On the acoustic flux of Sirius A

Fontaine, G.; Villeneuve, B.; Wilson, J. M.

doi:10.1086/158619

Cited by 29 publications

(18 citation statements)

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“…It has become customary to denote the specific version as, e.g., ML2/α = 0.6 or in shorthand as ML2/0.6. Here ML2 represents a choice of three dimensionless parameters, which are defined and explained in Fontaine et al (1981) and Tassoul et al (1990), the number (0.6) is the mixing-length as a multiple of the pressure scale height. Koester et al (1994) and Bergeron et al (1995) demonstrated that a version with "intermediate" efficiency of energy transport is the best choice for DAs in ensuring a consistent fit to optical and UV spectra simultaneously.…”

Section: The Convection Zonementioning

confidence: 99%

Accretion and diffusion in white dwarfs

Koester¹

2009

A&A

273

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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Section: The Convection Zonementioning

confidence: 99%

Accretion and diffusion in white dwarfs

Koester¹

2009

A&A

273

471

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“…Fontaine et al 1981;Tassoul et al 1990;or Koester et al 1994 for an explanation of nomenclature). The mass-radius relation predicted by these models is in reasonable agreement with more sophisticated calculations (e.g.…”

Section: Stellar Masses and Hydrogen Mass Fractionsmentioning

confidence: 99%

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey

Voß¹,

Koester

Napiwotzki

et al. 2007

A&A

119

234

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Context. We present a detailed spectroscopic analysis of the stars with helium-dominated spectra in the ESO Supernova Ia Progenitor Survey (SPY). Aims. Atmospheric parameters, masses, and abundances of trace hydrogen are determined and discussed in the context of spectral evolution of white dwarfs. Methods. The spectra are compared with theoretical model atmospheres using a χ 2 fitting technique, leading to determinations of effective temperature, surface gravity, and hydrogen abundance. Results. Our final sample contains 71 objects, of which 6 are new detections and 14 are reclassified from DB to DBA because of the presence of H lines. One is a cool DO with weak He ii lines, 2 are composite DB+dM. 55% of the DB sample show hydrogen and are thus DBA, a significantly higher fraction than found before. Conclusions. The large incidence of DBA, and the derived total hydrogen masses are compatible with the scenario that DBs "reappear" around 30 000 K from the DB gap by mixing and diluting a thin hydrogen layer of the order of 10 −15 M . This hydrogen mass is then during the evolution continuously increased by interstellar accretion. There are indications that the accretion rate increases smoothly with age or decreasing temperature, a trend which continuous even below the current low temperature limit (Dufour 2006). A remaining mystery is the low accretion rate of H compared to that of Ca observed in the DBZA, but a stellar wind extending down to the lowest temperatures with decreasing strength might be part of the solution.

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“…The models are in LTE, are line-blanketed, and include the transport of energy by convection. For consistency, the mixing-length theory was treated using a parameterization of ¼ 1:25 ( Fontaine et al 1981), already used by Beauchamp (1995) and Beauchamp et al (1999). The chemical composition includes only helium and hydrogen and no heavy elements.…”

Section: Model Atmospheresmentioning

confidence: 99%

FUSEObservations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

Desharnais¹,

Wesemaël²,

Chayer³

et al. 2008

ApJ

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We present a comprehensive analysis of the far-ultraviolet spectra of five DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270À124) target. The observed carbon abundances range from log N (C)/N ( He) $ À6:9 to $ À8:8. In addition, four of the five objects display photospheric lines of silicon. Other elements such as oxygen, iron, and sulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up.

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On the acoustic flux of Sirius A

Cited by 29 publications

References 0 publications

Accretion and diffusion in white dwarfs

Accretion and diffusion in white dwarfs

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey

FUSEObservations of Heavy Elements in the Photospheres of Cool DB White Dwarfs

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