The Size, Structure, and Variability of Late-Type Stars Measured ...

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Figure 4.2: Variation of the Diameter of α Ori with Date
a time-like change in the star, or a space-like asymmetry in the star which was manifested
as a size change due to the different position angle of the 2000 and 2001 baselines.
The non-repeatability of Miras’ light curves and spectral line shapes at similar phases in
succesive cycles is well-established and Bessell et al. (1996) [10] argue that non-periodic
radii variations are also likely in Miras. Visible diameters of o Cet were measured by
Tuthill et al. (1995) [102] and changes as large as 70% in the TiO bands and 9% in the
pseudo-continuum bands were seen which did not correspond to the 332 d periodic pulsation.
It is possible that the 11% change in the 11 µm size of o Cet between 2000 and
2001 is due to a non-regular photospheric variation or changing features on the surface of
the stellar disk such as a hotspot or convective cell. However, the more likely explanation
for the observed size change is a lack of spherical symmetry because the change is