OGLE-2009-BLG-076S - The most metal-poor dwarf star in the Galactic bulge

TL;DR

This study reports the discovery and detailed analysis of OGLE-2009-BLG-076S, the most metal-poor dwarf star in the Galactic bulge, challenging previous assumptions about bulge metallicity distribution and formation models.

Contribution

It provides the first detailed elemental abundance analysis of a very metal-poor dwarf star in the Galactic bulge, highlighting potential biases in previous metallicity measurements.

Findings

OGLE-2009-BLG-076S has [Fe/H]=-0.76, the most metal-poor bulge dwarf observed.

Results suggest possible biases in previous bulge metallicity distribution measurements.

Findings may impact models of bulge formation and evolution.

Abstract

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.