The goal of this proposal is to elucidate the evolutinary status of J-type
carbon stars by observing their extended dust shells in the far-infrared.
There are non-negligible number of cool carbon stars which show low C12/C13
carbon isotopic ratios in their atmosphere (J-type carbon stars), prototype
of which is Y CVn.  They show no s-process element enhancements, which
suggests that they have not experienced the third dredge-up which is the key
process to produce carbon stars on the aymptotic giant branch (AGB).  The lack
of s-process enhancements tends to be interpreted as the sign of them being not
on the AGB.
However, we have detected a very extended, detached dust shell around Y CVn
at 90 and 160 micron with ISOPHOT in our Guaranteed Time (GT) program,
HIZUMIUR_REDSTAR2.  Our results indicate that the mass-loss rate decreased by
about two orders of magnitude in the last 14,000 years.  The rapid, great
change of the mass-loss rate resembles to those observed toward carbon stars
U Hya and U Ant which are known to be on the AGB.  Such similarity suggests
that Y CVn is also an AGB star.
While the formation process of ordinary cool carbon stars on the AGB is
relatively well understood now, the evolutionary status of those J-type cool
carbon stars is still remains to be answered.
In this proposal we plan to study the dust shell of RY Dra, the second
brightest J-type carbon star next to Y CVn, both of which have extremely low
C12/C13 isotopic ratios (< 5) in the atmosphere, in order to study its
mass-loss history.  We will examine if RY Dra also has an extended, detached
dust shell similar to the one around Y CVn.  The results on the two stars will
provide us with important information on the mass-loss history of J-type carbon
stars, thus would help to understand their evolutionary status.
This proposal is an extension of our GT ISO observing program
HIZUMIUR_REDSTAR2, in which we examine the structure of extended dust shells
around both oxygen-rich and carbon-rich AGB stars in the far-infrared using
ISOPHOT.