We propose a SWS06 observation of the silicate carbon star V778
Cyg to obtain high-quality spectra in four selected wavelength regions.
The origin of the silicate emission in carbon stars has been a mystery
ever since their first discovery by the IRAS LRS. We obtained a
SWS01/speed 3 spectrum of V778 Cyg in our guaranteed time program. The
result confirms that the carbon-rich molecular features in the short
wavelength region and the silicate dust features at 10 and 20 microns
co-exist in one spectrum, with the transition from carbon-rich to
oxygen-rich occurring at 6.5 micron.
The similarity in shape of the silicate features in our SWS spectrum
compared to the 1983 IRAS/LRS spectrum indicates that the silicate
dust is probably not located in a rapidly expanding thin shell close
to the central carbon star but in a more stable structure, although
it is difficult to distinguish whether this is a disk or a shell.
The first purpose of this additional observation is to search for the
possible contribution of oxygen-rich molecules in the circumstellar
material by trying to detect H2O bands at 2.7 and 6.2 micron. The
second objective is to observe C2H2 and HCN molecular bands at 7.5
and 14 micron. Detailed modeling of these molecular bands will tell
us how the carbon-rich material is distributed around the star. Both
observations require a very high signal-to-noise ratio of about 100,
which cannot be reached in the ATO01 observation. The spectral
resolution of up to 2000 that can be attained in the SWS06
observation is essential to be able to measure weak molecular lines.
We also would like to observe the visual carbon star, V Aql, as the
reference for `normal' photospheric spectra of carbon stars. These
reference spectra are important to separate the weak H2O lines from
the strong carbon-rich molecular lines which dominate the spectrum of
V778 Cyg at these wavelengths.
V778 Cyg is the only silicate carbon star observed by ISO/SWS.