Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster

摘要

Aims. We aim to provide observational constraints on diffusion models thatpredict peculiar chemical abundances in the atmospheres of Am stars. We alsointend to check if chemical peculiarities and slow rotation can be explained bythe presence of a weak magnetic field. Methods. We have obtained high resolution, high signal-to-noise ratio spectraof eight previously-classified Am stars, two normal A-type stars and one BlueStraggler, considered to be members of the Praesepe cluster. For all of thesestars we have determined fundamental parameters and photospheric abundances fora large number of chemical elements, with a higher precision than was everobtained before for this cluster. For seven of these stars we also obtainedspectra in circular polarization and applied the LSD technique to constrain thelongitudinal magnetic field. Results. No magnetic field was detected in any of the analysed stars. HD73666, a Blue Straggler previously considered as an Ap (Si) star, turns out tohave the abundances of a normal A-type star. Am classification is not confirmedfor HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometrythat is in good agreement with the observations. There is a generally goodagreement between abundance predictions of diffusion models and values that wehave obtained for the remaining Am stars. However, the observed Na and Sabundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.Li appears to be overabundant in three stars of our sample.