Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

Abstract

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

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…From the analysis of OGLE-2008-BLG-209S, which was the first microlensed subgiant star with a metallicity below solar, we got the first hint from a chemically un-evolved star that the metal-poor parts of the bulge might be similar to those of the Galactic thick disk (Bensby et al 2009b). In Bensby et al (2010c) where the sample was expanded to 15 microlensed dwarf and subgiant stars, out of which 7 had [Fe/H] < 0, it was clear that the metal-poor bulge, in terms of both elemental abundance ratios as well as stellar ages, is similar to the Galactic thick disk.…”

“…Stellar parameters and elemental abundances have been determined through exactly the same methods as in our previous studies of microlensed bulge dwarfs (Bensby et al 2009b(Bensby et al , 2010c (Gustafsson et al 1975;Edvardsson et al 1993;Asplund et al 1997). (For consistency with our previous analyses, we continue to use the old MARCS models.…”

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…The determination of stellar parameters and calculation of elemental abundances were carried out as described in method 1 of Bensby et al (2009b). Briefly, this method is based on equivalent width measurements (W λ ) and one-dimensional LTE model stellar atmospheres calculated with the Uppsala MARCS code (Gustafsson et al 1975;Edvardsson et al 1993;Asplund et al 1997).…”

“…We also re-analyse the events previously studied by Cavallo et al (2003); Cohen et al (2009) and Epstein et al (2010). Combining these data with the results from Bensby et al (2009a) and Bensby et al (2009b) (which includes a re-analysis of the events from Johnson et al 2007Johnson et al , 2008Cohen et al 2008) we now have a sample of 15 microlensed dwarf stars in the Bulge that have been homogeneously analysed using the exact same methods. -1999-BLG-022S that has data from from both OGLE (circles) and binned MACHO data (crosses).…”

“…However, comparisons have been hampered because of the small number of microlensed stars that did not always cover the [Fe/H] range of interest. Also, individual age estimates were provided for stars near the turnoff subgiant branch, including some stars that could be younger than the canonical old Bulge population Bensby et al 2009b). …”