Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

Abstract

Lithium abundances are presented for 91 dwarf and subgiant stars in the Galactic bulge. The analysis is based on line synthesis of the 7Li line at 6707 Å in high-resolution spectra obtained during gravitational microlensing events, when the brightnesses of the targets were highly magnified. Our main finding is that bulge stars at sub-solar metallicities that are older than about eight billion years do not show any sign of Li production; that is, the Li trend with metallicity is flat or even slightly declining. This indicates that no lithium was produced during the first few billion years in the history of the bulge. This finding is essentially identical to what is seen for the (old) thick disk stars in the solar neighbourhood, and adds another piece of evidence for a tight connection between the metal-poor bulge and the Galactic thick disk. For the bulge stars younger than about eight billion years, the sample contains a group of stars at very high metallicities at [Fe/H] ≈ +0.4 that have lithium abundances in the range A(Li) = 2.6 − 2.8. In the solar neighbourhood the lithium abundances have been found to peak at A(Li) ≈ 3.3 at [Fe/H] ≈ +0.1 and then decrease by 0.4–0.5 dex when reaching [Fe/H] ≈ +0.4. The few bulge stars that we have at these metallicities seem to support this declining A(Li) trend. This could indeed support the recent claim that the low A(Li) abundances at the highest metallicities seen in the solar neighbourhood could be due to stars from the inner disk, or the bulge region, that have migrated to the solar neighbourhood.