Astron. Astrophys. 353, 557-568 (2000)

1. Introduction

The Beers et al. (1985, 1992) HK interference-filter/ objective-prism survey of very metal-poor stars has identified an unexpected number of carbon-rich and often also nitrogen-rich stars in the metallicity range (see for example Rossi et al. 1999). These stars are often (but not in every case) also enriched in neutron-capture s elements, suggesting an intrinsic post-AGB enrichment in the star, or mass transfer in a binary system in which one component is a former Asymptotic Giant Branch (AGB) star (McClure et al. 1980, McClure & Woodsworth 1990).

We have obtained high-resolution spectra for two such stars discovered in the HK survey, CS 22948-27 and CS 29497-34, at coordinates and (52.81^o, -87.67^o), J2000 () = (, ) and (, ), respectively. A preliminary analysis of the elemental abundances of these stars was given in Barbuy et al. (1997, hereafter Paper I).

In this paper we carry out a new analysis for these two stars, with the following main improvements relative to Paper I: (a) the spectral range of our high-resolution spectra was enlarged, in order to include the blue part of the spectrum as well as the red region. For many neutron-capture elements the most suitable lines for detailed abundance analysis lie in the blue. This has allowed us to increase the number of measurable neutron-capture elements to nine; (b) a new derivation of effective temperatures based on BVRI photometry obtained at the ESO 0.9 Dutch telescope was carried out, (c) a new grid of atmospheric models was used, with chemical compositions closer to the approximate elemental abundances for these stars, which are quite different from other non C- and N- enriched stars. New opacities and new models were computed for these peculiar chemical compositions, which have significant weight in molecular-line absorption due to C₂, CH and CN bands. (d) new high-resolution spectra were obtained in 1999 in order to check for possible radial velocity variations.

Finally, a new and important piece of information has been added. These two stars have measured proper motions from two independent sources. Both obtain a significant proper motion, excluding the possibility that these stars are distant, and thus very luminous.

In Sect. 2 the observations and reduction procedures are reported. In Sect. 3 the spectrum synthesis calculations and abundance determinations are described. In Sect. 4 the results are presented, and in Sect. 5 they are discussed and compared to other analyses of C-enriched stars. Finally, the concluding remarks are given in Sect. 6.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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