Extreme subdwarfs. I - Molecular band strengths - A theoretical approach

For subdwarf stars with effective temperatures below about 5500 K, the G-band region, which is a conglomerate of many atomic and molecular (CH) lines, is shown by spectrum synthesis to be an extremely sensitive metal-abundance discriminator. In particular, some of the spectra computed for metal-deficient model atmospheres in the wavelength interval 4290 to 4330 A show that this feature is stronger than that for the normal abundance model at the same effective temperature and gravity. Thus the 'CH strong' red subluminous stars classified by Greenstein (1977) can be adequately interpreted as belonging to this group of stars, rather than unevolved stars having an overabundance of carbon. The behavior of CO, OH, and H2O molecules in determining the amount of free carbon available for CH formation in stellar atmospheres is discussed for a grid of flux-constant model atmospheres. Synthetic G-bands constructed with this grid of dwarf and subdwarf model atmospheres enable metal abundances to be derived for subdwarf stars with effective temperatures between 4000 K and 6000 K.