We have derived abundances of 33 elements and upper limits for 6 additional elements for the metal-poor ([Fe/H] = −2.42) turn-off star HE 0338−3945 from high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ...). It is also heavily enriched in Eu, which is generally considered an r-element, and in other similar elements. It is less enriched, by about a factor of 10, in the lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesser degree, N and O. These abundance estimates are subject to severe uncertainties due to NLTE and thermal inhomogeneities which are not taken into detailed consideration. However, an interesting result, which is most probably robust in spite of these uncertainties, emerges: the abundances derived for this star are very similar to those of other stars with an overall enhancement of all elements beyond the iron peak. We have defined criteria for this class of stars, r + s stars, and discuss nine different scenarios to explain their origin. None of these explanations is found to be entirely convincing. The most plausible hypotheses involve a binary system in which the primary component goes through its giant branch and asymptotic giant branch phases and produces CNO and s-elements which are dumped onto the observed star. Whether the r-element Eu is produced by supernovae before the star was formed (perhaps triggering the formation of a low-mass binary), by a companion as it explodes as a supernova (possibly triggered by mass transfer), or whether it is possibly produced in a high-neutron-density version of the s-process is still unclear. Several suggestions are made on how to clarify this situation.Key words. stars: population ii -stars: fundamental parameters -stars: abundances -Galaxy: halo -Galaxy: abundances -Galaxy: evolution IntroductionElements with atomic numbers Z > 30 are believed to be almost exclusively synthesized in neutron-capture (n-capture) reactions. In the most metal-poor stars the overall abundance of these elements varies from star to star, by more than a factor of 100 at a given metallicity (McWilliam et al. 1995;Ryan et al. 1996). Also, the different abundance ratios vary, e.g. the Ba/Eu ratio tends to decline with decreasing [Fe/H]

Abstract.We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood, most of them subgiants or turn-off-point stars with iron abundances [Fe/H] ranging from −0.4 to −3.0. About half of this sample has not been spectroscopically analysed in detail before. Effective temperatures were estimated from uvby photometry, and surface gravities primarily from Hipparcos parallaxes. The analysis is differential relative to the Sun, and was carried out with plane-parallel MARCS models. Various sources of error are discussed and found to contribute a total error of about 0.1-0.2 dex for most elements, while relative abundances, such as [Ca/Fe], are most probably more accurate. For the oxygen abundances, determined in an NLTE analysis of the 7774 Å triplet lines, the errors may be somewhat larger. We made a detailed comparison with similar studies and traced the reasons for the, in most cases, relatively small differences. Among the results we find that [O/Fe] possibly increases beyond [Fe/H] = -1.0, though considerably less so than in results obtained by others from abundances based on OH lines. We did not trace any tendency toward strong overionization of iron, and find the excesses, relative to Fe and the Sun, of the α elements Mg, Si, and Ca to be smaller than those of O. We discuss some indications that also the abundances of different α elements relative to Fe vary and the possibility that some of the scatter around the trends in abundances relative to iron may be real. This may support the idea that the formation of Halo stars occurred in smaller systems with different star formation rates. We verify the finding by Gratton et al. (2003b, A&A, 406, 131) that stars that do not participate in the rotation of the galactic disk show a lower mean and larger spread in [α/Fe] than stars participating in the general rotation. The latter stars also seem to show some correlation between [α/Fe] and rotation speed. We trace some stars with peculiar abundances, among these two Ba stars, HD 17072 and HD 196944, the second already known to be rich in s elements. Finally we advocate that a spectroscopic study of a larger sample of halo stars with well-defined selection criteria is very important, in order to add to the very considerable efforts that various groups have already made.

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni and Ba for 43 metal-poor field stars, mostly stars at the turn-off point and on the subgiant branch, in the interval −3.0 < [Fe/H] < −0.4. The analysis is differential relative to the Sun. Oxygen abundances, with consideration of NLTE effects, were derived from the OI 777.4 nm triplet lines. We find [O/Fe] to gradually increase with decreasing [Fe/H], though considerably slower than has earlier been obtained from OH lines in the UV. A scatter in [O,Mg,Ca,Ti/Fe] at a given [Fe/H] is found and we argue that this scatter is partly real. The deviations from the mean trends of abundance ratios vs [Fe/H] are found to correlate in non-trivial ways for different abundances. Similar trends are found from results of accurate studies by other groups. This seems to give further evidence for the hypothesis that the stars once formed in different subsystems, with different star-formation rates.The paper is in press in A&A, and may be obtained as astro-ph/0505118.