STRUCTURE OF THE CIRCUMSTELLAR SHELL AROUND THE CARBON STAR S-SCUTI

We have observed the carbon star S Sct in the CO J = 1-0 line with the 16'' beam of the 45-m radio telescope at Nobeyama Radio Observatory. The spectrum towards the star exhibits a sharp double spike, and the s pectra at the offset positions (20'', 40'', and 60'') show that the in tensities and the velocity widths vary with the offset, indicating tha t the circumstellar envelope of S Sct is detached. Model calculations of the CO line intensities taking into account the non-LTE (Local Ther modynamical Equilibrium) effects lead us to conclude that the inner ra dius of the circumstellar shell of the star is 5.2 x 10(17) cm, that t he thickness of the circumstellar shell is less-than-or-equal-to 0.20 x 10(17) cm (which is by about a factor of 3 thinner than previously t hought), and that the shell was formed by an extremely high-mass-loss event (a mass-loss rate of greater-than-or-equal-to 1.3 x 10(-4) M. yr -1) about 10(4) yr ago. The unusually high rate of mass loss could exp lain the fact that only three carbon stars with detached shells have e ver been detected by CO observations. A new hypothesis concerning carb on star formation by Willems and de Jong (1988, AAA 065.070) suggests that optically bright carbon stars are objects in the intermediate pha se between the termination of the oxygen-rich mass-loss phase and the beginning of the mass loss of the infrared carbon star phase. The deri ved time scale after the mass-loss termination of S Sct (10(4) yr) is similar to the estimated lifetime of optically bright carbon stars in their scenario.