Interpretation of the gross molecular spectra of S stars

A grid of S star model atmospheres (Teff = 2500–3500 K, C/O = 0.6–1.0, log g = 0) is examined. Several tests are made to examine the effect of (1) atomic line blanketing, (2) gravity, (3) s-process enhancements and (4) non-solar C/O ratios. The models are used to generate column densities of molecules of particular interest in S stars. The column densities are used to develop guidelines by which the observed molecular spectra of S stars may be interpreted in terms of physical variables. The main conclusions are: (1) Molecular band strengths are not good temperature indicators from star to star because of their great C/O sensitivity. Carefully selected atomic lines or infrared colours should produce a reliable temperature index. (2) The band strength of TiO is a good C/O indicator in S stars when adjusted for its temperature variations. (3) The molecular spectra of S stars cannot be interpreted solely by C/O effects. When allowance is made for molecular equilibrium effects the band strength of ZrO could provide an abundance index for S stars.