Astron. Astrophys. 350, 163-180 (1999)

7. Conclusions

The main results of this study can be summarized as follows:

1. AFGL 4106 is a (double-lined spectroscopic) binary, consisting of a warm star with  K and a cool companion with  K.

2. The luminosity ratio of both stars , which indicates that both stars are evolved. The warm star is likely responsible for the dust shell.

3. The expansion velocity of the shell, the grain size distribution and the minimal variations in the photographic and V band over the last 90 years all suggest a high luminosity. We conclude that the stars are at kpc and have and respectively. These values correspond to main sequence masses of 15 to 20 . The warm star is probably evolving rapidly to the blue part of the HR-diagram and may evolve to a blue-supergiant (ending its life like SN 1987A) or to a Wolf-Rayet phase.

4. The main component of the dust are large (m) amorphous, Fe-rich olivines. If also amorphous pyroxenes are present, they will have a much lower Fe content. We find a rich spectrum of narrow solid state emission bands in the ISO-SWS and LWS spectra, which we identify with crystalline olivines and pyroxenes. These grains are Mg-rich and Fe-poor and have an abundance between 7 and 15% by mass, depending on the assumed enstatite to forsterite ratio. We also find evidence for the presence of FeO, Al₂O₃, melilite and crystalline H₂O-ice. Shape effects can have an important influence on the derived abundances.

5. The temperatures for the different dust species are quite different, this is caused by variations in their absorption characteristics at visual and near-IR wavelengths. These temperature differences imply that the different dust species are not thermally coupled, directly (e.g. as a composite grain) nor indirectly (e.g. via gas-dust interactions).

6. The crystalline silicate bands can be divided into narrow (FWHM/) and broad (FWHM/) features. The wavelengths of the broad features match well with forsterite, while those of most of the narrow features line up well with enstatite. The width of the features is significantly smaller than those observed in laboratory spectra. The abundance ratio of enstatite to forsterite is between 3 and 1.

7. The mean mass-loss rate was /yr for a period of about years and probably increased during this period. The mass loss stopped 450 years ago. The total mass expelled, assuming a gas to dust ratio of 100, is 3.9 .

8. We find a cool dust component, which cannot be fitted by our dust model. This component is either an older mass loss phase, or an incorrect background subtraction of the ISM. We also found indications from the spectrum between 4 and 7 µm and the 10 µm imaging that a third dust component, close to the binary is present. Whether this represents a recent mass loss phase or a more stable circumstellar dust configuration is not known.

Because of its (IR-)brightness AFGL 4106 will be a key object in the further study of (crystalline) dust formation. High resolution imaging and spectroscopy both in the optical and IR will allow us to constrain the circumstellar versus interstellar extinction, the photospheres of the components, density and abundance gradients through the CSE, and thus the conditions for dust formation and evolution.

© European Southern Observatory (ESO) 1999

Online publication: September 24, 1999
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