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