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

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3.1 Observed Complications to a Straight-Forward Interpretation
of Measured Diameters
Within the last decade, departures from uniform disk intensity distributions have
been observed by several high spatial resolution instruments. For several giant stars, including
α Ori, o Cet, and R Leo, limb-darkening, hints of surface non-uniformity, asymmetries
and elongations, and time-like changes have been detected at different wavelengths. These
second order corrections to a uniform disk are often very wavelength dependent and could
account for much of the discrepancies between diameter measurements at different wavelengths.
A circularly symmetric departure from a uniform disk whereby the intensity falls
off towards the edge of the star is known as limb darkening. It occurs when outer, cooler
layers of the stellar atmosphere, which are thicker when viewed edge on around the circumference
of the star, absorb the hotter continuum photospheric emission. This is reflected
in the image as a more gradual decrease in brightness near the edge of the star as compared
to a uniform disk. Some limb darkening has been measured in α Ori at 830 nm
(Burns et al. (1997) [18]), o Cet at 700 nm (Wilson et al. (1992) [111]), α Boo between
450 and 800 nm (Quirrenbach et al. (1996) [82]), and α Ari and α Cas between 520 and
850 nm (Hajian et al. (1998) [36]). For the α Ori and o Cet measurements (Burns et al. and
Wilson et al. ,) a brightness profile having a gaussian-like tail allowed for a much improved
fit with experimental data.
There have been some recent measurements of varying surface non-uniformities
on α Ori. Burns et al. (1997) [18] report no asymmetries, and no surface features on
α Ori in 1995 contributing more than 4% of the total flux at 830 nm. However, Wilson
et al. (1992) [111] observed non-uniformities on α Ori at 546 and 710 nm in 1991 and
Wilson et al. (1997) [112] claimed that at least three bright spots were observed on α Ori
at 700 nm in 1994 totalling more than 20% of the flux. Moreover, the positions of these
bright spots appeared to change on a weekly time-scale.
Asymmetry and elongation in o Cet has also been observed. Karovska et al. (1991)
and Karovska et al. (1997) [56, 57] report asymmetries in the visible which vary with wavelength
and have axial ratios from 0.77 to 0.93. Wilson et al. (1992) [111] claimed elongation
in o Cet at 700 nm with an axial ratio of 0.85. Josselin et al. (2000) [55] observed asymmetries
at somewhat larger radii (∼ 200 mas) in the molecular gas envelope surrounding