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We present two Keck HIRES spectra (R=40,000) of a bulge K3 giant taken onsuccessive nights during the second caustic crossing of the binary microlensingevent EROS BLG-2000-5. This caustic crossing served to effectively resolve thesurface of the source star: the spectrum from the second night is dominated bythe limb, while the spectrum from the first night is comprised of light from abroader range of radii. To demonstrate that the spectra are adequate to resolvethe differences between them, we analyze the H-alpha line. The equivalent widthis ~8% smaller on the second night, and the signal-to-noise ratio perresolution element (165 and 75 on the two nights respectively) is sufficient toshow that the difference is approximately constant over the ~2A (~15 resolutionelement) extent of the line. The sign of the difference is in the expecteddirection since the limb is the coolest part of the star and therefore shouldhave the weakest H-alpha. We invite atmosphere modelers to predict thedifference spectrum from the entire spectral range 5500<lambda<7900A so thatthese predictions can be compared to our observations.Comment: 12 pages, 3 figures, Accepted by ApJ Letters 2001 Jan

Spatial/Spectral Resolution of a Galactic Bulge K3 Giant Stellar Atmosphere via Gravitational Microlensing