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DOE OSTI2021

The Most Massive White Dwarfs in the Solar Neighborhood

We present an analysis of the most massive white dwarf candidates in the Montreal White Dwarf Database 100 pc sample. We identify 25 objects that would be more massive than 1.3M ⊙ if they had pure H atmospheres and CO cores, including two outliers with unusually high photometric mass estimates near the Chandrasekhar limit. We provide follow-up spectroscopy of these two white dwarfs and show that they are indeed significantly below this limit. We expand our model calculations for CO core white dwarfs up to M = 1.334 M ⊙ , which corresponds to the high-density limit of our equation-of-state tables, ρ = 109 g cm -3 . We find many objects close to this maximum mass of our CO core models. A significant fraction of ultramassive white dwarfs are predicted to form through binary mergers. Merger populations can reveal themselves through their kinematics, magnetism, or rapid rotation rates. We identify four outliers in transverse velocity, four likely magnetic white dwarfs (one of which is also an outlier in transverse velocity), and one with rapid rotation, indicating that at least 8 of the 25 ultramassive white dwarfs in our sample are likely merger products.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

J-PLUS: Spectral evolution of white dwarfs by PDF analysis

We estimated the spectral evolution of white dwarfs with effective temperature using the Javalambre Photometric Local Universe Survey (J-PLUS) second data release (DR2), which provides 12 photometric optical passbands over 2176 deg 2 . We analyzed 5926 white dwarfs with r ≤ 19.5 mag in common between a white dwarf catalog defined from Gaia EDR3 and J-PLUS DR2. We performed a Bayesian analysis by comparing the observed J-PLUS photometry with theoretical models of hydrogen- and helium-dominated atmospheres. We estimated the probability distribution functions for effective temperature (T eff ), surface gravity, parallax, and composition; and the probability of having a H-dominated atmosphere (p H ) for each source. We applied a prior in parallax, using Gaia EDR3 measurements as a reference, and derived a self-consistent prior for the atmospheric composition as a function of T eff . We described the fraction of white dwarfs with a He-dominated atmosphere (f He ) with a linear function of the effective temperature at 5000 < T eff < 30 000 K. We find f He = 0.24 ± 0.01 at T eff = 10 000 K, a change rate along the cooling sequence of 0.14 ± 0.02 per 10 kK, and a minimum He-dominated fraction of 0.08 ± 0.02 at the high-temperature end. We tested the obtained p H by comparison with spectroscopic classifications, finding that it is reliable. We estimated the mass distribution for the 351 sources with distance d < 100 pc, mass M > 0.45 M ⊙ , and T eff > 6000 K. The result for H-dominated white dwarfs agrees with previous studies, with a dominant M = 0.59 M ⊙ peak and the presence of an excess at M ~ 0.8 M ⊙ . This high-mass excess is absent in the He-dominated distribution, which presents a single peak. The J-PLUS optical data provide a reliable statistical classification of white dwarfs into H- and He-dominated atmospheres. We find a 21 ± 3% increase in the fraction of He-dominated white dwarfs from T eff = 20 000 K to T eff = 5000 K.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Primeval very low-mass stars and brown dwarfs – VIII. The first age benchmark L subdwarf, a wide companion to a halo white dwarf

ABSTRACT We report the discovery of five white dwarf + ultracool dwarf systems identified as common proper motion wide binaries in the Gaia Catalogue of Nearby Stars. The discoveries include a white dwarf + L subdwarf binary, VVV 1256−62AB, a gravitationally bound system located 75.6$^{+1.9}_{-1.8}$ pc away with a projected separation of 1375$^{+35}_{-33}$ au. The primary is a cool DC white dwarf with a hydrogen dominated atmosphere, and has a total age of $10.5^{+3.3}_{-2.1}$ Gyr, based on white dwarf model fitting. The secondary is an L subdwarf with a metallicity of [M/H] = $-0.72^{+0.08}_{-0.10}$ (i.e. [Fe/H] = $-0.81\pm 0.10$) and $T_{\rm eff}$ = 2298$^{+45}_{-43}$ K based on atmospheric model fitting of its optical to near infrared spectrum, and likely has a mass just above the stellar/substellar boundary. The subsolar metallicity of the L subdwarf and the system’s total space velocity of 406 km s−1 indicates membership in the Galactic halo, and it has a flat eccentric Galactic orbit passing within 1 kpc of the centre of the Milky Way every $\sim$0.4 Gyr and extending to 15–31 kpc at apogal. VVV 1256−62B is the first L subdwarf to have a well-constrained age, making it an ideal benchmark of metal-poor ultracool dwarf atmospheres and evolution.

Zhang, Z. H. (ORCID:000000033047607X)↗

DOE OSTI2021

A catalogue of white dwarfs in Gaia EDR3

ABSTRACT We present a catalogue of white dwarf candidates selected from Gaia Early Data Release 3 (EDR3). We applied several selection criteria in absolute magnitude, colour, and Gaia quality flags to remove objects with unreliable measurements while preserving most stars compatible with the white dwarf locus in the Hertzsprung–Russell diagram. We then used a sample of over 30 000 spectroscopically confirmed white dwarfs and contaminants from the Sloan Digital Sky Survey (SDSS) to map the distribution of these objects in the Gaia absolute magnitude–colour space. Finally, we adopt the same method presented in our previous work on Gaia Data Release 2 (DR2) to calculate a probability of being a white dwarf (PWD) for ≃1.3 million sources that passed our quality selection. The PWD values can be used to select a sample of ${\simeq} 359\,000$ high-confidence white dwarf candidates. We calculated stellar parameters (effective temperature, surface gravity, and mass) for all these stars by fitting Gaia astrometry and photometry with synthetic pure-H, pure-He, and mixed H–He atmospheric models. We estimate an upper limit of 93 per cent for the overall completeness of our catalogue for white dwarfs with G ≤ 20 mag and effective temperature (Teff) > 7000 K, at high Galactic latitudes (|b| > 20°). Alongside the main catalogue we include a reduced proper motion extension containing ${\simeq} 10\,200$ white dwarf candidates with unreliable parallax measurements that could, however, be identified on the basis of their proper motion. We also performed a cross-match of our catalogues with SDSS Data Release 16 (DR16) spectroscopy and provide spectral classification based on visual inspection for all resulting matches.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1990

White dwarf evolution - Cradle-to-grave constraints via pulsation

White dwarf evolution, particularly in the early phases, is not very strongly constrained by observation. Fortunately, white dwarfs undergo nonradial pulsation in three distinct regions of the H-R diagram. These pulsations provide accurate masses, surface compositional structure and rotation velocities, and help constrain other important physical properties. We demonstrate the application of the tools of stellar seismology to white dwarf evolution using the hot white dwarf star PG 1159-035 and the cool DAV (or ZZ Ceti) stars as examples. From pulsation studies, significant challenges to the theory of white dwarf evolution emerge.

Kawaler, Steven D.↗

DOE OSTI2020

The 100 pc White Dwarf Sample in the SDSS Footprint

In this work, we present follow-up spectroscopy of 711 white dwarfs within 100 pc, and we present a detailed model atmosphere analysis of the 100 pc white dwarf sample in the Sloan Digital Sky Survey footprint. Our spectroscopic follow-up is complete for 83% of the white dwarfs hotter than 6000 K, where the atmospheric composition can be constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen atmospheres. The DA mass distribution has an extremely narrow peak at 0.59 M ⊙ and reveals a shoulder from relatively massive white dwarfs with M = 0.7–0.9 M ⊙. Comparing this distribution with binary population synthesis models, we find that the contribution from single stars that form through mergers cannot explain the overabundance of massive white dwarfs. In addition, the mass distribution of cool DAs shows a near absence of M > 1 M ⊙ white dwarfs. The pile-up of 0.7–0.9 M ⊙ and the disappearance of M > 1 M ⊙ white dwarfs is consistent with the effects of core crystallization. Even though the evolutionary models predict the location of the pile-up correctly, the delay from the latent heat of crystallization by itself is insufficient to create a significant pile-up, and additional cooling delays from related effects like phase separation are necessary. We also discuss the population of infrared-faint (ultracool) white dwarfs and demonstrate for the first time the existence of a well-defined sequence in color and magnitude. Curiously, this sequence is connected to a region in the color–magnitude diagrams where the number of white dwarfs with a helium-dominated atmosphere is low. This suggests that the infrared-faint white dwarfs likely have mixed H/He atmospheres.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

The frequency of gaseous debris discs around white dwarfs

ABSTRACT A total of 1–3 per cent of white dwarfs are orbited by planetary dusty debris detectable as infrared emission in excess above the white dwarf flux. In a rare subset of these systems, a gaseous disc component is also detected via emission lines of the Ca ii 8600 Å triplet, broadened by the Keplerian velocity of the disc. We present the first statistical study of the fraction of debris discs containing detectable amounts of gas in emission at white dwarfs within a magnitude and signal-to-noise ratio limited sample. We select 7705 single white dwarfs spectroscopically observed by the Sloan Digital Sky Survey (SDSS) and Gaia with magnitudes g ≤ 19. We identify five gaseous disc hosts, all of which have been previously discovered. We calculate the occurrence rate of a white dwarf hosting a debris disc detectable via Ca ii emission lines as $0.067\, \pm \, ^{0.042}_{0.025}$ per cent. This corresponds to an occurrence rate for a dusty debris disc to have an observable gaseous component in emission as 4 ± $_{2}^{4}$ per cent. Given that variability is a common feature of the emission profiles of gaseous debris discs, and the recent detection of a planetesimal orbiting within the disc of SDSS J122859.93+104032.9, we propose that gaseous components are tracers for the presence of planetesimals embedded in the discs and outline a qualitative model. We also present spectroscopy of the Ca ii triplet 8600 Å region for 20 white dwarfs hosting dusty debris discs in an attempt to identify gaseous emission. We do not detect any gaseous components in these 20 systems, consistent with the occurrence rate that we calculated.

Manser, Christopher J.↗

DOE OSTI2023

The non-explosive stellar merging origin of the ultra-massive carbon-rich white dwarfs

We have investigated the origin of a sub-class of carbon-polluted white dwarfs (DQ) originally identified as the “hot DQ” white dwarfs. These objects are relatively hot ($10\, 000\lesssim T_{\rm eff}\lesssim 25\, 000$ K), have markedly higher carbon abundance (C-enriched), are more massive (M ≳ 0.8 M⊙) than ordinary DQs (M ∼ 0.6 M⊙), and display high space velocities. Hence, despite their young appearance their kinematic properties are those of an old white dwarf population. The way out of this dilemma is to assume that they formed via the merging of two white dwarfs. In this paper, we examine the observed characteristics of this population of “C-enriched” DQ white dwarfs and confirm that nearly half of the 63 known objects have kinematic properties consistent with those of the Galactic thick disc or halo. We have also conducted population synthesis studies and found that the merging hypothesis is indeed compatible with observations. Studies of this sub-class of white dwarfs have important implications for our understanding of Type Ia Supernovae (SNeIa), commonly used to determine the expansion history of the Universe, since the same formation channel applies to both kinds of objects. Hence, probing the properties of these white dwarfs that failed to explode may yield important constraints to the modelling of the mechanisms leading to a thermonuclear runaway.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

New Conductive Opacities for White Dwarf Envelopes

Thanks to their continuous cooling and relative simplicity, white dwarf stars are routinely used to measure the ages of stellar populations. The usefulness of white dwarfs as cosmochronometers depends on the availability of accurate cooling models. A key ingredient of those models is conductive opacities, which largely govern the cooling rate. In this work, we present improved conductive opacities for the regime of moderate coupling and moderate degeneracy that characterizes an important portion of the envelopes of DA and DB white dwarfs. Here, we find differences of up to a factor of 3 between our calculations and the commonly used opacities of Cassisi et al., which we attribute to an improved account of electron–electron scattering. The cooling models are strongly affected by those changes in the conductive opacities: the age of a 4000 K white dwarf can be reduced by as much as 2 Gyr. We provide analytical fits to our new opacities to facilitate the implementation of this important effect in white dwarf evolution codes.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1988

Temperature determinations of hot DA white dwarfs using IUE continuum fluxes

Effective temperatures of 15 DA white dwarfs hotter than 20,000 K were derived from low-dispersion far ultraviolet spectra obtained with IUE. The analysis was carried out by comparing the observed far ultraviolet fluxes with model fluxes scaled to the V-band flux. Accurate calibration of the IUE spectra is critical for this analysis. Observations at all epochs were corrected to the 1980 IUE calibration using the time-dependent corrections of Bohlin (1988). Taking advantage of the smooth and well-defined continuum fluxes provided by DA white dwarfs, seven white dwarfs for which accurate, independent temperature determinations were made from line profile analyses were used to improve the accuracy of the IUE flux calibration. The correction to the original calibration is as great as 20 percent in individual 5 A wavelength bins, while the average over the IUE wavelength range is 5 percent. The final calibration correction and the temperatures for the hot white dwarfs are presented.

Finley, David↗

NASA NTRS1987

Observations of hot DA white dwarfs

Observations of the hottest white dwarfs offer the opportunity to study the earliest stages of degenerate evolution. For those white dwarfs having pure or nearly pure hydrogen photosphere it is now possible with the current generation of high gravity model atmospheres to achieve substantial refinements in the estimation of atmospheric parameters, such as effective temperature, gravity and trace elements abundance. Such improvements can in turm lead to a clearer picture of DA cooling have a significant impact on the interpretation of a growing number of soft X-ray observations of hot DAs, and contribute to the understanding of He and trace metal abundance patterns in hot DAs.

Holberg, J. B.↗

DOE OSTI2022

Simulation of Stark-broadened Hydrogen Balmer-line Shapes for DA White Dwarf Synthetic Spectra

Abstract White dwarfs (WDs) are useful across a wide range of astrophysical contexts. The appropriate interpretation of their spectra relies on the accuracy of WD atmosphere models. One essential ingredient of atmosphere models is the theory used for the broadening of spectral lines. To date, the models have relied on Vidal et al., known as the unified theory of line broadening (VCS). There have since been advancements in the theory; however, the calculations used in model atmosphere codes have only received minor updates. Meanwhile, advances in instrumentation and data have uncovered indications of inaccuracies: spectroscopic temperatures are roughly 10% higher and spectroscopic masses are roughly 0.1 M ⊙ higher than their photometric counterparts. The evidence suggests that VCS-based treatments of line profiles may be at least partly responsible. Gomez et al. developed a simulation-based line-profile code Xenomorph using an improved theoretical treatment that can be used to inform questions around the discrepancy. However, the code required revisions to sufficiently decrease noise for use in model spectra and to make it computationally tractable and physically realistic. In particular, we investigate three additional physical effects that are not captured in the VCS calculations: ion dynamics, higher-order multipole expansion, and an expanded basis set. We also implement a simulation-based approach to occupation probability. The present study limits the scope to the first three hydrogen Balmer transitions (H α , H β , and H γ ). We find that screening effects and occupation probability have the largest effects on the line shapes and will likely have important consequences in stellar synthetic spectra.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2013

Chandra Grating Spectroscopy of Three Hot White Dwarfs

High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T(sub eff) = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models. Such models are therefore reliable tools to investigate the interior structure of GW Vir variables. Conclusions. Our soft X-ray study reveals that the understanding of metal abundances in hot DA white dwarf atmospheres is still incomplete. On the other hand, model atmospheres of hydrogen-deficient PG1159-type stars are reliable and reproduce well the observed spectra from soft X-ray to optical wavelengths.

Adamczak, J.↗

DOE OSTI2024

The DESI Early Data Release white dwarf catalogue

The Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI) comprises spectroscopy obtained from 2020 December 14 to 2021 June 10. White dwarfs were targeted by DESI both as calibration sources and as science targets and were selected based on Gaia photometry and astrometry. Here, we present the DESI EDR white dwarf catalogue, which includes 2706 spectroscopically confirmed white dwarfs of which approximately 60 per cent have been spectroscopically observed for the first time, as well as 66 white dwarf binary systems. We provide spectral classifications for all white dwarfs, and discuss their distribution within the Gaia Hertzsprung–Russell diagram. We provide atmospheric parameters derived from spectroscopic and photometric fits for white dwarfs with pure hydrogen or helium photospheres, a mixture of those two, and white dwarfs displaying carbon features in their spectra. We also discuss the less abundant systems in the sample, such as those with magnetic fields, and cataclysmic variables. The DESI EDR white dwarf sample is significantly less biased than the sample observed by the Sloan Digital Sky Survey, which is skewed to bluer and therefore hotter white dwarfs, making DESI more complete and suitable for performing statistical studies of white dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

The frequency of metal enrichment of cool helium-atmosphere white dwarfs using the DESI early data release

There is an overwhelming evidence that white dwarfs host planetary systems; revealed by the presence, disruption, and accretion of planetary bodies. A lower limit on the frequency of white dwarfs that host planetary material has been estimated to be ≃ 25–50 per cent; inferred from the ongoing or recent accretion of metals on to both hydrogen-atmosphere and warm helium-atmosphere white dwarfs. Now with the unbiased sample of white dwarfs observed by the Dark Energy Spectroscopic Instrument (DESI) survey in their Early Data Release (EDR), we have determined the frequency of metal enrichment around cool-helium atmosphere white dwarfs as 21 ± 3 per cent using a sample of 234 systems. This value is in good agreement with values determined from previous studies. With the current samples we cannot distinguish whether the frequency of planetary accretion varies with system age or host-star mass, but the DESI data release 1 will contain roughly an order of magnitude more white dwarfs than DESI EDR and will allow these parameters to be investigated.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

A spectrophotometric analysis of cool white dwarfs in the Gaia and pan-STARRS footprint

We present a spectrophotometric analysis of 2880 cool white dwarfs within 100 pc of the Sun and cooler than T eff ∼ 10 000 K, with grizy Pan-STARRS photometry and Gaia trigonometric parallaxes available. We also supplement our data sets with near-infrared JHK photometry, when available, which is shown to be essential for interpreting the coolest white dwarfs in our sample. We perform a detailed analysis of each individual object using state-of-the-art model atmospheres appropriate for each spectral type, including DA, DC, DQ, DZ, He-rich DA, and the so-called IR-faint white dwarfs. We discuss the temperature and mass distributions of each subsample, as well as revisit the spectral evolution of cool white dwarfs. We find little evidence in our sample for the transformation of a significant fraction of DA stars into He-atmosphere white dwarfs through the process of convective mixing between T eff = 10 000 and ∼6500 K, although the situation changes drastically in the range T eff = 6500–5500 K where the fraction of He-atmosphere white dwarfs reaches ∼45 per cent. However, we also provide strong evidence that at even cooler temperatures (T eff ≲ 5200 K), most DC white dwarfs have H atmospheres. We discuss a possible mechanism to account for this sudden transformation from He- to H-atmosphere white dwarfs involving the onset of crystallization and the occurrence of magnetism. Finally, we also argue that DQ, DZ, and DC white dwarfs may form a more homogeneous population than previously believed.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2020

From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing

ABSTRACT We present a study of the hypothesis that white dwarfs undergo a spectral change from hydrogen- to helium-dominated atmospheres using a volume-limited photometric sample drawn from the Gaia-DR2 catalogue, the Sloan Digital Sky Survey (SDSS), and the Galaxy Evolution Explorer (GALEX). We exploit the strength of the Balmer jump in hydrogen-atmosphere DA white dwarfs to separate them from helium-dominated objects in SDSS colour space. Across the effective temperature range from 20 000 to 9000 K, we find that 22 per cent of white dwarfs will undergo a spectral change, with no spectral evolution being ruled out at 5σ. The most likely explanation is that the increase in He-rich objects is caused by the convective mixing of DA stars with thin hydrogen layers, in which helium is dredged up from deeper layers by a surface hydrogen convection zone. The rate of change in the fraction of He-rich objects as a function of temperature, coupled with a recent grid of 3D radiation-hydrodynamic simulations of convective DA white dwarfs – which include the full overshoot region – lead to a discussion on the distribution of total hydrogen mass in white dwarfs. We find that 60 per cent of white dwarfs must have a hydrogen mass larger than MH/MWD = 10−10, another 25 per cent have masses in the range MH/MWD = 10−14–10−10, and 15 per cent have less hydrogen than MH/MWD = 10−14. These results have implications for white dwarf asteroseismology, stellar evolution through the asymptotic giant branch and accretion of planetesimals on to white dwarfs.

Cunningham, Tim↗

NASA NTRS1986

Stars of type MS with evidence of white dwarf companions

A search for white dwarf companions of MS-type stars was conducted, using IUE. The overendowments of these stars in typical S-process nuclides suggest that they, like the Ba II stars, may owe their peculiar compositions to earlier mass transfer. Short-wavelength IUE spectra show striking emission line variability in HD35155, HD61913, and 4 Ori; HD35155 and 4 Ori show evidence of white dwarf companions.

Peery, Benjamin F., Jr.↗