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

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Figure 4.3: A uniform intensity elliptical star will reproduce the observed 11.1±1.4% relative
size change only if its axial ratio and position angle (relative to the 2001 baseline) are within
the dashed lines shown above.
so well-correlated with switching baseline direction. No significant change was observed
between 1999 and 2000 on a single baseline even though the measurements were separated
in time by a year. Apparent asymmetries in Miras, including o Cet, have already
been observed (see Section 3.1). Elongation having an axial ratio as low as 0.77 in o Cet
(Karovska et al. (1997) [57]) in the visible and as low as 0.7 for R Tri (a Mira) in the nearinfrared
(Thompson et al. (2002) [97]) has been reported. A uniform intensity elliptical
stellar disk with an axial ratio less than 0.83 could be responsible for the observed increase
in diameter between the 1999/2000 and 2001 baselines. The possible elliptical models of
o Cet which would reproduce the 11.1% size change are illustrated in Figure 4.3. The axial
ratio and position angle of the ellipse are constrained to lie between the dashed lines in
order for an elongation to cause the observed change in apparent diameter. Elongation in
Mira variables could be due to the excitement of a non-radial mode of pulsation or possibly