White dwarf external layers. IV. Interpretation of spectra.
Abstract
The problems raised by the spectral classification of the White Dwarfs (WDs) are examined, and a systematic interpretation is attempted by means of envelope models. Since the stellar evolution theory is, in its present stage, unable to predict the chemical stratification in WDs, the orders of magnitude of the H and He-envelopes one expects in WDs have been computed on the hypothesis of steady nuclear burning. Convection is studied in detail as a function of the chemistry, and of the assumptions of convection theory. Stratified H-He envelopes are computed; the influence upon the convection and mixing of the zone in which a gradient of molecular weight is present is broadly investigated. Constraints on interstellar accretion are derived: it is suggested that accretion due to the simple geometric cross section of the star is the maximum compatible with the existence of He-WDs. Finally, the role of C-sedimentation and of its dredging-up due to He-convection is examined in an attempt to explain the C-rich WDs.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1979
- Bibcode:
- 1979A&A....74..161D
- Keywords:
-
- Stellar Envelopes;
- Stellar Models;
- Stellar Spectra;
- White Dwarf Stars;
- Helium;
- Hydrogen Atoms;
- Spectrum Analysis;
- Stellar Evolution;
- Stellar Mass Accretion;
- Stellar Structure;
- Astrophysics;
- Convection:White Dwarfs;
- White Dwarfs:Envelopes;
- White Dwarfs:Models