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Published June 1, 2009 | Published
Journal Article Open

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S


Aims. Our aims are twofold. First we aim to evaluate the robustness and accuracy of stellar parameters and detailed elemental abundances that can be derived from high-resolution spectroscopic observations of microlensed dwarf and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to investigate the abundance structure and chemical evolution of the Milky Way Bulge. Contrary to the cool giant stars, with their extremely crowded spectra, the dwarf stars are hotter, their spectra are cleaner, and the elemental abundances of the atmospheres of dwarf and subgiant stars are largely untouched by the internal nuclear processes of the star. Methods. We present a detailed elemental abundance analysis of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the Bulge, for which we obtained a high-resolution spectrum with the MIKE spectrograph on the Magellan Clay telescope. We have performed four different analyses of OGLE-2008-BLG-209S. One method is identical to the one used for a large comparison sample of F and G dwarf stars, mainly thin and thick disc stars, in the Solar neighbourhood. We have also re-analysed three previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and OGLE-2007-BLG-349S with the same method. This homogeneous data set, although small, enables a direct comparison between the different stellar populations. Results. We find that OGLE-2008-BLG-209S is a subgiant star that has a metallicity of [Fe/H] ≈ -0.33. It possesses [α/Fe] enhancements similar to what is found for Bulge giant stars at the same metallicity, and what also is found for nearby thick disc stars at the same metallicity. In contrast, the previous three microlensing dwarf stars have very high metallicities, [Fe/H] ≳ +0.4, and more solar-like abundance ratios, i.e. [α/Fe] ≈ 0. The decrease in the [α/Fe] ratio with [Fe/H] is the typical signature of enrichment from low and intermediate mass stars. We furthermore find that the results for the four Bulge stars, in combination with results from studies of giant stars in the Bulge, seem to favour a secular formation scenario for the Bulge.

Additional Information

© 2009 ESO. Received 8 January 2009; accepted 17 March 2009. We would like to thank Bengt Gustafsson, Martin Asplund, Bengt Edvardsson, and Kjell Eriksson for usage of the MARCS model atmosphere program and their suite of stellar abundance (EQWIDTH) programs. Paul Barklem is also thanked for helping us to enlarge the grid of stellar model atmospheres. S.F. is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation. Work by A.G. was supported by NSF Grant AST-0757888. J.C. and W.H. are grateful to NSF grant AST-0507219 for partial support. A.U. acknowledges support by the Polish MNiSW grant N20303032/4275. J.S. is supported by a Marie Curie Incoming International Fellowship. D.A. and S.F. thank the Swedish Research Council for a dedicated travel grant that enabled D.A. to travel to Las Campanas for this observing run.

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