Abundances of four very metal-poor stars of the BPS survey

Abstract

High resolution CASPEC and EMMI spectra (R$\approx$10$^4$) of the faint metal- deficient candidates CS 22891--209, CS 22897--8, CS 22948--66 and CS 22968--14 taken from the survey of Beers, Preston and Shectman (1985, BPS) are analyzed for abundances. The fine analysis with LTE model atmospheres gives an iron metallicity for the four stars between $-$3.15 and $-$3.45 in good agreement with the abundances inferred from CaII K index by Beers et al. (1992). These four objects considerably augment the number of spectroscopically studied stars at the low metallicity extreme. We found the $α$-elements Mg, Ca, Si, and Ti enhanced, and the iron-peak elements Sc, Mn and Cr tracking closely iron with the typical pattern of the more metallic halo stars. Aluminum results deficient with respect both to iron and to magnesium showing a strong {\it odd-even\/} effect, but for [Fe/H] $\leq$ $-$3.0 [Al/Mg] levels out into a plateau around $-$1.0, showing a primary behaviour. [Na/Fe] is $\approx$ $-$0.2 and only compared to magnesium it shows a mild odd-even effect. The heavy elements Sr and Ba show a large scatter which is not observed for other elements and cannot be due to observational errors only. In the case of CS 22891--209 they are particularly overabundant with [Sr,Ba/Fe] $\approx$ 0.9. The spread might reflect the presence of strong inhomogeneities in the interstellar gas at the earliest stages of Galaxy formation. This might be an important signature of the still elusive processes of the early formation of these elements observed in old stars.