EQUILIBRIUM MODELS FOR STARS WHICH DERIVE ENERGY FROM HELIUM-BURNING. II. HELIUM STARS WITH HYDROGEN-RICH ENVELOPES
Models are constructed for helium-burning stars which possess a hydrogen- rich envelope in radiative equilibrium. These inhomogeneous models are obtained from the models constructed for homogeneous, pure helium-burning stars by use of an approximation method. This method is applicable only if the fractions of the total mass and total energy production contained in the envelope are small. The bolometric luminosity is assumed to be the same as that of a homogeneous, pure helium star of the same mass M. If the hydrogen abundance X' (by weight) in the envelope is near unity, then the presence of a hydrogen-rich envelope containing a mass fraction Q' of only 0.1 is found to increase the radius of the star by a factor of about 10. The factor by which the radius is increased is independent of mass M but increases rapidly with both X' and Q'. The present approximate models are in good agreement with more accurate ones constructed for larger values of Q'. Assuming the 0-type subdwarf HZ 44 to be represented by a model of this type, its mass and absolute bolometric magnitude can be estimated. It is found that M approx equal 0.5 M/sub sun/, M/sub bol/ approx equal +0.8, and Q' approx equal 0.05. The possible evolutionary significance of the present models and their relation to the observed properties of the nuclei of planetary nebulae and of the hot subwarfs are discussed. The approximation method is described in general form and is applied to pure helium star models to show that inclusion of the free-free contribution to the opacity in the outer layers of the pure helium models would increase their radii by no more than 2 or 3 per cent. (auth)
- Research Organization:
- Cornell Univ., Ithaca, N.Y.
- NSA Number:
- NSA-15-024182
- OSTI ID:
- 4028575
- Journal Information:
- Astrophysical Journal (U.S.), Vol. Vol: 133; Other Information: Orig. Receipt Date: 31-DEC-61
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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