Direct multi-wavelength limb-darkening measurements of three late-type giants with the Navy Prototype Optical Interferometer

We present direct measurements of the limb-darkened intensity profiles of t he late-type giant stars HR5299, HR7635, and HR8621 obtained with the Navy Prototype Optical Interferometer (NPOI) at the Lowell Observatory. A triang le of baselines with lengths of 18.9 m, 22.2 m, and 37.5 m was used. We uti lized squared visibility amplitudes beyond the first minimum, as well as tr iple amplitudes and phases in up to 10 spectral channels covering a wavelen gth range of similar to 650 nm to similar to 850 nm. We find that our data can best be described by featureless symmetric limb-darkened disk models wh ile uniform disk and fully darkened disk models can be rejected. We derive high-precision angular limb-darkened diameters for the three stars of 7.44 mas +/- 0.11 mas, 6.18 mas +/- 0.07 mas, and 6.94 mas +/- 0.12 mas, respect ively. Using the HIPPARCOS parallaxes, we determine linear limb-darkened ra dii of 114 R. +/- 13 R., 56 R. +/- 4 R., and 98 R. +/- 9 R., respectively. We compare our data to a grid of Kurucz stellar model atmospheres, with the m derive the effective temperatures and surface gravities without additiona l information, and find agreement with independent estimates derived from e mpirical calibrations and bolometric fluxes. This confirms the consistency of model predictions and direct observations of the limb-darkening effect.