Radiative Opacity in Stellar Atmospheres. III. Effect of the Hydrogen-To Ratio on the Atmospheric Structure

A grid of line-blanketed model atmospheres with different hydrogen-to-helium ratios is constructed for wide ranges of Teff and log g in order to investigate the effect of differing hydrogen content on the structure of stellar atmospheres. The changes in the emergent flux distribution with wavelength, UBV color indices, and the fly and Ho profiles are also considered. It is found that a small reduction in the H-to-He ratio produces a large increase in the electron pressure as well as in the gas pressure, and that the change in Pe produces considerable effects on the observable features. On the other hand, the effect of hydrogen content on the temperature stratification is small until the H-to-He ratio is reduced to about 0.1. Theoretical interpretations are presented to account for the variations in T, Pe, and P with varying hydrogen content in terms of differential variations in the relative importance of H, H, and He as sources of atmospheric opacity coupled with Lyman- and Balmer-line blanketing. Since an increase in Fe also results from an increased surface gravity, the variations of the observable quantities with hydrogen content or with surface gravity are generally indistinguishable. A general for- mula is derived to relate the errors due to uncertainties in log g and those in hydrogen content. In a hydrogen-rich atmosphere, the accuracies in the determinations of the surface gravity and hydrogen content from the color indices are estimated to be `-~.-`O 1 in log g and about 15 per cent in the hydrogen content for a model with Teff 150000 K and logg 7. Since a small change in hydrogen content has lit- tle effect on Teff, the determination of Teff from observed color indices appears to be a promising method for white dwarfs. For stars with lower g, however, the use of hydrogen line profiles may be superior to the use of color indices in determining the atmospheric parameters, although in this case it is not possible to determine Teff independently from g and the H-to-He ratio