DA transition occurs at substantially different temperatures (75,000 K > > 30,000 K) for different objects, implying a broad range of hydrogen content within the DO population. Furthermore, we identify 127 hybrid white dwarfs, including 31 showing evidence of chemical stratification, and we discuss how these stars fit in our understanding of the spectral evolution. Finally, we uncover significant problems in the spectroscopic mass scale of very hot ( > 60,000 K) white dwarfs.
We present a detailed spectroscopic and photometric analysis of 219 DA and DB white dwarfs for which trigonometric parallax measurements are available. Our aim is to compare the physical parameters ...derived from the spectroscopic and photometric techniques, and then to test the theoretical mass-radius relation for white dwarfs using these results. The agreement between spectroscopic and photometric parameters is found to be excellent, especially for effective temperatures, showing that our model atmospheres and fitting procedures provide an accurate, internally consistent analysis. The values of surface gravity and solid angle obtained, respectively, from spectroscopy and photometry, are combined with parallax measurements in various ways to study the validity of the mass-radius relation from an empirical point of view. After a thorough examination of our results, we find that 73% and 92% of the white dwarfs are consistent within 1 and 2 confidence levels, respectively, with the predictions of the mass-radius relation, thus providing strong support to the theory of stellar degeneracy. Our analysis also allows us to identify 15 stars that are better interpreted in terms of unresolved double degenerate binaries. Atmospheric parameters for both components in these binary systems are obtained using a novel approach. We further identify a few white dwarfs that are possibly composed of an iron core rather than a carbon/oxygen core, since they are consistent with Fe-core evolutionary models.
The Pulsating White Dwarf Stars Fontaine, G.; Brassard, P.
Publications of the Astronomical Society of the Pacific,
10/2008, Letnik:
120, Številka:
872
Journal Article
Recenzirano
Odprti dostop
We present a summary of what is currently known about the three distinct families of isolated pulsating white dwarfs. These are the GW Vir stars (He/C/O-atmosphere stars with
T
eff ≃ 120,000 K
T
eff
...≃
120
,
000
K
), the V777 Her stars (He-atmosphere,
T
eff ≃ 25,000 K
T
eff
≃
25
,
000
K
), and the ZZ Ceti stars (H-atmosphere,
T
eff ≃ 12,000 K
T
eff
≃
12
,
000
K
), all showing multiperiodic luminosity variations caused by low-order and low-degree
g
g
-mode instabilities. We also provide, in anAppendix, a very brief overview of the newly found evidence in favor of the existence of a fourth category of oscillating white dwarfs bearing strong similarities with these families of pulsators. We begin our survey with a short historical introduction, followed by a general discussion of pulsating white dwarfs as compact pulsators. We then discuss the class properties of these objects, including an updated census. We next focus on the instability domains for each family of pulsators in thelog g - T
eff
log
g
-
T
eff
diagram, and present their time-averaged properties in more detail. This is followed by a section on excitation physics, i.e., the causes of the pulsational instabilities, with emphasis on the common properties of the different types of pulsator. We then discuss the time-dependent properties of the pulsating white dwarfs featuring, among other things, a brief “picture tour” across the ZZ Ceti instability strip. We next review the methods used to infer or constrain the angular geometry of a pulsation mode in a white dwarf. These include multicolor photometry and time-resolved spectroscopy, the exploitation of the nonlinear features in the observed light curves, and rotational splitting. We also consider basic adiabatic asteroseismology starting with a discussion of the reaction of the period spectrum to variations of model parameters. We next review the various asteroseismological inferences that have so far been claimed for white dwarfs. We also discuss the potential of exploiting the rates of period change. We finally provide some concluding remarks, including a list with several suggestions for future progress in the field.
Starch has been employed via layer by layer assembly for building an efficient and sustainable biobased coatings capable of protecting cotton from fire. In order to obtain a better understanding of ...the coating to substrate relationship, the coating efficiency has been tested on cotton fabrics having different densities (i.e., 100, 200, and 400 g/m2). The adopted deposition conditions allow for the buildup of a homogeneous coating even at a low number of deposition steps. The physical and chemical mechanisms are described and related to the achieved results. The coating can greatly enhance the char forming ability of cellulose, nearly doubling the amount of thermally stable organic residue produced by cotton at high temperatures, as assessed by thermogravimetric analyses. After only 2 bilayers deposited, this biobased system is capable of self-extinguishing a flame during flammability tests with less than 5% in weight deposited on cotton. This high efficiency is kept even when the coating is deposited on cotton with the highest density. By cone calorimetry, all treated cottons showed significant reductions (up to 40%) of the total heat released during combustion, thus demonstrating the high efficiency achieved.
We present a critical review of the determination of fundamental parameters of white dwarfs discovered by the Gaia mission. We first reinterpret color-magnitude and color-color diagrams using ...photometric and spectroscopic information contained in the Montreal White Dwarf Database (MWDD), combined with synthetic magnitudes calculated from a self-consistent set of model atmospheres with various atmospheric compositions. The same models are then applied to measure the fundamental parameters of white dwarfs using the so-called photometric technique, which relies on the exquisite Gaia trigonometric parallax measurements, and photometric data from Panoramic Survey Telescope And Rapid Response System, Sloan Digital Sky Survey, and Gaia. In particular, we discuss at length the systematic effects induced by these various photometric systems. We then study in great detail the mass distribution as a function of effective temperature for the white dwarfs spectroscopically identified in the MWDD, as well as for the white dwarf candidates discovered by Gaia. We pay particular attention to the assumed atmospheric chemical composition of cool, non-DA stars. We also briefly revisit the validity of the mass-radius relation for white dwarfs and the recent discovery of the signature of crystallization in the Gaia color-magnitude diagram for DA white dwarfs. We finally present evidence that the core composition of most of these white dwarfs is, in bulk, a mixture of carbon and oxygen, an expected result from stellar evolution theory, but never empirically well established before.
ABSTRACT We present the second of a two-part seismic analysis of the bright, hot ZZ Ceti stars GD 165 and Ross 548. In this second part, we report the results of detailed searches in parameter space ...for identifying an optimal model for each star that can account well for the observed periods, while being consistent with the spectroscopic constraints derived in our first paper. We find optimal models for each target that reproduce the six observed periods well within ∼0.3% on the average. We also find that there is a sensitivity on the core composition for Ross 548, while there is practically none for GD 165. Our optimal model of Ross 548, with its thin envelope, indeed shows weight functions for some confined modes that extend relatively deep into the interior, thus explaining the sensitivity of the period spectrum on the core composition in that star. In contrast, our optimal seismic model of its spectroscopic sibling, GD 165 with its thick envelope, does not trap/confine modes very efficiently, and we find weight functions for all six observed modes that do not extend into the deep core, hence accounting for the lack of sensitivity in that case. Furthermore, we exploit after the fact the observed multiplet structure that we ascribe to rotation. We are able to map the rotation profile in GD 165 (Ross 548) over the outermost ∼20% (∼5%) of its radius, and we find that the profile is consistent with solid-body rotation.
We revisit the problem of the formation of DB white dwarfs, as well as the origin of hydrogen in DBA stars, using a new set of envelope model calculations with stratified and mixed hydrogen/helium ...compositions. We first describe an approximate model to simulate the so-called convective dilution process, where a thin, superficial hydrogen radiative layer is gradually eroded by the underlying and more massive convective helium envelope, thus transforming a DA white dwarf into a DB star. We show that this convective dilution process is able to account for the large increase in the number of DB white dwarfs below Teff ∼ 20,000 K, but that the residual hydrogen abundances expected from this process are still orders of magnitude lower than those observed in DBA white dwarfs. Scenarios involving the accretion of hydrogen from the interstellar medium or other external bodies have often been invoked to explain these overabundances of hydrogen. In this paper, we describe a new paradigm where hydrogen, initially diluted within the thick stellar envelope, is still present and slowly diffuses upward in the deeper layers of a Teff ∼ 20,000 K white dwarf. When the convective dilution process occurs, the bottom of the mixed H/He convection zone sinks deep into the star, resulting in large amounts of hydrogen being dredged up to the stellar surface, a phenomenon similar to that invoked in the context of DQ white dwarfs.
Asteroseismology is a powerful tool to unravel the chemical composition and stratification inside white dwarfs, as recently achieved by Giammichele et al. (2018, Nature, 554, 73) for the pulsating DB ...star KIC 08626021. However, Timmes et al. (2018, ApJ, 867, L30) pointed out that neglecting the effects of neutrino cooling, such as in the models used in Giammichele et al. study, could significantly impact the derived seismic solution and compromise conclusions drawn upon it. In this context, we perform a complete reevaluation of the seismic solution uncovered for KIC 08626021, using improved static models which incorporate more realistic luminosity profiles that reflect the still significant energy losses induced by neutrino emission mechanisms in hot DB white dwarfs. We find that including (or neglecting) neutrino cooling for the specific case of KIC 08626021 induces frequency differences of ∼35 μHz on average (with variations up to ∼84 μHz) for the relevant g-modes, that is, similar to the frequency shifts estimated in Timmes et al. study. However, we show that the propagation of these variations into the derived seismic model properties remain limited and mainly trigger changes of the C/O and C/He composition ratio in the intermediate layers of the seismic model, while other important parameters are only slightly affected. In particular, the derived central oxygen mass fraction and extent of the homogeneous inner part of the core are essentially unchanged. Hence, as found by Timmes et al., seismic investigations of hot pulsating DB white dwarfs that rely on parameterized static models should include the non-negligible effects of neutrino cooling to provide more accurate solutions, but all the important conclusions brought by Giammichele et al. from the analysis of KIC 08626021 remain entirely valid.
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