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Low Mass Hydrogen Envelopes and the DB Gap

Published online by Cambridge University Press:  12 April 2016

James MacDonald
James MacDonald*
Affiliation:
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716

Extract

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The existence of a gap in the distribution of helium atmosphere white dwarfs at the effective temperature interval 30,000 K ≤ Te ≤ 45,000 K is well documented (Wesemael, Green and Liebert 1985; Liebert et al 1986; Green and Liebert 1987).

To explain the presence of this gap and other variations of the non-DA to DA ratio, it has been proposed (Fontaine and Wesemael 1987; Liebert, Fontaine and Wesemael 1987) that the helium-rich PG 1159 stars are the progenitors of essentially all white dwarfs. Some minute quantity of hydrogen of total mass, MH, is assumed to be mixed in the outer helium envelope, and settle upward as the star cools from the PG 1159 stage. Eventually enough hydrogen is at the surface to make the star appear a DA. The star has to become DA before it cools down to ≈ 45,000 K and cools to ≈ 30,000 K as a DA. At this temperature, depending on MH, the underlying helium convection zone may break into the hydrogen layer, diluting it and making the star a DB.

Type
Research Article
Information
International Astronomical Union Colloquium , Volume 114: White Dwarfs , 1989 , pp. 172 - 175
Copyright
Copyright © Springer-Verlag 1989

References

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