Interferometry on Mira in the Mid‐Infrared: Cyclic Variability of the Continuum Diameter and the Effect of Spectral Lines on Apparent Size

The size and variability of the continuum photosphere of o Ceti have been measured with 11 μm heterodyne interferometry to an accuracy of about 1%. Narrow bandwidths (~0.17 cm -1 ) were used to avoid spectral lines and measure continuum only. The resulting 11 μm diameter of o Cet is larger than the previously measured visible and near-infrared sizes. In addition, variations in the diameter with phase and a possible elongation have been observed. Visibilities were also measured at wavelengths known to contain strong H 2 O spectral contamination, giving larger apparent stellar sizes and information on the distribution of hot H 2 O gas.