Aims. White dwarfs with helium-dominated atmospheres (spectral types DO, DB) comprise approximately 20% of all white dwarfs. There are fewer studies than of their hydrogen-rich counterparts (DA) and ...thus several questions remain open. Among these are the total masses and the origin of the hydrogen traces observed in a large number and the nature of the deficit of DBs in the range from 30 000−45 000 K. We use the largest-ever sample (by a factor of 10) provided by the Sloan Digital Sky Survey (SDSS) to study these questions. Methods. The photometric and spectroscopic data of 1107 helium-rich objects from the SDSS are analyzed using theoretical model atmospheres. Along with the effective temperature and surface gravity, we also determine hydrogen and calcium abundances or upper limits for all objects. The atmosphere models are extended with envelope calculations to determine the extent of the helium convection zones and thus the total amount of hydrogen and calcium present. Results. When accounting for problems in determining surface gravities at low Teff , we find an average mass for helium-dominated white dwarfs of 0.606 ± 0.004 M⊙, which is very similar to the latest determinations for DAs. There are 32% of the sample with detected hydrogen, but this increases to 75% if only the objects with the highest signal-to-noise ratios are considered. In addition, 10−12% show traces of calcium, which must come from an external source. The interstellar medium (ISM) is ruled out by the fact that all polluted objects show a Ca/H ratio that is much larger than solar. We also present arguments that demonstrate that the hydrogen is very likely not accreted from the ISM but is the result of convective mixing of a residual thin hydrogen layer with the developing helium convection zone. It is very important to carefully consider the bias from observational selection effects when drawing these conclusions.
Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body ...within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims. We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20–200 Myr and 17 000 K <Teff < 27 000 K. Methods. The sample was observed with the Cosmic Origins Spectrograph on board the Hubble Space Telescope in the far ultraviolet range between 1130 and 1435 Å. The atmospheric parameters were obtained using these spectra and optical observations from the literature. Element abundances were determined using theoretical models, which include the effects of element stratification due to gravitational settling and radiative levitation. Results. We find 48 of the 85 DA white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 105 g s-1 to a few 108 g s-1, with no evident trend in cooling age from ≈40 Myr to ≈2 Gyr. Only a single, modest case of metal pollution (Ṁ < 106 g s-1) is found among ten white dwarfs with Teff > 23 000 K, in excellent agreement with the absence of infrared excess from dust around these warmer stars. The median, main sequence progenitor of our sample corresponds to an A-type star of ≈2 M⊙, and we find 13 of 23 white dwarfs descending from main sequence 2–3 M⊙, late B- and A-type stars to be currently accreting. Only one of 14 targets with Mwd > 0.8 M⊙ is found to be currently accreting, which suggests a large fraction of these stars result from double-degenerate mergers, and the merger disks do not commonly reform large planetesimals or otherwise pollute the remnant. We reconfirm our previous finding that two 625 Myr Hyades white dwarfs are currently accreting rocky planetary debris. Conclusions. At least 27% of all white dwarfs with cooling ages 20–200 Myr are accreting planetary debris, but that fraction could be as high as ≈50%. At Teff > 23 000 K, the luminosity of white dwarfs is probably sufficient to vaporize circumstellar dust grains, so no stars with strong metal-pollution are found. Planetesimal disruption events should occur in this cooling age and temperature range as well, and they are likely to result in short phases of high mass-transfer rates. It appears that the formation of rocky planetary material is common around 2–3 M⊙ late B- and A-type stars.
Context. Among the spectroscopically identified white dwarfs, a fraction smaller than 2% have spectra dominated by carbon lines, mainly molecular C2, but also a smaller group dominated by C I and ...C II lines. These are together called DQ white dwarfs. Aims. We want to derive atmospheric parameters Teff, log g, and carbon abundances for a large sample of these stars and discuss implications for their spectral evolution. Methods. Sloan Digital Sky Survey spectra and ugriz photometry were used, together with Gaia Data Release 2 parallaxes and G band photometry. These were fitted to synthetic spectra and theoretical photometry derived from model atmospheres. Results. We found that the DQ hotter than Teff ~ 10 000 K have masses ~ 0.4 M⊙ larger than the classical DQ, which have masses typical for the majority of white dwarfs (~ 0.6 M⊙). We found some evidence that the peculiar DQ below 10 000 K also have significantly larger masses and may thus be the descendants of the hot and warm DQ above 10 000 K. A significant fraction of the hotter objects with Teff > 14 500 K have atmospheres dominated by carbon.
Abstract
In a previous study, we analysed the spectra of 230 cool (Teff < 9000 K) white dwarfs exhibiting strong metal contamination, measuring abundances for Ca, Mg, Fe and in some cases Na, Cr, Ti, ...or Ni. Here, we interpret these abundances in terms of the accretion of debris from extrasolar planetesimals, and infer parent body compositions ranging from crust-like (rich in Ca and Ti) to core-like (rich in Fe and Ni). In particular, two white dwarfs, SDSS J0823+0546 and SDSS J0741+3146, which show log Fe/Ca > 1.9 dex, and Fe to Ni ratios similar to the bulk Earth, have accreted by far the most core-like exoplanetesimals discovered to date. With cooling ages in the range 1–8 Gyr, these white dwarfs are among the oldest stellar remnants in the Milky Way, making it possible to probe the long-term evolution of their ancient planetary systems. From the decrease in maximum abundances as a function of cooling age, we find evidence that the arrival rate of material on to the white dwarfs decreases by three orders of magnitude over a ≃ 6.5 Gyr span in white dwarf cooling ages, indicating that the mass-reservoirs of post-main sequence planetary systems are depleted on a ≃ 1 Gyr e-folding time-scale. Finally, we find that two white dwarfs in our sample are members of wide binaries, and both exhibit atypically high abundances, thus providing strong evidence that distant binary companions can dynamically perturb white dwarf planetary systems.
We report the discovery of 6576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain T
eff, log g and mass for hydrogen atmosphere ...white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra (DQs). We found one central star of a planetary nebula, one ultracompact helium binary (AM CVn), one oxygen line-dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium-dominated white dwarfs, 148 white dwarf + main-sequence star binaries, 236 metal-polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2936 new hydrogen-dominated white dwarf stars, and 2675 cool hydrogen-dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N ≥ 15 in DR12, including the ones in DR7 and DR10, with an average S/N = 26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/V
max.
The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted ...from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A-and F-type stars that end their lives as white dwarfs.
We report the discovery of 9088 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain T sub(eff), log g and mass for hydrogen ...atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon-dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main-sequence star binaries, 206 magnetic DAHs, 327 continuum-dominated DCs, 397 metal-polluted white dwarfs, 450 helium-dominated white dwarfs, 647 subdwarfs and 6887 new hydrogen-dominated white dwarf stars.
Close double degenerate binaries are one of the favoured progenitor channels for type Ia supernovae, but it is unclear how many suitable systems there are in the Galaxy. We report results of a large ...radial velocity survey for double degenerate (DD) binaries using the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY – SPY). Exposures taken at different epochs are checked for radial velocity shifts indicating close binary systems. We observed 689 targets classified as DA white dwarfs (displaying hydrogen-rich atmospheres), of which 46 were found to possess a cool companion. We measured radial velocities (RV) of the remaining 643 DA white dwarfs. We managed to secure observations at two or more epochs for 625 targets, supplemented by eleven objects meeting our selection criteria from literature. The data reduction and analysis methods applied to the survey data are described in detail. The sample contains 39 double degenerate binaries, only four of which were previously known. Twenty are double-lined systems, in which features from both components are visible, the other 19 are single-lined binaries. We provide absolute RVs transformed to the heliocentric system suitable for kinematic studies. Our sample is large enough to sub-divide by mass: 16 out of 44 low mass targets (≤0.45 M⊙) are detected as DDs, while just 23 of the remaining 567 targets with multiple spectra and mass > 0.45 M⊙ are double. The detected fraction amongst the low mass objects (36.4 ± 7.3%) is significantly higher than for the higher-mass, carbon-oxygen core dominated part of the sample (3.9 ± 0.8%), but it is much lower than expected from the detection efficiency for companion masses of 0.05 M⊙ or higher and a 100% binary fraction. This suggests either companion stars of with a mass below 0.05 M⊙ or some of the low mass white dwarfs are single.
We present Hubble Space Telescope (HST) ultraviolet spectroscopy of the white dwarfs PG 0843+516, PG 1015+161, SDSS 1228+1040, and GALEX 1931+0117, which accrete circumstellar planetary debris formed ...from the destruction of asteroids. Combined with optical data, a minimum of five and a maximum of 11 different metals are detected in their photospheres. With metal sinking time-scales of only a few days, these stars are in accretion/diffusion equilibrium, and the photospheric abundances closely reflect those of the circumstellar material. We find C/Si ratios that are consistent with that of the bulk Earth, corroborating the rocky nature of the debris. Their C/O values are also very similar to those of bulk Earth, implying that the planetary debris is dominated by Mg and Fe silicates. The abundances found for the debris at the four white dwarfs show substantial diversity, comparable at least to that seen across different meteorite classes in the Solar system. PG 0843+516 exhibits significant overabundances of Fe and Ni, as well as of S and Cr, which suggests the accretion of material that has undergone melting, and possibly differentiation. PG 1015+161 stands out by having the lowest Si abundance relative to all other detected elements. The Al/Ca ratio determined for the planetary debris around different white dwarfs is remarkably similar. This is analogous to the nearly constant abundance ratio of these two refractory lithophile elements found among most bodies in the Solar system.
Based on the detection of all major elements of the circumstellar debris, we calculate accretion rates of ≃1.7 × 108 to ≃1.5 × 109 g s−1. Finally, we detect additional circumstellar absorption in the Si iv 1394, 1403 Å doublet in PG 0843+516 and SDSS 1228+1040, reminiscent to similar high-ionization lines seen in the HST spectra of white dwarfs in cataclysmic variables. We suspect that these lines originate in hot gas close to the white dwarf, well within the sublimation radius.
The cool white dwarf SDSS J124231.07+522626.6 exhibits photospheric absorption lines of eight distinct heavy elements in medium resolution optical spectra, notably including oxygen. The T
eff = 13 ...000 K atmosphere is helium-dominated, but the convection zone contains significant amounts of hydrogen and oxygen. The four most common rock-forming elements (O, Mg, Si, and Fe) account for almost all the accreted mass, totalling at least 1.2 × 1024 g, similar to the mass of Ceres. The time-averaged accretion rate is 2 × 1010 g s−1, one of the highest rates inferred among all known metal-polluted white dwarfs. We note a large oxygen excess, with respect to the most common metal oxides, suggesting that the white dwarf accreted planetary debris with a water content of ≈38 per cent by mass. This star, together with GD 61, GD 16, and GD 362, form a small group of outliers from the known population of evolved planetary systems accreting predominantly dry, rocky debris. This result strengthens the hypothesis that, integrated over the cooling ages of white dwarfs, accretion of water-rich debris from disrupted planetesimals may significantly contribute to the build-up of trace hydrogen observed in a large fraction of helium-dominated white dwarf atmospheres.
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