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A&A Supplement series, Vol. 127, January II 1998, 277-294

Received October 30, 1996; accepted May 9, 1997

Are metallic A-F giants evolved Am stars? Rotation and rate of binaries among giant F starsgif,gif

M. Künzli - P. North

Send offprint request: P. North
Institut d'Astronomie de l'Université de Lausanne, CH-1290 Chavannes-des-Bois, Switzerland


We test the hypothesis of Berthet (1992) which foresees that Am stars become giant metallic A and F stars (defined by an enhanced value of the blanketing parameter tex2html_wrap_inline2467 of the Geneva photometry) when they evolve.

If this hypothesis is right, Am and metallic A-FIII stars need to have the same rate of binaries and a similar distribution of tex2html_wrap_inline2469. From our new spectroscopic data and from tex2html_wrap_inline2469 and radial velocities in the literature, we show that it is not the case. The metallic giant stars are often fast rotators with tex2html_wrap_inline2469 larger than 100 kms-1, while the maximum rotational velocity for Am stars is about 100 kms-1. The rate of tight binaries with periods less than 1000 days is less than 30% among metallic giants, which is incompatible with the value of 75% for Am stars (Abt & Levy 1985). Therefore, the simplest way to explain the existence of giant metallic F stars is to suggest that all normal A and early F stars might go through a short "metallic" phase when they are finishing their life on the main sequence.

Besides, it is shown that only giant stars with spectral type comprised between F0 and F6 may have a really enhanced tex2html_wrap_inline2467 value, while all A-type giants seem to be normal.

keywords: stars: binaries: spectroscopic -- stars: chemically peculiar -- stars: evolution -- stars: rotation -- stars: tex2html_wrap_inline2481 Scuti

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