Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992jrasc..86..309b&link_type=abstract
Journal of the Royal Astronomical Society of Canada (ISSN 0035-872X), vol. 86, no. 6, p. 309-324.
Astronomy and Astrophysics
Astronomy
2
Abundance, Stellar Atmospheres, Stellar Composition, Stellar Structure, White Dwarf Stars, A Stars, Helium, Stellar Convection, Stellar Gravitation, Stellar Interiors, Stellar Temperature
Scientific paper
The problem of the evolution of the surface abundance of cool DA white dwarf stars is considered. Since these stars exhibit only hydrogen lines in their spectra, it has long been assumed that their atmospheres were composed of pure hydrogen. However, it is demonstrated that helium, which is spectroscopically invisible at these temperatures (Teff not greater than about 12,000 K), is present in large amounts at the surface of these stars. The presence of helium is interpreted as strong evidence for convective mixing between the thin superficial hydrogen layer with the more massive underlying helium layer. In particular, it is found that most stars mix at a temperature of 11,500 K, which corresponds to the location of the cool edge of the ZZ Ceti instability strip.
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