Comparison of Stellar Angular Diameters from the NPOI, the Mark III Optical Interferometer, and the Infrared Flux Method

The Navy Prototype Optical Interferometer (NPOI) has been used to measure the angular diameters of 41 late-type giant and supergiant stars previously observed with the Mark III optical interferometer. Sixteen of these stars have published angular diameters based on model atmospheres (infrared flux method, IRFM). Comparison of these angular diameters shows that there are no systematic offsets between any pair of data sets. Furthermore, the reported uncertainties in the angular diameters measured using both interferometers are consistent with the distribution of the differences in the diameters. The distribution of diameter differences between the interferometric and model atmosphere angular diameters are consistent with uncertainties in the IRFM diameters of 1.4%. Although large differences in angular diameter measurements are seen for three stars, the data are insufficient to determine whether these differences are due to problems with the observations or are due to temporal changes in the stellar diameters themselves.