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Detection of Lithium in a Main-Sequence Bulge Star Using Keck I as a 15 Meter Diameter Telescope

The bulge contains the most chemically evolved old stellar population in the Milky Way. Thanks to microlensing, it is now possible to obtain high-resolution echelle spectra of bulge stars near the main-sequence turnoff and study the abundance of elements that are affected by stellar evolution, such as lithium. We have observed with the HIRES spectrograph on the Keck I 10 m telescope the source star of the MACHO microlensing event 97 BLG 45, while it was magnified by ~1 mag. Here we report the detection of the Li I line at 6707.8 Å in the echelle spectrum of this star and measure the bulge lithium abundance for the first time: A(Li)=2.25±0.25.