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

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Mira.
Finally, variations in time of stellar features have been seen in several stars from
repeated measurements using a single instrument. Gray (2000) [35] reports non-regular
changes occuring in the spectrum of α Ori on time scales ranging from days to years.
Changes in the brightness distribution of α Ori over a period of 22 months have been observed
by Wilson et al. (1992) [111]. Karovska et al. (1991) [56] have seen changes in the size
and asymmetry in o Cet over the course of a year. A phase-related change in size by 35% of
some of the atmospheric layers of R Leo was reported by Burns et al. (1998) [19]. In contrast,
radio measurements of flux density for o Cet and R Leo by Reid and Menten (1997) [84]
show no variability in the “radio photospheres” of these stars (around 2R ∗ ) indicating that
outwardly propagating pulsational disturbances must be damped by some mechanism before
reaching 2R ∗ .
3.2 Dust
The cool temperatures and relatively high densities in the circumstellar environment
of these AGB stars can lead to the precipitation of small solids. These sub-micron
sized particles made up of carbon or oxides are collectively known as dust. The carbon
monoxide molecule, having a strong binding energy, is readily formed in the atmospheres
of stars. The formation of this molecule occurs until almost all of the limiting reactant,
carbon or oxygen, is bound. The result is that stars with a slight carbon overabundance
end up with no free oxygen in the cool regions of its atmosphere, and its dust is made up
entirely of amorphous carbon particles and graphites. Conversely, O-rich stars, such as all
of the stars we’re considering, are surrounded by dust shells made up of oxides of Si, Mg,
Ti, Al, S, Fe, etc.
The optical properties of the dust are somewhat dependent on the chemistry and
distribution of grain sizes. Measurements of laboratory-formed dust have been performed
and compared with astronomical data. Martin and Rogers (1987) [67] describe some models
for carbon dust compared with spectral observations of the carbon star IRC+10216.
Suh (1999) [96] presents the optical properties of silicate dust and data supporting the
conclusion that silicates are the dominant form of dust in O-rich AGB stars. The same conclusion
is reached in Danchi et al. (1994) [22] for the five stars considered here. Figure 3.1
reproduces Figure 1 from Suh (1999) [96] comparing the absorptivity of 0.1 µm silicate dust