I. Yamamura 1, F.J. Molster 1, L.B.F.M. Waters 1,3, T. de Jong 2,1
1 Astronomical Institute ``Anton Pannekoek'', University of Amsterdam, Kruislaan 403, NL-1098 SJ, Amsterdam, the Netherlands
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA, Utrecht, the Netherlands
3 SRON-Groningen, Landleven 12, 9747 AD, Groningen, the Netherlands
We report the result of the SWS observations of silicate carbon stars, a group of carbon stars which show prominent silicate emission features. Because silicate dust is normally formed and observed in the oxygen-rich environment, the origin of the silicate emission in these stars has been a big puzzle as well as the evolution of these stars, since their first discovery by the IRAS/LRS (Willems and de Jong 1986; Little-Marenin 1986). The central stars are probably all 13C rich (i.e. J-type carbon stars; Willems and de Jong 1986, Lloyd Evans 1990), suggesting an unique evolutionary procedure of the stars. We observed one of the brightest silicate carbon stars, V778 Cyg, by ISO/SWS. The obtained spectrum covers from 2.38 to 45.2 micron with a resolution of about 500. The spectrum is clearly divided into two parts; the carbon-rich part between 2.4 and 6.5 micron with many carbonaceous molecular bands, and the oxygen-rich part longward of 6.5 micron dominated by the silicate dust emission features. No remarkable change is found in the shape of the silicate features since the IRAS/LRS observation about 13 years ago, i.e. the silicate dust must be in a stable structure such as a disk. In the ISO/SWS observations, we found a few new carbon-rich objects with silicate dust features. In those ``new-type'' silicate carbon stars, the separation between carbon-rich and oxygen-rich spectral parts takes place around 15 micron, much longer than the ``traditional'' silicate carbon stars. These objects show prominent crystalline silicate features in the 30 micron region, in contrast to the ``traditional'' silicate carbon stars in which 10 and 18 micron amorphous silicate bands are dominant. One of the ``new-type'' objects with silicate emission is Red Rectangle, a well know proto-planetary nebula (Waters et al. 1998). In this object, silicate dust is probably in the equatorial disk. Presumably, there is an evolutionary connection between these objects. A possible scenario is as follows: The oxygen-rich material is stored in a circumbinary disk while the star was oxygen-rich. The star would evolves to a carbon-rich object (``traditional'' carbon star). The silicate grains crystallize and grow in this debris disk, resulting a lower dust temperature. The amorphous features decrease their intensity and crystalline features become prominent (``new-type'' silicate carbon star). Finally, the central star exits the AGB phase and evolves towards the white dwarf phase (Red Rectangle). The grain processing in the circumbinary disks of these objects may resemble that seen in disks around young stars.