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Lyman alpha wing opacity effect on temperature scale and helium content in F and G subdwarf atmospheres

Completion of long-term astrometric plate series yields revised results on the orbits and parallaxes. The subdwarf 'i Cassiopeiae seems to be overluminous; the distance of the supergiant 6 Aurigae is near 600 pc. The unresolved companion in the red dwarf pair BD +6°398 is of very low mass, and the companion of BD +67°552 massive but probably underluminous.

A grid of model atmospheres has been used to perform an abundance analysis of these two extreme Population II stars. The abundances determined from the models confirm the general results of previous investigations, namely that metals are deficient by a factor of about 100 relative to average Population I abundances. A marginal deficiency of s-process material relative to the average deficiency is found; the carbon deficiencies are comparable to those of the other elements and for HD 19445 the α-process elements are enhanced. From the analysis it also appears that convection plays a significant role in determining the temperature structure of subdwarf atmospheres.

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,more » 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)« less

As an aid in classifying spectra of 40 stars from which radial velocities were measured at 36 Å/mm, two numerical indices were formed, which depend respectively on average differences in central absorption of eight pairs of lines sensitive to pressure, and of nine pairs sensitive to temperature in the atmosphere.

HARTWICK1 has used the method of semi-empirical evolution due to Sandage2, together with homologously transformed stellar models of Iben and Faulkner3, to deduce that the helium content of M 92 is probably close to Y=0.35. This result was obtained by plotting the fuel consumed (expressed in terms of mass-fraction) against the bolometric magnitude and comparing this theoretical quantity with the equivalent quantity deduced by the semi-empirical method. Good agreement was obtained for Y=0.35, but not for Y=0.10 or 0.01. The disagreement in the last two cases was particularly striking in the regions just above the turn-off point. As Hartwick noted, this was in part a reflexion of the fact, first pointed out by Simoda and Kimura4, that the slope of the luminosity function ϕ in this region is extremely sensitive to the helium content. Indeed, this sensitivity was dramatically exhibited when Hartwick, on “unevolving” the M 92 data, found that the initial luminosity function would be required to decrease quite sharply to fainter magnitudes for Y=0.10 and 0.01, whereas it increased slightly for Y=0.35. The former behaviour is most unpalatable, but the latter is what one might expect on other statistical grounds5.

A series of model stellar atmospheres, representing stars of low heavy- element content, is tabulated. Characteristics of the continua of the models, including (U,B,V) colors and bolometric corrections, are discussed and compared with observations. (auth)

Primordial helium and horizontal branch star luminosity, obtaining luminosities in He burning phase for different He and metal abundances from models