  Context: In order to study the evolution of Li in the Galaxy it is necessary
to observe dwarf or subgiant stars. Abridged Although Li has been extensively
studied in the Galactic disk and halo, to date there is only one uncertain
detection of Li in an unevolved bulge star. AIMS: Our aim with this study is to
provide the first clear detection of Li in the Galactic bulge, based on an
analysis of a dwarf star that has largely retained its initial Li abundance.
METHODS: We have performed a detailed elemental abundance analysis of the bulge
dwarf star MOA-2010-BLG-285S using a high-resolution, and high signal-to-noise
spectrum obtained with the UVES spectrograph at the VLT when the object was
optically magnified during a gravitational microlensing event (visual
magnification A~550 during observation). The lithium abundance was determined
through synthetic line profile fitting of the 7Li resonance doublet line at
670.8 nm. The results have been corrected for departures from LTE. Results:
MOA-2010-BLG-285S is, at [Fe/H]=-1.23, the most metal-poor dwarf star detected
so far in the Galactic bulge. Its old age (12.5 Gyr) and enhanced [alpha/Fe]
ratios agree well with stars in the thick disk at similar metallicity. This
star represents the first unambiguous detection of Li in a metal-poor dwarf
star in the Galactic bulge. We find an NLTE corrected Li abundance of log
e(Li)=2.16, which is consistent with values derived for Galactic disk and halo
dwarf stars at similar metallicities and temperatures. Conclusion: Our results
show that there are no signs of Li enrichment or production in the Galactic
bulge during its earliest phases. Observations of Li in other galaxies (omega
Cen) and other components of the Galaxy suggest further that the Spite plateau
is universal.
