Self-absorption curves

In previous work (Friedjung & Muratorio 1987, Muratorio & Friedjung 1988), we developed methods using self-absorption curves (SACs) to study stars having Fe II emission lines in their spectra. Such a curve is obtained by plotting log(Fλ3/gf) against log(gfλ), where F is the total flux, λ the wavelength, g the lower level statistical weight, f the oscillator strength. gfλ is proportional to the optical thickness. If no selective excitation mechanisms exist for particular levels, and the levels inside a term have populations proportional to their statistical weights, such a plot for emission lines of the same multiplet will have points lying on the same self-absorption curve. The shape of the curve is characteristic of the nature of the medium where the line is formed. Shifting the curves for different multiplets (which should have the same shape) relative to each other so as to superpose them, will give at the same time the relative populations of their upper and also their lower terms. Until now, we have calculated SACs for various simplified cases, and a comparison was made with observations of luminous stars whose spectra contained many Fe II emission lines. It was found that observations of certain Magellanic cloud stars could not be fitted by spherically symmetric wind models. Another line emitting medium seemed to be present (a slab or a thin disc with constant opening angle), which is also suggested by the continuum energy distributions.