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^*

¹ Institut fur Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria e-mail: [fossati;weiss]@astro.univie.ac.at
² Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland e-mail: sba@arm.ac.uk
³ Groupe de Recherches en Astronomie et Astrophysique du Languedoc, UMR 5024, Université Montpellier II, Place E. Bataillon, 34095 Montpellier, France e-mail: Richard.Monier@graal.univ-montp2.fr
⁴ Department of Physics & Astronomy, University of Western Ontario, London, N6A 3K7, Ontario, Canada e-mail: [skhan;jlandstr]@astro.uwo.ca
⁵ Department of Astronomy and Space Physics, Uppsala University, 751 20, Uppsala, Sweden e-mail: oleg@astro.uu.se
⁶ Physics Dept., Royal Military College of Canada, PO Box 17000, Station Forces, K7K 4B4, Kingston, Canada e-mail: Gregg.Wade@rmc.ca

Received: 20 July 2007
Accepted: 19 September 2007

Abstract

Aims.We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field.

Methods.We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified Am stars, two normal A-type stars and one Blue Straggler, considered to be members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements, with a higher precision than was ever obtained before for this cluster. For seven of these stars we also obtained spectra in circular polarization and applied the LSD technique to constrain the longitudinal magnetic field.

Results.No magnetic field was detected in any of the analysed stars. HD 73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to have the abundances of a normal A-type star. Am classification is not confirmed for HD 72942. For HD 73709 we have also calculated synthetic photometry that is in good agreement with the observations. There is a generally good agreement between abundance predictions of diffusion models and values that we have obtained for the remaining Am stars. However, the observed Na and S abundances deviate from the predictions by 0.6 dex and ≥0.25 dex respectively. Li appears to be overabundant in three stars of our sample.