Clues to the Metallicity Distribution in the Galactic Bulge: Abundances in MOA-2008-BLG-310S and MOA-2008-BLG-311S

Abstract

We present abundance analyses based on high dispersion and high signal-to-noise ratio Magellan spectra of two highly microlensed Galactic bulge stars in the region of the main-sequence turnoff with T_(eff) ~ 5650 K. We find that MOA-2008-BLG-310S has [Fe/H]^8 = +0.41 ± 0.09 dex. We adopt the usual spectroscopic notations that [A/B] ≡log_(10)(N_A /N_B )^* – log_10(N_A /N_B )_⊙, and that log[є(A)] ≡ log_10(N_A /N_H ) + 12.00, for elements A and B. and MOA-2008-BLG-311S has +0.26 ± 0.09 dex. The abundance ratios for the ~20 elements for which features could be detected in the spectra of each of the two stars follow the trends with [Fe/H] found among samples of bulge giants. Combining these two bulge dwarfs with the results from previous abundance analysis of four other Galactic bulge turnoff region stars, all highly magnified by microlensing, gives a mean [Fe/H] of +0.29 dex. This implies that there is an inconsistency between the Fe-metallicity distribution of the microlensed bulge dwarfs and that derived by the many previous estimates based on surveys of cool, luminous bulge giants, which have mean [Fe/H] ~ –0.1 dex. A number of possible mechanisms for producing this difference are discussed. If one ascribes this inconsistency to systematic errors in the abundance analyses, we provide statistical arguments suggesting that a substantial systematic error in the Fe metallicity for one or both of the two cases, bulge dwarfs versus bulge giants, is required which is probably larger than can realistically be accommodated.