INTERSTELLAR SULFUR ISOTOPES AND STELLAR OXYGEN BURNING

A (CS)-C-12-S-32, (CS)-C-13-S-32, (CS)-C-12-S-34, and (CS)-C-12-S-33 J =2-1 line survey has been made to study interstellar S-32/S-34 and S-3 4/S-33 ratios from the galactic disk. The four CS isotopomers were det ected in 20 star forming regions with galactocentric distances between 3 and 9 kpc. From a comparison of line velocities, the (CS)-S-33 J=2- 1 rest frequency is similar to 250 kHz below the value given in the Lo vas (1992) catalog. Taking C-12/C-13 ratios from Wilson & Rood (1994) and assuming equal (CS)-C-12-S-32 and (CS)-C-13-S-32 excitation temper atures and beam filling factors, (CS)-C-12-S-32 opacities are in the r ange 3 to 15; average S-32/S-34 and S-34/S-33 isotope ratios are-24.4 +/- 5.0 and 6.27 +/- 1.01, respectively. While no systematic variation in the S-34/S-33 isotope ratio is found, the S-32/S-34 ratio increase s with galactocentric distance when accounting for the C-12/C-13 gradi ent of the galactic disk. A fit to the unweighted data yields S-32/S-3 4 = 3.3 +/- 0.5 (D-GC/kpc) + 4.1 +/- 3.1 with a correlation coefficien t of 0.84. Since the interstellar sulfur (S) isotopes are synthesized by oxygen burning in massive stars, consequences for nucleosynthesis a nd models of chemical evolution are briefly discussed(1).