Clues to the Metallicity Distribution in the Galactic Bulge: Abundances in OGLE-2007_BLG-349S

TL;DR

This study analyzes the high metallicity of a microlensed Galactic bulge dwarf star, suggesting the median metallicity in the bulge is higher than previously estimated, which impacts understanding of stellar populations.

Contribution

It provides detailed abundance measurements of a highly metal-rich bulge dwarf star, challenging prior estimates of the bulge's metallicity distribution.

Findings

Star has [Fe/H] = +0.51, indicating very high metallicity.

Abundance ratios are solar, with no alpha-element enhancement.

Implication that previous metallicity estimates of the bulge are underestimated.

Abstract

We present an abundance analysis based on high dispersion and high signal-to-noise ratio Keck spectra of a very highly microlensed Galactic bulge dwarf, OGLE-2007-BLG-349S, with Teff ~ 5400 K. The amplification at the time the spectra were taken ranged from 350 to 450. This bulge star is highly enhanced in metallicity with [Fe/H] = +0.51 \pm 0.09 dex. The abundance ratios for the 28 species of 26 elements for which features could be detected in the spectra are solar. In particular, there is no evidence for enhancement of any of the alpha-elements including O and Mg. We conclude that the high [Fe/H] seen in this star, when combined with the equally high [Fe/H] derived in previous detailed abundance analysis of two other Galactic bulge dwarfs, both also microlensed, implies that the median metallicity in the Galactic bulge is very high. We thus infer that many previous estimates of the metallicity distribution in the Galactic bulge have substantially underestimated the mean Fe-metallicity there due to sample bias, and suggest a candidate mechanism for such. If our conjecture proves valid, it may be necessary to update the calibrations for the algorithms used by many groups to interpret spectra and broad band photometry of the integrated light of very metal-rich old stellar populations, including luminous elliptical galaxies.