Abundances in metal-rich stars - Detailed abundance analysis of 47 G and K dwarf stars with [Me/H] >> 0.10 dex

We have derived elemental abundances of O, Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni for 47 G and K dwarf, with [Me/H]>0.1 dex. The selection of stars was based on their kinematics as well as on their uvby-photometry. One sample of stars on rather eccentric orbits traces the chemical evolution interior to the solar orbit and another, on circular orbits, the evolution around the solar orbit. A few Extreme Pop I stars were included in the latter sample. The stars have -0.1<Fe/H]<0.42 dex. We find that the elemental abundances of Mg, Al, Si, Ca, Ti, Cr and Ni all follow [Fe/H]. Our data put further constraints on models of galactic chemical evolution, in particular of Cr, Mn and Co which has not previously been studied for dwarf stars with [Me/H]>0.1 dex. The increase in [Na/Fe] and [Al/Fe] as a function of [Fe/H] found previously by Edvardsson et al. (1993a) has been confirmed for [Na/Fe]. This upturning relation, and the scatter around it, are shown not to be due to a mixture of populations with different mean distances to the galactic centre. We have not been able to trace any tendency for relative abundances of O, Si, and Ti relative to Fe to vary with the stellar velocities, i.e. the stars present mean distance to the galactic centre. These results imply that there is no significant difference in the chemical evolution of the different stellar populations for stars with [Me/H]>0.1 dex. SMR stars on highly eccentric orbits, alleged to trace the evolution of the chemical evolution in the galactic Bulge, have previously been found overabundant in O, Mg and Si. We have included 3 such stars from the study by Barbuy & Grenon (1990). We find them to be less metal rich and the other elemental abundances remain puzzling. Our study includes 5 K dwarf stars and reveal a striking example of overionization.Comment: Accepted for publication in Astronomy and Astrophysics Supplement Series. RGO preprint No. 28

The case for high precision in elemental abundances of stars in the era of large spectroscopic surveys

A number of large spectroscopic surveys of stars in the Milky Way are under way or are being planned. In this context it is important to discuss the extent to which elemental abundances can be used as discriminators between different (known and unknown) stellar populations in the Milky Way. We aim to establish the requirements in terms of precision in elemental abundances, as derived from spectroscopic surveys of the Milky Way's stellar populations, in order to detect interesting substructures in elemental abundance space. We present a simple relation between the minimum number of stars needed to detect a given substructure and the precision of the measurements. The results are in agreement with recent small- and large-scale studies, with high and low precision, respectively. Large-number statistics cannot fully compensate for low precision in the abundance measurements and each survey should carefully evaluate what the main science drivers are for the survey and ensure that the chosen observational strategy will result in the precision necessary to answer the questions posed.Comment: 6 pages, 6 figures. Accepted for publication in Astronomy & Astrophysic

Carbon Abundances in the Galactic Thin and Thick Disks

Although carbon is, together with oxygen and nitrogen, one of the most important elements in the study of galactic chemical evolution its production sites are still poorly known and have been much debated (see e.g. Gustafsson et al. 1999; Chiappini et al. 2003). To trace the origin and evolution of carbon we have determined carbon abundances from the forbidden [C I] line at 8727 A and made comparisons to oxygen abundances from the forbidden [O I] line at 6300 A in a sample of 51 nearby F and G dwarf stars. These data and the fact that the forbidden [C I] and [O I] lines are very robust abundance indicators (they are essentially insensitive to deviations from LTE and uncertainties in the stellar parameters, see, e.g., Gustafsson et al. 1999; Asplund et al. 2005) enable us to very accurately measure the C/O ratio as well as individual C and O abundances. Our first results indicate that the time-scale for the main source that contribute to the carbon enrichment of the interstellar medium operate on the same time-scale as those that contribute to the iron enrichment (and can possibly be AGB stars...)Comment: 2 pages, to appear in Proceedings IAUS Symposium 228, From Li to U: Elemental Tracers of Early Cosmic Evolution (Paris, May 23- 27, 2005), eds. V. Hill, P. Francois, and F. Prima

Oxygen in the Galactic thin and thick disks

First results from a study into the abundance trends of oxygen in the Galactic thin and thick disks are presented. Oxygen abundances for 21 thick disk and 42 thin disk F and G dwarf stars based on very high resolution spectra (R\sim 215000) and high signal-to-noise (S/N>400) of the faint forbidden oxygen line at 6300 A have been determined. We find that [O/Fe] for the thick disk stars show a turn-down, i.e. the ``knee'', at [Fe/H] between -0.4 and -0.3 dex indicating the onset of SNe type Ia. The thin disk stars on the other hand show a shallow decrease going from [Fe/H] \sim -0.7 to the highest metallicities with no apparent ``knee'' present indicating a slower star formation history.Comment: To be published in "CNO in the Universe", ASP Conference Series, C. Charbonnel, D. Schaerer & G. Meynet (eds.

Stellar Abundances in Giant Stars in the Metal-Rich Globular Cluster NGC6528

We present the first results of a detailed abundance analysis, based on VLT observations, of giant stars in the very metal-rich globular cluster NGC 6528. We will be able to tie the horizontal branch abundances (see e.g. Carretta et al. 2001) to those of the more luminous giants (see CMDs). For the very similar cluster NGC 6553 studies of different types of stars have yielded very disparate results. Our first analysis of three of our stars seem to indicate that indeed the different sorts of stars do show similar abundances if one homogeneous set of models and parameters are being used.Comment: 2 pages, 2 diagrams, to be published in the proceedings of "New Horizons in Globular Cluster Astronomy" held in Padua 2002, eds. Piotto, Meylan, Djorgovsk