A study of metal abundance patterns in cool white dwarfs. II - Simulations of accretion episodes

Dupuis, J.; Fontaine, G.; Pelletier, C.; Wesemaël, F.

doi:10.1086/191746

Cited by 100 publications

(107 citation statements)

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“…7). The size of these average rates is reasonable (Dupuis et al 1993), although the huge spread and the increase toward lower temperatures might be difficult to explain. An alternative proposal, the continuous accretion of comets from an Oort cloud (Veras et al 2014), could also explain such an increase toward older white dwarfs, but this faces the same problems.…”

Section: Hydrogenmentioning

confidence: 92%

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

Koester¹,

Kepler²

2015

A&A

125

193

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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 T eff , 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.

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Section: Hydrogenmentioning

confidence: 92%

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

Koester¹,

Kepler²

2015

A&A

125

193

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“…The possibility of this scenario providing an explanation of the observed metals in cool white dwarfs was studied extensively in a series of three papers by Dupuis and collaborators (Dupuis et al 1992(Dupuis et al , 1993a. In the first two of those papers, the authors solved the complete problem of time-dependent accretion and diffusion, by considering the spatial and temporal distribution of a heavy element during the white dwarf cooling evolution.…”

Section: Elementary Considerations For the Accretion/diffusion Scenariomentioning

confidence: 99%

“…In their study of accretion from the interstellar matter, Dupuis et al (1993a) assumed schematically a duration of the accretion phase within a dense ISM cloud of 10 6 yrs. This would correspond to the time required to complete all of the process indicated in Fig.…”

Section: Gd 362mentioning

confidence: 99%

“…Assuming the interstellar matter to be the outer source, this so-called accretion/diffusion scenario was discussed in great detail in three fundamental papers by Dupuis et al (1992Dupuis et al ( , 1993a. However, there are some severe problems with this scenario: the apparent lack or at least significant underabundance of hydrogen in the accreted matter, and the lack of any correlation between the location of the white dwarfs and the conditions of the ISM.…”

Section: Introductionmentioning

confidence: 99%

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Accretion and diffusion in white dwarfs

Koester¹

2009

A&A

282

471

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Context. A number of cool white dwarfs with metal traces, of spectral types DAZ, DBZ, and DZ have been found to exhibit infrared excess radiation due to circumstellar dust. The origin of this dust is possibly a tidally disrupted asteroid that formed a debris disk now supplying the matter accreting onto the white dwarf. To reach any clear conclusions from the observed composition of the white dwarf atmosphere to that of the circumstellar matter, we need a detailed understanding of the accretion and diffusion process, in particular the diffusion timescales. Aims. We aim to provide data for a wide range of white dwarf parameters and all possible observed chemical elements. Methods.Starting from atmosphere models, we calculate the structure of the outer envelopes, obtaining the depth of the convection zone and the physical parameters at the lower boundary. These parameters are used to calculate the diffusion velocities using calculations of diffusion coefficients available in the literature. Results. With a simple example, we demonstrate that the observed element abundances are not identical to the accreted abundances. Reliable conclusions are possible only if we know or can assume that the star has reached a steady state between accretion and diffusion. In this case, most element abundances differ only by factors in the range 2−4 between atmospheric values and the circumstellar matter. Knowing the diffusion timescales, we can also accurately relate the accreted abundances to the observed ones. If accretion has stopped, or if the rates vary by large amounts, we cannot determine the composition of the accreted matter with any certainty.

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“…For decades the standard explanation has been accretion from interstellar matter (ISM), with subsequent diffusion downward out of the atmosphere. This scenario has been discussed in detail and compared with observations in a series of three fundamental papers by Dupuis et al (1992Dupuis et al ( , 1993a. A special case is the DQ spectral class, with pollution by the element carbon.…”

Section: Introductionmentioning

confidence: 99%

Cool DZ white dwarfs in the SDSS

Koester

Girven²,

Gänsicke³

et al. 2011

A&A

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Aims. We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) (u − g) vs. (g − r) two-color diagram; 21 of these stars are new discoveries. Methods. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of logg = 8. Results. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around T eff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, probably because of our selection procedures. The interpretation is complicated in terms of the accretion/diffusion scenario, because we do not know if accretion is still occurring or has ended long ago. Independent of that uncertainty, the masses of the metals currently present in the convection zones -and thus an absolute lower limit of the total accreted masses -of these stars are similar to the largest asteroids in our solar system.

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A study of metal abundance patterns in cool white dwarfs. II - Simulations of accretion episodes

Cited by 100 publications

References 0 publications

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

DB white dwarfs in the Sloan Digital Sky Survey data release 10 and 12

Accretion and diffusion in white dwarfs

Cool DZ white dwarfs in the SDSS

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