2000A&A...363..629Y

The origin of silicate carbon stars has been a mystery ever since their discovery. We discuss here a full grating spectrum between 2.4 and 45µm of the silicate carbon star V778 Cyg obtained by the ISO/SWS. The spectrum, taken about 14 years after the IRAS LRS observation, confirms the complex nature of the object. The spectrum is clearly divided into a short wavelength (λ<6.5µm), carbon-rich part and long-wavelength, oxygen-rich part. No obvious change of the 10 and 18µm silicate features is observed between IRAS and ISO spectra, indicating that the silicate dust is in a steady structure. The 2.7µm H₂O band and the 15µm CO₂ bands are tentatively detected. The near-infrared part of the spectrum indicates that the present-day mass-loss rate is very low. The silicate features can only be fitted by optically thin dust emission from sub-micron size grains. The total oxygen-rich dust mass seen at infrared wavelengths is 2-10x10^–6M_☉, of which 3-50x10^–8M_☉ is warm (300-600K). If the dust is heated by radiation from the central star, the dust should be located as close as about 12 stellar radii from the star. We suggest that the dust responsible for the emission features is in a steady outflow from the system. We show that the dust cannot be located in a circum-binary disk, but is stored in a disk around the companion star during the previous O-rich mass-loss phase. The duration of silicate emission is estimated as ∼10⁴yr. It is compatible with the fact that not all J-type carbon stars show silicate emission. The evolution of the central star and formation of the disk in AGB binary systems largely depends on the orbital separation. V778 Cyg and other ``IRAS discovered'' silicate carbon stars probably have wide orbits. In such a case, a disk is formed around the companion. Close-binary systems such as the Red Rectangle form massive equatorial O-rich disks, and the evolution of the central star is largely influenced by the binarity.