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-arcsec beam of the 45-m radio telescope at Nobeyama Radio Observatory. The spectrum towards the star exhibits a sharp double spike, and the spectra at the offset positions (20, 40, and 60 arcsec) show that the intensities and the velocity widths vary with the offset, indicating that the circumstellar envelope of S Sct is detached. Model calculations of the CO line intensities taking into account the non-LTE effects lead us to conclude that the inner radius of the circumstellar shell of the star is 5.2 x 10 exp 17 cm, that the thickness of the circumstellar shell is greater than about 0.20 x 10 exp 17 cm (which is by about a factor of 3 thinner than previously thought), and that the shell was formed by an extremely high-mass-loss event about 10,000 yr ago. The unusually high rate of mass loss could explain the fact that only three carbon stars with detached shells have ever been detected by CO observations. A new hypothesis concerning carbon star formation by Willeros and de Jong (1988) suggests that optically bright carbon stars are objects in the intermediate phase between the termination of the oxygen- rich mass-loss phase and the beginning of the mass loss of the IR carbon star phase. The derived time scale after the mass-loss termination of S Sct (104 yr) is similar to the estimated lifetime of optically bright carbon stars in their scenario.