1996A&A...305..960C

A ¹²C³²S, ¹³C³²S, ¹²C³⁴S, and ¹²C³³S J=2-1 line survey has been made to study interstellar ³²S/³⁴S and ³⁴S/³³S 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 C³³S J=2-1 rest frequency is ∼250kHz below the value given in the Lovas (1992) catalog. Taking ¹²C/¹³C ratios from Wilson & Rood (1994) and assuming equal ¹²C³²S and ¹³C³²S excitation temperatures and beam filling factors, ¹²C³²S opacities are in the range 3 to 15; average ³²S/³⁴S and ³⁴S/³³S isotope ratios are 24.4±5.0 and 6.27±1.01, respectively. While no systematic variation in the ³⁴S/³³S isotope ratio is found, the ³²S/³⁴S ratio increases with galactocentric distance when accounting for the ¹²C/¹³C gradient of the galactic disk. A fit to the unweighted data yields ³²S/³⁴S=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.