2000A&A...354..216B

In the third of a series of papers on the A stars in open clusters, the Coma and Hyades clusters are revisited; in the first and second papers, the Pleiades and Praesepe were respectively investigated. All the spectra were secured 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. All the A stars with enough-sharp lines to be studied for Li were observed in the four clusters. Abundance results are summarized for 31 cluster members, 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 1000K. Compared to normal A stars, Li is significantly deficient in Am stars (by a factor of 3), Al marginally overabundant, Si, S, and Fe are the same, Ni and Eu (with only a few results) overabundant. 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 velocity. For both Am and normal A samples, no abundance trend as a function of age and/or evolution is detected in the case of Li, Al, Si, S, or Fe. The ages considered are in the range 0.8-7x10⁸ years; the evolution is limited from the ZAMS to the cluster turn-off. The build-up of the chemical abundances studied, in particular the Li differentiation between Am and normal A stars, could have taken place very early when the stars arrive on the Main Sequence. 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 slightly evolved out of the Main Sequence. The Li abundance does not change as soon as the star evolves through the subgiant phase and the convection zone 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 obvious circumstances that can distinguish both stars from others in the very same region of their respective cluster sequence. In each of the four clusters, the maximum Li abundance is found in A stars, generally in normal A stars. The Fe abundance of both Am and normal A groups is found to be twice the original Fe value (on the ZAMS) in each of the four clusters, independently of 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 unexpected 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 predictions of new models coupling atomic diffusion with turbulent transport (Richer et al., 2000, ApJ, in press).