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Composite Spectra. XII. o Leonis: An Evolving Am Binary

It is shown by techniques of spectrum disentangling that both components of the composite-spectrum binary ο Leo (V = 3.52 mag) have Am characteristics, even though the primary is an evolving giant (log g = 3.25) with Teff ∼ 6100 K. This is believed to be the first isolation of such a cool Am star, and the finding challenges the theories of diffusion, which are widely accepted as the cause of metallicism. The primary component of ο Leo appears to be deficient in Ca and Sc, as are classical Am stars—though the δ Del stars which are described as evolved Am stars are not—and therefore seems to be in a class on its own. It is suggested that the unusual state of the primary component is attributable either to its current state of rapid evolution or to regular Am-star evolution that is difficult to recognize spectroscopically. This paper describes the technical problems that have hitherto prevented the identification of such unexpected properties in this easily observed binary, determines the physical parameters of the component stars, examines their evolutionary states, and debates the possible classification of the giant component. Future directions for this work will include detailed chemical composition analyses and an observing program designed to search for other cases of substantially evolved Am stars.