=====================================================================
==> In this proposal, more time is being requested for SKWOK.EX_CSTAR
=====================================================================
Carbon stars have traditionally been identified by either optical
spectroscopy or by infrared spectroscopy through the detection of the 11.3
micron SiC emission feature.  The latter group, which are called "infrared
carbon stars" or "SiC carbon stars" are undergoing significant mass loss and
are likely to represent a more evolved stage of evolution than the former
group.  More recently, we have identified another group of carbon stars which
we call "extreme carbon stars".  These are stars that have thick circumstellar
envelopes with a featureless dust continuum in the 10-20 micron region.  The
derived mass loss rates of these extreme carbon stars are so high that they
must be near the end of their asymptotic giant branch of evolution.
However, the nature and composition of the dust grains in these extreme
carbon stars are not certain.  ISO observations, with better spectral coverage
and higher spectral resolutions than previously possible from ground-based
observations, will yield important data to answer this question.  We request
SWS and LWS observations of two extreme carbon stars which are observable in
the remaining lifetime of ISO.  One is a very bright object for which high
quality spectra will be easily obtained.  The other object appears to have a
dust shell of higher optical depth than for any other extreme carbon star and
it may also have a distinct dust composition (Volk, Kwok and Langill 1992;
ApJ, 391, 285).  Radiative transfer modeling of the observed spectra can then
be carried out to derive the dust opacity properties and mass loss history of
these objects.