ISOPHOT MAPPING OBSERVATIONS OF CARBON STARS

Mapping observations have been made toward five carbon stars in the fa r-infrared using ISOPHOT, an imaging photo-polarimeter onboard the Inf rared Space Observatory. Cold, very extended dust shells are clearly r evealed in two of them. Y CVn is surrounded by a very extended, detach ed dust shell, which indicates a sudden decline of the mass-loss by tw o orders of magnitude in the last (1-2)x10(4) years on a short time sc ale. The Hipparcos parallax reinforces our previous conclusion that Y CVn is a J-type carbon star on the asymptotic giant branch. U Ant show s a double shell structure, a compact dust shell surrounded by a very extended one. The outer shell has a brightness comparable to the dust shell of Y CVn. The structure indicates that there were two different high mass-loss phases separated by a period with a much lower mass-los s rate in between the two. The structure is consistent with the double dust shell proposed for this star by Izumiura et al. (1997) based on a detailed investigation of IRAS survey data.