Predicting Stellar Angular Diameters from V, I_C, H, K Photometry

Determining the physical properties of microlensing events depends on having accurate angular sizes of the source star. Using long-baseline optical interferometry we are able to measure the angular sizes of nearby stars with uncertainties $\leq 2\%$. We present empirically derived relations of angular diameters that are calibrated using both a sample of dwarfs/subgiants and a sample of giant stars. These relations are functions of five color indices in the visible and near-infrared, and have uncertainties of 1.8-6.5% depending on the color used. We find that a combined sample of both main-sequence and evolved stars of A-K spectral types is well fit by a single relation for each color considered. We find that in the colors considered, metallicity does not play a statistically significant role in predicting stellar size, leading to a means of predicting observed sizes of stars from color alone.