OGLE-2009-BLG-076S - The most metal-poor dwarf star in the Galactic
  bulge
  Measurements based on a large number of red giant stars suggest a broad
metallicity distribution function (MDF) for the Galactic bulge, centered on
[Fe/H]=-0.1. However, recently, a new opportunity emerged to utilize 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 disfavoring 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 analyzed, it may require modification of our
understanding of either Bulge formation models, or the behavior of metal-rich
giant stars.
