On the evolutionary status of the carbon-band white dwarfs

Sion, E. M.; Fragola, J. L.; Odonnell, W. C.

doi:10.1086/130520

Cited by 3 publications

(1 citation statement)

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“…About one of every six known non-DA stars in the 6,000 -10,000 K temperature range shows Co (A4670) bands (Sion, Fragola and O'Donnell 1979)• Since carbon lines have not been reliably identified in hotter helium-atmosphere stars, it has been tempting to propose that the helium envelope in the A 4670 cases has reached a carbon-rich core. D' Antona and Mazzitelli (1979ab) argue that pre-white-dwarf mass loss should not leave a helium envelope thin enough for the convection to penetrate all the way through; however, Vauclair and Fontaine's (1979) calculations indicate that, if the helium layer were small enough, the/<-discontinuity would be penetrated and carbon should be mixed upwards.…”

Section: White Dwarfmentioning

confidence: 99%

The Luminosity Function and Evolution of Cool White Dwarfs

Liebert

1979

International Astronomical Union Colloquium

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In the review, I attempt to focus on what we have recently learned from the observations about three general topics of great theoretical interest:1. evidence for evolution of white dwarf surface abundances, 2. the special problems of determining atmospheric parameters and abundances at the cool end of the sequence, and 3. evidence for a peaking of the white dwarf luminosity function at M. , = +15 ± 1, with possible implications for cooling theory and the history of galactic star formation. 1.Spectroscopic Temperature/Abundance Sequences Figure 1 shows the parallel spectroscopic sequences of stars having hydrogen-dominated or helium-dominated atmospheric compositions. Greenstein (1960 and in subsequent papers)has provided the definitions and modern framework of spectral types. The non-DA sequence of course breaks into DC, metallic-line and carbon band (C2) stars at temperatures too cool for the DB helium lines to be seen. The middle sequence of intermediate abundance objects or "hybrids" generally contains stars with helium-dominated atmospheres and detectable trace abundances of hydrogen. The DA,F stars are included in this category since have argued that those few objects truly deserving this classification may be hydrogen-poor. The potential DCO^) classification recognizes the possibility of detecting H_ in cool hydrogen-rich atmospheres-see §3.We should not expect that each of these sequences represent the actual evolutionary stages for a star with a given atmospheric abundance. Because of several physical processes (to be reviewed next by G. Vauclair), we expect that atmospheric abundances can change as the stars cool, converting a given star from one sequence to another. I leave for him the detailed documentation of the impressive, recent theoretical work on these processes. Gravitational settling, thermal diffusion and radiation pressure may be important for hot stars, but we must especially investigate the anticipated roles of convective mixing and accretion for the cooler stars. We should ask ourselves how it can be shown whether these predicted processes really are at work in the stars.-146-https://www.cambridge.org/core/terms. https://doi

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Section: White Dwarfmentioning

confidence: 99%