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

TL;DR

This study analyzes microlensed dwarf and subgiant stars in the Galactic bulge to assess their elemental abundances, revealing insights into the bulge's chemical evolution and supporting a secular formation scenario.

Contribution

It provides a homogeneous detailed abundance analysis of microlensed dwarf stars, comparing their chemical signatures to giant stars and thick disc stars, offering new evidence on bulge formation.

Findings

OGLE-2008-BLG-209S is a subgiant with [Fe/H] ~ -0.33 and alpha enhancements similar to bulge giants.

Previous microlensed dwarfs show higher metallicities ([Fe/H]>+0.4) with solar-like abundance ratios.

Results support a secular formation scenario for the Galactic bulge.

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. [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.