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

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Figure 3.22: Visibility Calculated from the Image in Figure 3.21 as a Function of the
Magnitude of the Spatial Frequency. The range of values corresponds to different position
angles of observation. Also plotted is the Visibility of the Uniform Stellar Disk without the
Hotspot
The visible sizes vary by roughly a factor of two and their lower limit is close to the near-
IR apparent sizes. The 11 µm ISI measurement for both o Cet and R Leo is about twice
the near-IR size, however there are significant variations in apparent size between 1 and
2.2 µm which were not seen in α Ori. That the relative disagreement in size is larger
for the Miras is not unexpected since they are believed to be cooler, less massive, and
dynamic, all of which tend to produce more extension in the stellar atmosphere. The lower
atmospheric temperatures of the Miras allows for the formation of molecules and dust in
larger quantities than for α Ori. The smaller surface gravity of the Miras causes the gas
density to drop off more slowly, and the pulsation in the interior of a Mira variable and
subsequent formation of shocks raises the density by many orders of magnitude in the outer
regions of the atmosphere. The result is that at wavelengths corresponding to molecular
transitions, the atmosphere can very easily become opaque at radii much larger than the
continuum photosphere. These molecular lines are likely the cause of the diameter variations