The chemical composition of field RR Lyrae stars .1. Iron and calcium

Iron and calcium abundances for 18 local field RR Lyrae stars are obtained from high-resolution spectra taken near minimum light. Abundance analyses are presented using model atmospheres defined by published photometry. Analyses were also undertaken using model atmospheres chosen to fit the excitation equilibrium of Fe I and the ionization equilibrium of iron. Systematic differences between the two analyses are discussed. Contributions of non-LTE effects are assessed quantitatively. The Fe abundance is given most reliably by Fe II lines analyzed with the photometric models, and the Ca abundance is probably best given by combining this Fe abundance with the Ca/Fe ratio given by the photometric(or the spectroscopic) model atmospheres and the Ca I and Fe I lines. The Ca/Fe ratio of these RR Lyrae stars follows the well-known variation of [Ca/Fe] with [Fe/H] found for dwarfs and giants; most notably, the metal-rich ([Fe/H] similar or equal to 0)RR Lyrae stars do not have an anomalous Ca/Fe ratio. New calibrations are presented of the Delta S-[Fe/H] and M(V)-[Fe/H] relations. Appendices provide critical evaluations of published gf-values for the Fe I and Fe II lines used in this analysis.