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 oCeti have beenmeasured with 11 lmheterodyne interferometry to an accuracy of about 1%.Narrowbandwidths ( 0.17 cm 1) were used to avoid spectral lines and measure continuum only. The resulting 11 lm 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 alsomeasured atwavelengths known to contain strongH2O spectral contamination, giving larger apparent stellar sizes and information on the distribution of hotH2O gas. Subject headings: infrared: stars — stars: AGB and post-AGB — stars: atmospheres — stars: individual (oCeti) — techniques: interferometric