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

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wavelengths which would cause the diameter measurement to be larger than in the continuum.
It should be noted that other mira atmospheric models in Jacob et al. (2000) [50]
result in characteristic increases in apparent size in all of the TiO bands modelled. The
possibility of spectral opacity causing increases or decreases in the apparent size of the star
is also exhibited by the simple thin shell models considered in Section 5.2.
The importance of unbiased continuum diameters makes avoiding spectral lines in
stellar size measurements quite valuable. This can be accomplished by choosing spectral
bands which do not contain significant absorption. The use of narrow bandpasses facilitates
extracting continuum diameter measurements provided that a high resolution spectrum can
be used to choose an observing region free of spectral contamination. Wide bandwidths, on
the other hand, effectively average over wavelength and are incapable of separating out the
effect of lines which may be present.
The ISI uses an assortment of narrow (∼0.17 cm −1 ) bandpasses and through the
use of high resolution spectra obtained with the IRTF telescope can select line-free observing
regions in which to make continuum diameter measurements. The detailed analysis of the
11 µm spectra and the possible effects of spectral contamination on ISI observations are
discussed in Sections 5.1 and 5.2. Considerations based on known H 2 O spectral lines within
the chosen ISI observing bandpass imply only a small correction to our measured diameters
of between 0.0% and +0.2%. The effect of spectral contamination due to gases other than
H 2 O is more uncertain, but should be no more than 1% for the 11.149 µm bandpass for any
of the stars.
3.4 Analysis of AGB Atmospheres Including Continuum
Opacity
In order to fully understand the variety of interferometric measurements obtained
for AGB stars, it is necessary to construct a model complete enough to explain the observations.
Such a model should contain adequate descriptions of the dust formation and
molecular line opacities as it was shown previously that these result in important modifications
to the intensity distribution emerging from the star at some wavelengths. However,
there are limits to treating the photosphere itself as a spherical blackbody. A physical star
has no discontinuity in its density as a function of radius. Rather, the density and temper-