Gaseous Metal Disk Around a White Dwarf Gänsicke, B.T; Marsh, T.R; Southworth, J ...
Science (American Association for the Advancement of Science),
12/2006, Letnik:
314, Številka:
5807
Journal Article
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The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of ...the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.
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.
Context. As endpoints of stellar evolution, white dwarfs (WDs) are powerful tools to study the evolutionary history of the Galaxy. In particular, the multiplicity of WDs contains information ...regarding the formation and evolution of binary systems. Aims. Can we understand the multiplicity of the local WD sample from a theoretical point of view? Population synthesis methods are often applied to estimate stellar space densities and event rates, but how well are these estimates calibrated? This can be tested by a comparison with the 20 pc sample, which contains ≃100 stars and is minimally affected by selection biases. Methods. We model the formation and evolution of single stars and binaries within 20 pc with a population synthesis approach. We construct a model of the current sample of WDs and differentiate between WDs in different configurations, that is single WDs, and resolved and unresolved binaries containing a WD with either a main-sequence (MS) component or with a second WD. We also study the effect of different assumptions concerning the star formation history, binary evolution, and the initial distributions of binary parameters. We compile from the literature the available information on the sample of WDs within 20 pc, with a particular emphasis on their multiplicity, and compare this to the synthetic models. Results. The observed space densities of single and binary WDs are well reproduced by the models. The space densities of the most common WD systems (single WDs and unresolved WD-MS binaries) are consistent within a factor two with the observed value. We find a discrepancy only for the space density of resolved double WDs. We exclude that observational selection effects, fast stellar winds, or dynamical interactions with other objects in the Milky Way explain this discrepancy. We find that either the initial mass ratio distribution in the solar neighbourhood is biased towards low mass-ratios, or more than ten resolved DWDs have been missed observationally in the 20 pc sample. Furthermore, we show that the low binary fraction of WD systems (~25%) compared to solar-type MS-MS binaries (~50%) is consistent with theory, and is mainly caused by mergers in binary systems, and to a lesser degree by WDs hiding in the glare of their companion stars. Lastly, Gaia will dramatically increase the size of the volume-limited WD sample, detecting the coolest and oldest WDs out to ≃50 pc. We provide a detailed estimate of the number of single and binary WDs in the Gaia sample.
White dwarfs whose atmospheres are polluted by terrestrial-like planetary debris have become a powerful and unique tool to study evolved planetary systems. This paper presents results for an unbiased ...Spitzer Infrared Array Camera search for circumstellar dust orbiting a homogeneous and well-defined sample of 134 single white dwarfs. The stars were selected without regard to atmospheric metal content but were chosen to have (1) hydrogen-rich atmospheres, (2) 17 000 < T
eff < 25 000 K and correspondingly young post-main-sequence ages of 15–270 Myr, and (3) sufficient far-ultraviolet brightness for a corresponding Hubble Space Telescope COS snapshot. Five white dwarfs were found to host an infrared bright dust disc, three previously known, and two reported here for the first time, yielding a nominal
$3.7^{+2.4}_{-1.0}$
per cent of white dwarfs in this post-main-sequence age range with detectable circumstellar dust. Remarkably, the complementary Hubble observations indicate that a fraction of 27 per cent show metals in their photosphere that can only be explained with ongoing accretion from circumstellar material, indicating that nearly 90 per cent of discs escape detection in the infrared, likely due to small emitting surface area. This paper also presents the distribution of disc fractional luminosity as a function of cooling age for all known dusty white dwarfs, suggesting possible disc evolution scenarios and indicating an undetected population of circumstellar discs.
Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound–bound opacities of hydrogen and helium ...are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g. planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of seven strongly metal polluted, cool (T
eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13 ± 4 per cent, noticeably much higher than among hot DA white dwarfs.
A catalogue of white dwarfs in Gaia EDR3 Gentile Fusillo, N P; Tremblay, P-E; Cukanovaite, E ...
Monthly notices of the Royal Astronomical Society,
12/2021, Letnik:
508, Številka:
3
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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.
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 present a detailed study of the metal-polluted DB white dwarf SDSS J0845+2257 (Ton 345). Using high-resolution Hubble Space Telescope/Cosmic Origins Spectrograph and Very Large Telescope ...spectroscopy, we have detected hydrogen and 11 metals in the atmosphere of the white dwarf. The origin of these metals is almost certainly the circumstellar disc of dusty and gaseous debris from a tidally disrupted planetesimal, accreting at a rate of 1.6 × 1010 g s−1. Studying the chemical abundances of the accreted material demonstrates that the planetesimal had a composition similar to the Earth, dominated by rocky silicates and metallic iron, with a low water content. The mass of metals within the convection zone of the white dwarf corresponds to an asteroid of at least ∼130–170 km in diameter, although the presence of ongoing accretion from the debris disc implies that the planetesimal was probably larger than this. While a previous abundance study of the accreted material has shown an anomalously high mass fraction of carbon (15 per cent) compared to the bulk Earth, our independent analysis results in a carbon abundance of just 2.5 per cent. Enhanced abundances of core material (Fe, Ni) suggest that the accreted object may have lost a portion of its mantle, possibly due to stellar wind stripping in the asymptotic giant branch. Time series spectroscopy reveals variable emission from the orbiting gaseous disc, demonstrating that the evolved planetary system at SDSS J0845+2257 is dynamically active.
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.
The field white dwarf mass distribution Tremblay, P.-E; Cummings, J; Kalirai, J. S ...
Monthly notices of the Royal Astronomical Society,
09/2016, Letnik:
461, Številka:
2
Journal Article
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We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best ...established completeness and most precise atmospheric parameters, the volume-complete survey within 20 pc and the Sloan Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disc vertical scale height as a function of stellar age, and binary evolution. Each of these input ingredients has a moderate effect on the predicted mass distributions, and we must also take into account biases owing to unidentified faint objects (20 pc sample), as well as unknown masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS sample). Nevertheless, we find that fixed standard assumptions for the above parameters result in predicted mean masses that are in good qualitative agreement with the observed values. It suggests that derived masses for both studied samples are consistent with our current knowledge of stellar and Galactic evolution. Our simulations overpredict by 40–50 per cent the number of massive white dwarfs (M > 0.75 M⊙) for both surveys, although we can not exclude a Salpeter IMF when we account for all biases. Furthermore, we find no evidence of a population of double white dwarf mergers in the observed mass distributions.
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