A detached dust shell surrounding the J-type carbon star Y Canum Venaticorum

This paper reports the first clear detection of a detached dust shell surrounding the prototypical J-type carbon star Y CVn in 90 m and 160 m maps taken with the ISO/ISOPHOT1. The projected inner radius of the shell is 180–190, corresponding to (6.4–7.1) 1017 cm at a distance of 250 pc. The shell thickness is obtained to be (2–5) 1017 cm. The mass-loss rate at the formation of the shell is estimated to be in the range (7–20) 10 6 M yr 1, about two orders of magnitude higher than the present-day mass-loss rate derived from CO gas observations. The obtained mass in the shell is (4–14) 10 2 M . It is concluded that the mass-loss rate decreased by two orders of magnitude on a short time scale 1.4 104 years ago assuming an average shell expansion velocity of 15 km s 1, and that Y CVn has been staying at the low mass-loss state. The duration of the previous higher mass-loss phase would be at most 2 104 years even if the asymmetry in the shell geometry is taken into account. The evolutionary status of Y CVn is also discussed.