Astron. Astrophys. 338, 1073-1079 (1998)

1. Introduction

This paper is the second in a series about the abundances of the trace light element Li for the normal A and Am stars in open clusters of different ages; the abundances of Al, Si, and S; the abundances of iron peak elements, Fe and Ni; and the abundances of the rare earth Eu. This paper is concerned with the A stars of the Praesepe cluster. The first paper (Burkhart & Coupry 1997) dealt with the A stars in the Pleiades. The objectives are, first, the origin and evolution of the anomalous abundances in the Am stars and, secondly, Li abundances in clusters on the hot side of the Li dip. As the age of the Pleiades is about   years and the A stars are at the beginning of the Main Sequence evolution when the age of Praesepe is nearly ten times, about 7  10⁸ years and the A stars are well advanced in their Main Sequence evolution, we may be able to detect if some surface abundances change with time and/or evolution.

The first paper defined our observation and reduction techniques, abundance determination procedure, kept the same throughout the paper series. (See, also, Burkhart & Coupry (1991), Coupry & Burkhart (1992), and references therein). The spectroscopic data have been obtained at the Canada-France-Hawaii (CFH) 3.6m telescope and the f/7.4 coudé spectrograph camera equipped with a Reticon detector (spectra covering 135 Å   at a dispersion of 4.83 Å  mm^-1, with typical signal-to-noise ratios in the range 200-400 at the 2 level). The equivalent width results were checked both internally with CMi observed during each run and externally with CMi (Steffen 1985) and the Sun (Rutten & van der Zalm 1984). The good agreement between the different equivalent width scales supports this work, the computed solar logf values (except for Li with laboratory log gf values), and the stray-light effects neglected since they are comparable for all spectrographs at work. The data reduction was carried out with codes written by M.Spite (private communication); abundances were derived using model atmospheres (Kurucz 1979a, 1979b) with temperatures derived from uvby, photometry (Moon 1985; Moon & Dworetsky 1985)

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
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