Chemical abundances in 43 metal-poor stars

Abstract

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.

Chemical enrichment and physical conditions in I Zw 18

“…The Ni/Fe ratio in the neutral gas of I Zw 18 is solar, with [Ni/Fe] = −0.05 ± 0.23. We note that [Ni/Fe] remains essentially null over a wide range of metallicities in nearby stars (also in DLAs; Izotov et al 2001b) while at the same time [α/Fe] increases at low-metallicity (Jonsell et al 2005). This implies that Ni and Fe evolve in parallel, and it is therefore not surprising that we also find a solar ratio in I Zw 18.…”

“…It is thus likely that the enrichment from SNe Ia, if any, is identical in the H  phase and in the H  phase. This value of [O S /Fe] lies between 0 (solar O/Fe ratio) and the value in Galactic halo stars (Barbuy 1988) and in metal-poor stars in the solar neighborhood (Jonsell et al 2005), suggesting an early enrichment by high-mass stars (see Izotov & Thuan 1999).…”

Deep SDSS optical spectroscopy of distant halo stars

“…Fulbright (2002), for example, found a correlation between the observed space velocities of a relatively small sample of stars (with metallicities in the range −2.0 ≤ [Fe/H] < −1.0) and their [α/Fe]; stars with lower [α/Fe] were argued to be associated with faster space motions. Gratton et al (2003) reported that the stars in their sample with substantial prograde rotation about the Galactic center exhibited higher [α/Fe] than those with small or retrograde rotation, as confirmed later by Jonsell et al (2005). Ishigaki et al (2010) found that in the metallicity range −2 < [Fe/H] < −1, stars on orbits reaching a maximum distance from the Galactic plane |Z| > 5 kpc possess [Mg/Fe] ∼ 0.1 dex lower than those that only reach |Z| < 5 kpc.…”

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?

“…Stellar abundance data were compiled from several different studies. For the solar neighbourhood, the data were compiled from several authors (Bensby & Feltzing 2006;Ecuvillon et al 2004;Fabbian et al 2009a;Gratton et al 2000;Gustafsson et al 1999, Jonsell et al 2005, Spite et al 2005, while the Cepheid abundances in Fig. 7 were taken from the works by Andrievski et al (2002aAndrievski et al ( ,b,c, 2004 and Luck et al (2003Luck et al ( , 2006.…”