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

Abstract

This paper reports the first clear detection of a detached dust shell surrounding the prototypical J-type carbon star Y CVn in 90μm and 160μm maps taken with the ISO/ISOPHOT. The projected inner radius of the shell is 180"-190", corresponding to (6.4-7.1)x10^17^cm at a distance of 250pc. The shell thickness is obtained to be (2-5)x10^17^cm. The mass-loss rate at the formation of the shell is estimated to be in the range (7-20)x10^-6^M_sun_/yr, 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)x10^-2^M_sun_. It is concluded that the mass-loss rate decreased by two orders of magnitude on a short time scale 1.4x10^4^ years ago assuming an average shell expansion velocity of 15km/s, 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 2x10^4^ years even if the asymmetry in the shell geometry is taken into account. The evolutionary status of Y CVn is also discussed.