Helium Atmosphere White Dwarfs Research Articles

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

ABSTRACT 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.

A Systematic Study of the Connection Between White Dwarf Period Spectra and Model Structure

To date, pulsational variability has been measured for nearly 70 pulsating helium-atmosphere white dwarfs (DBVs) and 500 pulsating hydrogen-atmosphere white dwarfs (DAVs), with only a fraction of these having been the subject of asteroseismic analysis. One way to approach white dwarf asteroseismology is forward modeling, where one assumes an interior structure and calculates the model’s periods. Many such models are calculated, in the search for the one that best matches the observed period spectrum. It is not computationally manageable, nor necessary, to vary every possible parameter for every object. We engage in a systematic study, based on a sample of 14 hydrogen-atmosphere white dwarfs, chosen to be representative of the types of pulsation spectra we encounter in white dwarf asteroseismology. These white dwarfs are modeled with carbon and oxygen cores. Our goal is to draw a connection between the period spectra and what parameters to which they are most sensitive. We find that the presence of longer-period modes generally muddies the mass and effective temperature determinations, unless continuous sequences of ℓ = 1 and ℓ = 2 modes are present. All period spectra are sensitive to the structure in the helium and hydrogen envelopes and most to at least some features of the oxygen abundance profile. Such sensitivity can be achieved either by the presence of specific low-radial-overtone modes, or by the presence of longer-period modes. Convective efficiency only matters when fitting periods greater than 800 s. The results of this study can be used to inform parameter selection and pave the way to pipeline asteroseismic fitting of white dwarfs.

Gaia Astrometry Challenges the High Central Oxygen Abundance Claimed for the Pulsating Helium-Atmosphere White Dwarf KIC 08626021

The asteroseismic radius determination previously reported for the pulsating helium-atmosphere white dwarf star KIC 08626021 is 6σ discrepant with constraints from Gaia astrometry. This calls into question the other results of the asteroseismic analysis, especially the high (central) oxygen abundance that stellar evolutionary models have been unable to reproduce.

The Pulsating Helium-atmosphere White Dwarfs. I. New DBVs from the Sloan Digital Sky Survey

Abstract We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1 m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000 K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of DBVs known between 20,000 and 24,000 K. We do not find any new DBVs hotter than PG 0112+104, the current hottest DBV is at T eff ≈ 31,000 K, but do find pulsations in four DBVs with temperatures between 27,000 and 30,000 K, improving empirical constraints on the poorly defined blue edge. We investigate the ensemble pulsation properties of all currently known DBVs, finding that the weighted mean period and total pulsation power exhibit trends with effective temperature that are qualitatively similar to the pulsating hydrogen-atmosphere white dwarfs.

EPIC 228782059: Asteroseismology of What Could Be the Coolest Pulsating Helium-atmosphere White Dwarf (DBV) Known

Abstract We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC 228782059, discovered from 55.1 days of K2 photometry. The long-duration, high-quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of 34.1 ± 0.4 hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns M * = 0.685 ± 0.003M ⊙, T eff = 21,910 ± 23 K, and log g = 8.14 ± 0.01 dex. These values are comparable to those derived from spectroscopy by Koester & Kepler (20,860 ± 160 K, and 7.94 ± 0.03 dex). If these values are confirmed or better constrained by other independent works, it would make EPIC 228782059 one of the coolest pulsating DB white dwarf stars known, and would be helpful for testing different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.

Discovery of Beryllium in White Dwarfs Polluted by Planetesimal Accretion

Abstract The element beryllium is detected for the first time in white dwarf stars. This discovery in the spectra of two helium-atmosphere white dwarfs was made possible only because of the remarkable overabundance of Be relative to all other elements, heavier than He, observed in these stars. The measured Be abundances, relative to chondritic, are by far the largest ever seen in any astronomical object. We anticipate that the Be in these accreted planetary bodies was produced by spallation of one or more of O, C, and N in a region of high fluence of particles of MeV or greater energy.

GD 424 – a helium-atmosphere white dwarf with a large amount of trace hydrogen in the process of digesting a rocky planetesimal

ABSTRACT The photospheric metal pollution of white dwarfs is now well established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still debated. Here, we report the analysis of GD 424: a metal-polluted, helium-atmosphere white dwarf with a large amount of trace hydrogen. We determined the atmospheric parameters using a hybrid analysis that combines the sensitivity of spectroscopy to the atmospheric composition, log(H/He), with that of photometry and astrometry to the effective temperature, Teff, and surface gravity, log g. The resulting white dwarf mass, radius, and cooling age are $\mbox{$M_{\mathrm{WD}}$}=0.77\pm 0.01\, \mbox{$\mathrm{M}_{\odot }$}$, $\mbox{$R_{\mathrm{WD}}$}=0.0109\pm 0.0001\, \mbox{$\mathrm{R}_{\odot }$}$, and τcool = 215 ± 10 Myr, respectively. We identified and measured the abundances of 11 photospheric metals and argue that the accretion event is most likely either in the increasing or in steady state, and that the disrupted planetesimal resembles either CI chondrites or the bulk Earth in terms of its composition. We suggest that the observed 1.33 × 1022 g of trace hydrogen in GD 424 was at least partly acquired through accretion of water-rich planetary debris in an earlier accretion episode.

Magnesium abundances in cool metal-polluted white dwarfs

ABSTRACT The accretion of rocky material is responsible for the presence of heavy elements in the atmospheres of a large fraction of white dwarf stars. Those objects represent a unique opportunity to infer the bulk composition of exoplanetesimals. This chemical characterization requires the use of detailed atmosphere models to determine the elemental abundances at the photospheres of white dwarfs. In this work, we use a state-of-the-art model atmosphere code to reanalyse the first large survey of metal-polluted white dwarfs for which abundances are found for multiple elements. We show that the improved constitutive physics of our models lead to systematically higher Mg abundances than previous analyses. We find an average $\log \, {\rm Mg/Ca}$ number abundance ratio of 1.5. This value is significantly above the reference abundance for chondrites, which is expected as current diffusion models predict that for the cool helium-atmosphere white dwarfs of our sample, Mg should remain in the atmosphere longer than Ca. This helps resolve a recently identified Mg depletion problem, where the planetesimals accreted by white dwarfs were reported to be Mg-deficient compared to the expected composition of their planetary systems.

The Canada–France Imaging Survey: Reconstructing the Milky Way Star Formation History from Its White Dwarf Population

Abstract As the remnants of stars with initial masses ≲8 M ⊙, white dwarfs contain valuable information on the formation histories of stellar populations. In this paper, we use deep, high-quality, u-band photometry from the Canada–France Imaging Survey, griz photometry from Pan-STARRS1, as well as proper motions from Gaia DR2, to select 25,156 white dwarf candidates over ∼4500 deg2 using a reduced proper motion diagram. We develop a new white dwarf population synthesis code that returns mock observations of the Galactic field white dwarf population for a given star formation history, while simultaneously taking into account the geometry of the Milky Way (MW), survey parameters, and selection effects. We use this model to derive the star formation histories of the thin disk, thick disk, and stellar halo. Our results show that the MW disk began forming stars (11.3 ± 0.5) Gyr ago, with a peak rate of (8.8 ± 1.4) M ⊙ yr −1 at (9.8 ± 0.4) Gyr, before a slow decline to a constant rate until the present day—consistent with recent results suggesting a merging event with a satellite galaxy. Studying the residuals between the data and best-fit model shows evidence for a slight increase in star formation over the past 3 Gyr. We fit the local fraction of helium-atmosphere white dwarfs to be (21 ± 3)%. Incorporating this methodology with data from future wide-field surveys such as the Large Synoptic Survey Telescope, Euclid, The Cosmological Advanced Survey Telescope for Optical and ultraviolet Research, and the Wide Field Infrared Survey Telescope should provide an unprecedented view into the formation of the MW at its earliest epoch through its white dwarfs.

TESS first look at evolved compact pulsators

Context.Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky.Aims.As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3.Methods.We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies were compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models fromLPCODEand the structural models fromWDEC.Results.We detected and measured the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158−160 to ∼0.01 μHz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole (ℓ = 1) modes with a mean period spacing of 38.1 ± 1.0 s. The global best-fit seismic models from bothLPCODEandWDEChave effective temperatures that are ≳3000 K hotter than archival spectroscopic values of 24 100–25 500 K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints fromGaiaastrometry.Conclusions.Our results demonstrate the value of the TESS data for DBV white dwarf asteroseismology. The extent of the short-cadence photometry enables reliably accurate and extremely precise pulsation frequency measurements. Similar subsets of both theLPCODEandWDECmodels show good agreement with these measurements, supporting that the asteroseismic interpretation of DBV observations from TESS is not dominated by the set of models used. However, given the sensitivity of the observed set of pulsation modes to the stellar structure, external constraints from spectroscopy and/or astrometry are needed to identify the best seismic solutions.

A Massive Magnetic Helium Atmosphere White Dwarf Binary in a Young Star Cluster

We have searched the Gaia DR2 catalogue for previously unknown hot white dwarfs in the direction of young open star clusters. The aim of this experiment was to try and extend the initial-final mass relation (IFMR) to somewhat higher masses, potentially providing a tension with the Chandrasekhar limit currently thought to be around 1.38 M$_{\odot}$. We discovered a particularly interesting white dwarf in the direction of the young $\sim$150 Myr old cluster Messier 47 (NGC 2422). All Gaia indicators (proper motion, parallax, location in the Gaia colour-magnitude diagram) suggest that it is a cluster member. Its spectrum, obtained from Gemini South, yields a number of anomalies: it is a DB (helium-rich atmosphere) white dwarf, it has a large magnetic field (2.5 MG), is of high mass ($\sim$1.06 M$_\odot$) and its colours are very peculiar --- particularly the redder ones ($r$, $i$, $z$ and $y$), which suggest that it has a late-type companion. This is the only magnetized, detached binary white dwarf with a non-degenerate companion of any spectral type known in or out of a star cluster. If the white dwarf is a cluster member, as all indicators suggest, its progenitor had a mass just over 6 M$_\odot$. It may, however, be telling an even more interesting story than the one related to the IFMR, one about the origin of stellar magnetic fields, Type I supernovae and gravitational waves from low mass stellar systems.

White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14

ABSTRACT White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with 3σ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g, and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15 793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below $T_\mathrm{eff}\simeq 10\, 000$ K; most are likely low-mass metal-poor main-sequence stars, but some could be the result of interacting binary evolution.

GD358: Three Decades of Observations for the In-depth Asteroseismology of a DBV Star

Abstract We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982 to 2006, and 1195.2 hr of new observations from 2007 to 2016. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ℓ = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex, carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 ± 500 K and a of 7.8 ± 0.08 dex. The surface gravity translates to a mass range of 0.455–0.540 . Our best-fit model has a temperature of 23,650 K and a mass of 0.5706 . That is slightly more massive than what is suggested by the most recent spectroscopy. We find a pure helium layer mass of 10−5.50, consistent with the result of previous studies and the outward diffusion of helium over time.

Fundamental parameter accuracy of DA and DB white dwarfs inGaiaData Release 2

We report on a comparison of spectroscopic analyses for hydrogen (DA) and helium atmosphere (DB) white dwarfs with Gaia Data Release 2 (DR2) parallaxes and photometry. We assume a reddening law and a mass-radius relation to connect the effective temperatures (Teff) and surface gravities (log g) to masses and radii. This allows the comparison of two largely independent sets of fundamental parameters for 7039 DA and 521 DB stars with high-quality observations. This subset of the Gaia white dwarf sample is large enough to detect systematic trends in the derived parameters. We find that spectroscopic and photometric parameters generally agree within uncertainties when the expectation of a single star is verified. Gaia allows the identification of a small systematic offset in the temperature scale between the two techniques, as well as confirming a small residual high-mass bump in the DA mass distribution around 11,000-13,000 K. This assessment of the accuracy of white dwarf fundamental parameters derived from Gaia is a first step in understanding systematic effects in related astrophysical applications such as the derivation of the local stellar formation history, initial-to-final mass relation, and statistics of evolved planetary systems.

Ensemble Properties of the White Dwarf Population of the Old, Solar Metallicity Open Star Cluster Messier 67

White dwarfs are excellent forensic tools for studying end-of-life issues surrounding low- and intermediate-mass stars, and the old, solar-metallicity open star cluster Messier 67 is a proven laboratory for the study of stellar evolution for solar-type stars. In this paper, we present a detailed spectroscopic study of brighter (M_g < 12.4) white dwarfs in Messier 67, and, in combination with previously-published proper motion membership determinations, we identify a clean, representative sample of cluster white dwarfs, including 13 members with hydrogen-dominated atmospheres, at least one of which is a candidate double degenerate, and 5 members with helium-dominated atmospheres. Using this sample we test multiple predictions surrounding the final stages of stellar evolution in solar type stars. In particular, the stochasticity of the integrated mass lost by ~1.5 solar mass stars is less than 7% of the white dwarf remnant mass. We identify white dwarfs likely resulting from binary evolution, including at least one blue straggler remnant and two helium core white dwarfs. We observe no evidence of a significant population of helium core white dwarfs formed by enhanced mass loss on the red giant branch of the cluster. The distribution of white dwarf atmospheric compositions is fully consistent with that in the field, limiting proposed mechanisms for the suppression of helium atmosphere white dwarf formation in star clusters. In short, the white dwarf population of Messier 67 is fully consistent with basic predictions of single- and multiple-star stellar evolution theories for solar metallicity stars.

A New Generation of Cool White Dwarf Atmosphere Models. I. Theoretical Framework and Applications to DZ Stars

Abstract The photospheres of the coolest helium-atmosphere white dwarfs are characterized by fluidlike densities. Under those conditions, standard approximations used in model atmosphere codes are no longer appropriate. Unfortunately, the majority of cool He-rich white dwarfs show no spectral features, giving us no opportunities to put more elaborate models to the test. In the few cases where spectral features are observed (such as in cool DQ or DZ stars), current models completely fail to reproduce the spectroscopic data, signaling shortcomings in our theoretical framework. In order to fully trust parameters derived solely from the energy distribution, it is thus important to at least succeed in reproducing the spectra of the few coolest stars exhibiting spectral features, especially since such stars possess even less extreme physical conditions due to the presence of heavy elements. In this paper, we revise every building block of our model atmosphere code in order to eliminate low-density approximations. Our updated white dwarf atmosphere code incorporates state-of-the-art constitutive physics suitable for the conditions found in cool helium-rich stars (DC and DZ white dwarfs). This includes new high-density metal-line profiles, nonideal continuum opacities, an accurate equation of state, and a detailed description of the ionization equilibrium. In particular, we present new ab initio calculations to assess the ionization equilibrium of heavy elements (C, Ca, Fe, Mg, and Na) in a dense helium medium and show how our improved models allow us to achieve better spectral fits for two cool DZ stars, Ross 640 and LP 658-2.

On the Spectral Evolution of Helium-atmosphere White Dwarfs Showing Traces of Hydrogen

Abstract 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, M H, that is contained in the outer convection zone as a function of effective temperature and atmospheric H/He ratio. We find that some (T eff, M H) 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 M H 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.

Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion

A handful of white dwarfs with helium-dominated atmospheres contain exceptionally large masses of hydrogen in their convection zones, with the metal-polluted white dwarf GD 16 being one of the earliest recognised examples. We report the discovery of a similar star: the white dwarf coincidentally named GD 17. We obtained medium-resolution spectroscopy of both GD 16 and GD 17 and calculated abundances and accretion rates of photospheric H, Mg, Ca, Ti, Fe and Ni. The metal abundance ratios indicate that the two stars recently accreted debris which is Mg-poor compared to the composition of bulk Earth. However, unlike the metal pollutants, H never diffuses out of the atmosphere of white dwarfs and we propose that the exceptionally high atmospheric H content of GD 16 and GD 17 ($2.2\times 10^{24}$g and $2.9\times 10^{24}$g respectively) could result from previous accretion of water bearing planetesimals. Comparing the detection of trace H and metal pollution among 729 helium atmosphere white dwarfs, we find that the presence of H is nearly twice as common in metal-polluted white dwarfs compared to their metal-free counterparts. This highly significant correlation indicates that, over the cooling age of the white dwarfs, at least some fraction of the H detected in many He atmospheres (including GD 16 and GD 17) is accreted alongside metal pollutants, where the most plausible source is water. In this scenario, water must be common in systems with rocky planetesimals.

A Deep Test of Radial Differential Rotation in a Helium-atmosphere White Dwarf.I. Discovery of Pulsations in PG 0112+104

Abstract We present the detection of non-radial oscillations in a hot, helium-atmosphere white dwarf using 78.7 days of nearly uninterrupted photometry from the Kepler space telescope. With an effective temperature &gt;30,000 K, PG 0112+104 becomes the hottest helium-atmosphere white dwarf known to pulsate. The rich oscillation spectrum of low-order g-modes includes clear patterns of rotational splittings from consecutive sequences of dipole and quadrupole modes, which can be used to probe the rotation rate with depth in this highly evolved stellar remnant. We also measure a surface rotation rate of 10.17404 hr from an apparent spot modulation in the K2 data. With two independent measures of rotation, PG 0112+104 provides a remarkable test of asteroseismic inference.

Solar abundances of rock-forming elements, extreme oxygen and hydrogen in a young polluted white dwarf

The Teff = 20,800 K white dwarf WD 1536+520 is shown to have broadly solar abundances of the major rock forming elements O, Mg, Al, Si, Ca, and Fe, together with a strong relative depletion in the volatile elements C and S. In addition to the highest metal abundances observed to date, including log(O/He) = -3.4, the helium-dominated atmosphere has an exceptional hydrogen abundance at log(H/He) = -1.7. Within the uncertainties, the metal-to-metal ratios are consistent with the accretion of an H2O-rich and rocky parent body, an interpretation supported by the anomalously high trace hydrogen. The mixed atmosphere yields unusually short diffusion timescales for a helium atmosphere white dwarf, of no more than a few hundred yr, and equivalent to those in a much cooler, hydrogen-rich star. The overall heavy element abundances of the disrupted parent body deviate modestly from a bulk Earth pattern, and suggest the deposition of some core-like material. The total inferred accretion rate is 4.2e9 g/s, and at least 4 times higher than any white dwarf with a comparable diffusion timescale. Notably, when accretion is exhausted in this system, both metals and hydrogen will become undetectable within roughly 300 Myr, thus supporting a scenario where the trace hydrogen is related to the ongoing accretion of planetary debris.