Astron. Astrophys. 363, 629-639 (2000)

9. Conclusions

The ISO/SWS observation of the silicate carbon star V778 Cyg revealed that there has been little change in the spectral profile of the silicate features in the last 14 years, since the IRAS/LRS observation. H₂O and CO₂ molecules marginally detected in emission, are probably located in the circumstellar environment. The near-infrared spectrum indicates that the present-day mass-loss rate is small. The silicate features can only be fitted by optically thin emission from small (sub-micron size) dust grains. We propose that the features arise from the continuous dust outflow from a companion disk, as dust grains spill out from the gravitational restriction and are blown away by the radiation pressure from the central star. We discussed formation of a disk in a binary system under the strong radiation field from the central star. It turned out that the orbital separation is a key parameter which determines the fate of the system. A heavy circum-binary disk is formed when the system is a close-binary, while a disk around the companion star results for the wide separation system. V778 Cyg is of the latter case. Objects such as the Red Rectangle and IRAS 09425-6040 are probably formed by the former mechanism. The primary of V778 Cyg is likely in the AGB phase. We assume that the oxygen-rich material was stored during the previous AGB mass-loss phase, but the conclusion does not change even if the material was provided in the earlier phase, during RGB or at the He-core flash. The dust mass in the disk of V778 Cyg is of the order of , which will be consumed in about years after the primary star became a luminous carbon star. We suspect that all luminous J-type stars have once been silicate carbon stars. Those who show little or no silicate emission might have exhausted the dust in the disk. Presumably, binarity influences the evolution of the primary star even for wide orbital separation.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
helpdesk.link@springer.de