Limb Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97‐BLG‐28

We present the PLANET photometric dataset for the binary-lens microlensingevent MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyzeit to determine the radial surface brightness profile of the Galactic bulgesource star. The microlensed source, demonstrated to be a K giant by ourindependent spectroscopy, crossed the central isolated cusp of the lensingbinary, generating a sharp peak in the light curve that was well-resolved bydense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile,South Africa, and Australia. Our modeling of these data has produced stellarprofiles for the source star in the I and V bands that are in excellentagreement with those predicted by stellar atmospheric models for K giants. Thelimb-darkening coefficients presented here are the first derived frommicrolensing, among the first for normal giants by any technique, and the firstfor any star as distant as the Galactic bulge. Modeling indicates that thelensing binary has a mass ratio q = 0.23 and an (instantaneous) separation inunits of the angular Einstein ring radius of d = 0.69 . For a lens in theGalactic bulge, this corresponds to a typical stellar binary with a projectedseparation between 1 and 2 AU. If the lens lies closer, the separation issmaller, and one or both of the lens objects is in the brown dwarf regime.Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-sourceproper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulgelens. If the non-lensed blended light is due to a single star, it is likely tobe a young white dwarf in the bulge, consistent with the blended light comingfrom the lens itself.