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

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Figure 3.14: Dominant Continuum 11.15 µm Opacity Source as a Function of Temperature
and Density
as the state of the gas approaches the opacity contour of 10 −10 m −1 . For an effective
temperature of 3000 K, this occurs at a density ≈ 5 × 10 20 m −3 . The dominant source of
opacity as a function of temperature and density is displayed in Figure 3.14. We see that for
a stellar atmosphere near 3000 K, the 11 µm monochromatic radius is determined primarily
by H − ff opacity.
Although the Rosseland radius (radius at which τ Rosseland = 1) is usually taken
to represent the stellar radius, its computation involves a complete frequency-dependent
description of all opacity sources including spectral lines. In addition, it is not a directly
observable quantity. Although quite natural for theoretical work, it cannot be calculated
solely from continuum sources and is beyond the scope of these calculations. For the models
considered here, we will calculate instead monochromatic radii and apparent diameters