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

In the rst 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 spectral reso- lution and 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 decient 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. TheseFeresultsareunexpectedsince,rstly,normalAstarsare 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 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 compara- ble 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.