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. [ABRIDGED] 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. [ABRIDGED] 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 [alpha/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. [alpha/Fe]~0. The
decrease in the [alpha/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.
