Chemical abundances of A and F-type stars: the Hyades open cluster

Abundances of Il chemical elements have been determined for 29 F and 19 A d warfs ("normal" and chemically peculiar with apparent rotational velocities ranging from 11 to 237 km.s(-1)) members of the Hyades cluster using Taked a's (1995) iterative procedure well suited for fast rotators. High quality spectra of high to moderate resolution have been synthesized to derive the apparent rotational velocities, microturbulent velocities and abundances of C, O, Na, Mg, Si, Ca, Sc, Ee, Ni, Y and Ba. For the F stars, the found abundances are compared to the predictions of Tu rcotte et al.'s (1998) evolutionary models for the age of the Hyades. We fa il to find the expected underabundances for light elements nor the overabun dances for the iron-peak elements for stars having effective temperatures g reater than 6500 K. This suggests that diffusion alone cannot account for t he derived abundances: other transport processes which counteract diffusion are probably at play. For the A stars, abundances of Na, Mg, Si, Ca, Fe, Ni and Ba are anticorrel ated (correlated for O) with the apparent equatorial velocities for v(e) si n i higher than similar to 100 km.s(-1). Below this value, we find no corre lation in agreement with the predictions of Charbonneau & Michaud (1991) ex cept possibly for the abundance of Ni which might be anticorrelated with v( e) sin i. The fastest rotators (v(e) sin i greater than or equal to 150km.s (-1)) are strongly depletted in Mg, Si, Ca, Fe, Ni and Ba(between -0.3 dex and -0.94 dex) while their O abundance is normal.