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 lm 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 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 also measured at wavelengths known to contain strong H2O spectral con- tamination, giving larger apparent stellar sizes and information on the distribution of hot H2O gas. Subject headings: infrared: stars — stars: AGB and post-AGB — stars: atmospheres — stars: individual (o Ceti) — techniques: interferometric Mira variables are extremely large and cool asymptotic giant branch stars that pulsate more or less regularly. Their visual brightness can vary by as much as 8 mag, with a period on the order of a year. Pulsationally induced shock waves, formed in the lower atmosphere, significantly affect the density and temperature structure of the star. The stellar '' surface '' becomes extended and obscured by dust and molecular opacity of the atmosphere. Measurements of the angular diameters of Mira variables provide a direct way to test and differentiate between the dynamical models constructed to understand Mira struc- ture. In particular, simultaneous measurements at multiple wavelengths probe different layers of the stellar atmosphere (because of the varying opacity), and measurements of the changes in size over the course of a cycle provide a direct measure of the pulsation occurring. Within the past two decades, an impressive quantity of stellar diameters have been measured. Published size meas- urements of o Cet are listed in Table 2. There are large dis- crepancies between some of these values at different wavelengths. Variations in the visible and near-infrared measurements of up to a factor of 2 are present. The wave- length-dependent size fluctuations evident in Table 2 illus- trate the importance of the interpretation of diameter measurements for large, extended asymptotic giant branch (AGB) stars such as Mira. These stars are not adequately described by compact atmosphere models. Still more impor- tant is the opacity arising from dust and molecular transi- tions in the atmosphere that can, at some wavelengths, induce large variations in the apparent size of the star. To further complicate matters, asymmetries (in some cases,