The Puzzle of the Metallic Line Stars

Abstract

In the puzzle of the metallic line (Am) stars, there still seem tobe missing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak.The Am stars have no ultraviolet emission lines. They are binariesthat, with very few exceptions, have rotational velocities vsinilower than 100 km s-1. Of the main-sequence A stars, 20% to 30% areAm stars. Here we rediscuss previous suggestions that tried to explainthe peculiar line strengths in the Am star spectra. In particular,we compare the well-studied properties of Hyades A and Am stars inorder to identify reasons that can or cannot explain the differences.We find that accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres. Charge-exchangereactions also seem to be important.