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Astron. Astrophys. 318, 870-878 (1997)

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The Pleiades open cluster: abundances of Li, Al, Si, S, Fe, Ni, and Eu in normal A and Am stars *

C. Burkhart 1 and M.F. Coupry 2

1 Observatoire de Lyon (Un. Lyon I) and UMR 5574 (CNRS), F-69561 Saint-Genis-Laval Cédex, France
2 Observatoire de Paris and URA 335 (CNRS), 61 Av. de l'Observatoire, F-75014 Paris, France

Received 15 April 1996 / Accepted 26 June 1996

Abstract

In the first of a series of papers on the A stars in open clusters, normal A and Am stars in the Pleiades were observed with the Canada-France-Hawaii telescope at high spectral resolution and high signal-to-noise ratios. Photospheric abundances have been determined for Li, Al, Si, S, Fe, Ni, and Eu from model atmosphere abundance analysis.

The A stars of the Pleiades are at the beginning of their Main Sequence evolution. At this stage, Li is clearly deficient in the Am stars compared with the normal A stars (-0.65 dex), and the abundance of Fe is the same for both stellar groups, twice its original solar value as given by the Pleiades F stars. These Fe results are unexpected since, firstly, normal A stars are thought to have normal abundances and, secondly, Am stars are classically said to be overabundant in Fe compared with normal A stars. The maximum Li abundance of the cluster is found in the normal A stars with log N (Li) = 3.55 [FORMULA] 0.1 on the scale log N (H) = 12.0. These stars seem to have preserved their original Li better than any other cooler stars of the Pleiades cluster.

The behavior of Li in the slow-rotator Am stars versus the rapid-rotator normal A stars in the Pleiades is exactly comparable with that found in the "low v sin i" versus "high v sin i" stars with similar masses in the Orion association. Could it be that Li differentiation between Am and normal A stars takes place during their pre-Main-Sequence evolution?

The Li results in the Am stars challenge predictions from model envelopes coupling diffusion and evolution (plus mass loss) in non rotating stars since only strong underabundances are expected at the age of the Pleiades.

Key words: open clusters: Pleiades – stars: abundances – stars: rotation

* Based on observations collected at the Canada-France-Hawaii telescope (Hawaii)

Send offprint requests to: C. Burkhart

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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