Interstellar S isotopes

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.