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

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tries in α Ori as well. Lobel and Dupree (2001) [62] model the velocity field surrounding
α Ori through spectroscopy. They predict a time-varying asymmetric velocity field containing
at times infalls of several km/s on one side and outflows on the other. Asymmetries in
the atmosphere of α Ori have also been observed by Harper et al. (2001) [39].
We can estimate the effect a clump of dust might have on our diameter measurements
by synthesizing visibility measurements from a model having an asymmetric dust
shell, and fitting a uniform disk to these data. For the case of a small clump of dust
(gaussian in shape with a FWHM of 15 mas) located at 2R ∗ to the side of the star and
contributing 15% of the stellar flux, the apparent diameter will be reduced by 2.2% to 4.1%
depending on the position angle of the visibility measurements. If, however, the dust clump
is located at 1R ∗ to the side, or equivalently, in front of the star and offset from the line of
sight by 1R ∗ , the apparent diameter will change by -2.6% to +4.1% depending on position
angle. Clumps of dust this severe would certainly be observable in the spectrum of the
star and would lead to stronger than observed non-periodicity in their infrared light curves.
Furthermore, the model from Lopez et al. (1997) [63] which fits 11 µm visibilities contained
an inner clump which contributed only 6% as much flux as the star; about 3 times less than
the model presented above. Hence, we expect any clumping present to be less than the
above estimates.
3.3 Spectral Lines
The low effective temperatures of AGB stars result in relatively cool circumstellar
material close to the star. In these regions, relatively high densities and cool temperatures
induce a variety of molecules to form which do not exist around stars like the sun. Some are
simple oxides and form in large quantities around O-rich stars, such as CO, TiO, SiO, and
H 2 O. These di- and polyatomic molecules often have “forests” of lines with large wavelengthaveraged
opacities that can significantly affect the emerging spectral energy distribution
(SED) from the star and cause dramatic changes in the apparent size of the star. The
redistribution of flux due to H 2 O and TiO is evident in Figure 3.5 which is a reproduction
of Figure 8 from Allard et al. (2000) [3]. The plot shows synthetic near-infrared spectra
calculated for a stellar atmosphere for four different values of its effective temperature. The
solid and dotted curves stem from using two different lists of spectral lines for the H 2 O
transitions. We see that for a 3500 K star, only small amounts of the molecules form, and