The composition and nature of the dust shell surrounding the binary AFGL 4106 ? ? ?

We present infrared spectroscopy and imaging of AFGL 4106. The 2.4–5 μm ISO-SWS spectrum reveals the presence of a cool, luminous star (Teff ≈ 3750 K) in addition to an almost equally luminous F star (Teff ≈ 7250 K). The 5–195 μm SWS and LWS spectra are dominated by strong emission from circumstellar dust. We find that the dust consists of amorphous silicates, with a minor but significant contribution from crystalline silicates. The amorphous silicates consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be excluded but if present they contain much less Fe than the amorphous olivines. Comparison with laboratory data shows that the pure Mg-end members of the crystalline olivine and pyroxene solid solution series are present. In addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as crystalline H2O ice. Several narrow emission features remain unidentified. Modelling of the dust emission using a dust radiation transfer code shows that large grains (≈ 1μm) must be present and that the abundance of the crystalline silicates is between 7 and 15% of the total dust mass, depending on the assumed enstatite to forsterite ratio, which is estimated to be between 1 and 3. The amorphous and crystalline dust components in the shell do not have the same temperature, implying that the different dust species are not thermally coupled. We find a dust mass of ≈ 3.9 · 10−2 M expelled over a period of ≈ 4 · 103 years for a distance of 3.3 kpc. The F-star in the AFGL 4106 binary is likely a post-red-supergiant in transition to a blue supergiant or WR phase. Send offprint requests to: F.J. Molster ([email protected]) ? Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA ?? Based on observations collected at the European Southern Observatory, La Silla, Chile