Precision measurements of the diameters of alpha Orionis and o Ceti at 11 microns

The angular diameters of alpha Orionis and o Ceti were measured at a wavele ngth of 11.15 mum using the two-telescope Infrared Spatial Interferometer ( ISI). Based on fitting the visibility data to uniform disk models, the diam eter of alpha Orionis is 54.7 +/- 0.3 mas and that of o Ceti at phase 0.90 is 47.8 +/- 0.5 mas. These diameters are the most precise ever measured in the mid-infrared, due in part to the addition of a 56 m baseline to the ISI , which provided sufficient resolution to observe the first zero in the vis ibility function of the stellar disks. Moreover, the effects of limb darken ing and stellar hot spots are small at these wavelengths. Theoretically est imated limb-darkening effects indicate the actual diameters are approximate ly 1% +/- 0.5% larger than the uniform disk approximation, or 55.2 +/- 0.5 and 48.2 +/- 0.6 mas for alpha Orionis and o Ceti, respectively.