Further determinations of the gravities of cool giant stars using Mg I and MgH features

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Abstract
Surface gravities are deduced for Arcturus and five other cool giant stars. A temperature and an overall abundance are adopted for each object and the surface gravity is then found by requiring the magnesium abundance derived from Mg I features to match that derived from MgH. The contamination of the MgH features by lines of the C2 system is allowed for. The values obtained by this method are compared with those found by other spectroscopic methods that use Fe ionization equilibria and the profiles of strong lines. The errors inherent in the molecular equilibrium methods are discussed. The surface gravities found from radii derived from parallaxes and angular diameters, and an assumed mass of 1 M⊙, are compared with the spectroscopic results. The results for Arcturus are also compared with those found from the molecular equilibrium method applied to oxygen features. All the determinations discussed here use a consistent set of theoretical, LTE, flux-constant, line-blanketed model atmospheres, and the differences in the results are taken to be evidence for non-LTE effects. It is suggested that these comparisons indicate a range of spectral types for which non-LTE effects are important. A supplemental abundance analysis for one of the giant stars (ϕ2 Ori) is also presented.
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