* Astronomy

Title: OGLE-2009-BLG-076S - The most metal-poor dwarf star in the Galactic bulge
Authors: T. Bensby (1), S. Feltzing (2), J.A. Johnson (3), A. Gal-Yam (4), A. Udalski (5) A. Gould (3), C. Han (6), D. Adén (2), J. Simmerer (2) ((1) European Southern Observatory, Chile, (2) Lund Observatory, Sweden, (3) Dept. of Astronomy, Ohio State Univ., USA, (4) Benoziyo Center for Astrophyics, Weizmann Institute of Science, Israel, (5) Warsaw University Observatory, Poland, (6) Department of Physics, Chungbuk National University, Republic of Korea)

Measurements based on a large number of red giant stars suggest a broad metallicity distribution function (MDF) for the Galactic bulge, centred on [Fe/H]=-0.1. However, recently, a new opportunity emerged to utilise temporary flux amplification (by factors of ~100 or more) of faint dwarf stars in the Bulge that are gravitationally lensed, making them observable with high-resolution spectrographs during a short observational window. Surprisingly, of the first 6 stars measured, 5 have [Fe/H]>+0.30, suggesting a highly skewed MDF, inconsistent with observations of giant stars. Here we present a detailed elemental abundance analysis of OGLE-2009-BLG-076S, based on a high-resolution spectrum obtained with the UVES spectrograph at the ESO Very Large Telescope. Our results indicate it is the most metal-poor dwarf star in the Bulge yet observed, with [Fe/H]=-0.76. Our results argue against a strong selection effect disfavouring metal-poor microlensed stars. It is possible that small number statistics is responsible for the giant/dwarf Bulge MDF discrepancy. Should this discrepancy survive when larger numbers of Bulge dwarf stars (soon to be available) are analysed, it may require modification of our understanding of either Bulge formation models, or the behaviour of metal-rich giant stars.

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