The Pleiades, Coma, Hyades, and Praesepe open clusters: Li, Al, Si, S, Fe,Ni, and Eu abundances compared in A stars

In the third of a series of papers on the A stars in open clusters, the Com a and Hyades clusters an revisited; in the first and second papers, the Ple iades and Praesepe were respectively investigated. All the spectra were sec ured with the Canada-France-Hawaii telescope at high spectral resolution an d high signal-to-noise ratios. Photospheric abundances have been determined for Li, Al, Si, S, Fe, Ni, and Eu from model atmosphere abundance analysis . All the A stars with enough-sharp lines to be studied for Li were observe d in the four clusters. Abundance results are summarized for 31 cluster mem bers, including 21 Am, 7 normal A, and 3 early-A stars. The Am stars have very uniform Li, Al, Si, S, and Fe abundances in a large temperature range of nearly 1000 K. Compared to normal A stars, Li is signi ficantly deficient in Am stars (by a factor of 3), Al marginally overabunda nt, Si, S, and Fe are the same, Ni and Eu (with only a few results) overabu ndant. Those uniform abundances of Li, Al, Si, S, or Fe in Am stars involve that abundances are little affected by the magnitude of the rotational vel ocity. For both Am and normal A samples, no abundance trend as a function of age a nd/or evolution is detected in the case of Li, Al, Si, S, or Fe. The ages c onsidered are in the range 0.8 - 7 10(8) years; the evolution is limited fr om the ZAMS to the cluster turn-off. The build-up of the chemical abundance s studied, in particular the Li differentiation between Am and normal A sta rs, could have taken place very early when the stars arrive on the Main Seq uence. The spread in lithium found for the A stars is reminiscent of that reported in the field and one open cluster for stars of nearly the same mass and sl ightly evolved out of the Main Sequence. The Li abundance does not change a s soon as the star evolves through the subgiant phase and the convection zo ne becomes deeper. There are two exceptional Am stars: one, in the Hyades, is Li-deficient and the other, in Praesepe, Li-overabundant. They are no ob vious circumstances that can distinguish both stars from others in the very same region of their respective cluster sequence. In each of the four clus ters, the maximum Li abundance is found in A stars, generally in normal A s tars. The Fe abundance of both Am and normal A groups is found to be twice the or iginal Fe value (on the ZAMS) in each of the four clusters, independently o f their age or metallicity. It is well established for the Am group and in only a narrow range of Teff for the normal A group. This behavior is unexpe cted for normal A stars which are thought to have their original abundances and Fe abundance different from that of Am stars. A larger normal A sample is needed to conclude anything. Our abundance results for cluster Am stars quantitatively agree with predic tions of new models coupling atomic diffusion with turbulent transport (Ric her et al. 1999).