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Astron. Astrophys. 338, 1073-1079 (1998)

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5. Conclusion

In Praesepe cluster, all A stars with enough sharp lines for Li to be studied were observed. They are ten Am stars.

- The Am stars have the same Li content as the F stars on the hot side of the Li dip. This result relies upon only a few stars. One star stands out with Li more abundant and similar to that of normal cluster stars. Extending the abundance analysis of this star and the others of the same effective temperature would be interesting.

- Al, Si, and S are marginally overabundant compared with the Sun, keeping in mind that weak Al blends may be difficult to manage and strong S lines are dependent on the microturbulent velocity, not well determined for some stars of this work.

- Compared with the Sun or the Praesepe F stars, the Fe overabundance by a factor of 2 at least is well established.

- Ni, one element of the Fe group, too, is overabundant (by a factor of 4) compared with the Sun. It is well established but for a few stars.

- The rare earth Eu is largely overabundant compared with the Sun: the factor of 10 is not accurate taking into account difficulties encountered in studying the Eu blend as explained in the first paper in the series.

There is remarkably an only abundance value for the studied elements, even if some star may have a little outstanding element. To the elements studied in this series of papers we can add Sc and Ca, each of which exactly shows uniform underabundance for five stars in common with Hui-Bon-Hoa and co-workers (1997; 1998). At the age of Praesepe (log t(years) = 8.8), must we think that there is enough time for interior transport processes to act and to reach the same stable photospheric chemical composition for stars with different structure envelopes (since so are the effective temperatures and stages of evolution)?

From the age of the Pleiades till that of the Hyades and Praesepe, Hui-Bon-Hoa & Alecian (1998) claim that they find significant differences in abundance patterns of A stars, especially for Sc and Ca. In this paper, on the contrary, the behavior of Li and Fe in Am stars of Praesepe is found very similar to that of the Pleiades. New constraints are, so, put on abundance evolution between both ages of the clusters, i.e., in the log t interval of 8.0-8.8. Time dependence of the abundances would be better tested in the next paper in the series dealing with new results in Coma and Hyades clusters, which have ages similar to Praesepe.

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998