EXTENDED DUST SHELL AROUND THE J-TYPE CARBON STAR RY DRACONIS

the goal of this proposal is to elucidate the evolutinary status of jtype carbon stars by observing their extended dust shells in the farinfrared. there are nonnegligible number of cool carbon stars which show low c12/c13 carbon isotopic ratios in their atmosphere (jtype carbon stars), prototype of which is y cvn. they show no sprocess element enhancements, which suggests that they have not experienced the third dredgeup which is the key process to produce carbon stars on the aymptotic giant branch (agb). the lack of sprocess 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 massloss rate decreased by about two orders of magnitude in the last 14,000 years. the rapid, great change of the massloss 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 jtype 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 jtype 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 massloss 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 massloss history of jtype 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 extende truncated!, P [truncated!, Please see actual data for full text]