The infrared angular diameter of omicron Ceti near maximum light

We present infrared angular diameter measurements of o Ceti, the prototype Mira star. The measurements were obtained near maximum light (Sept. 1990) with IRMA (the Infrared Michelson Array), a near-infrared, broad-band, two telescope interferometer. Here we report visibilities obtained with telescope separations of 8.0 and 12.0 m. The data were fit with both uniform disk and limb darkened models. For the time of our observations, we find that the uniform disk angular diameter of o Cet at 2.2 μm is 36.1 ± 1.4 milliarcsec. By using the limb darkening models of Scholz & Takeda [A&A, 186, 200 (1987)] and our angular diameter measurement, we have derived a stellar physical angular diameter of 43.3±1.7 milliarcsec. From this angular diameter and contemporaneous photometry we have further determined the effective temperature of o Cet during our observations to be 2270±140 K. This value agrees well with the infrared color temperature for Mira, but is considerably lower than T_eff indicated by molecular excitation temperatures.