Direct measurements of carbon and sulfur isotope ratios in the Milky Way

Context. Isotope abundance ratios provide a powerful tool for tracing stellar nucleosynthesis, evaluating the composition of stellar ejecta, and constraining the chemical evolution of the Milky Way.
Aims: We aim to measure the ¹²C/¹³C, ³²S/³⁴S, ³²S/³³S, ³²S/³⁶S, ³⁴S/³³S, ³⁴S/³⁶S, and ³³S/³⁶S isotope ratios across the Milky Way.
Methods: With the IRAM 30 meter telescope, we performed observations of the J = 2−1 transitions of CS, C³³S, C³⁴S, C³⁶S, ¹³CS, ¹³C³³S, and ¹³C³⁴S as well as the J = 3−2 transitions of C³³S, C³⁴S, C³⁶S, and ¹³CS toward a large sample of 110 high-mass star-forming regions.
Results: We measured the ¹²C/¹³C, ³²S/³⁴S, ³²S/³³S, ³²S/³⁶S, ³⁴S/³³S, ³⁴S/³⁶S, and ³³S/³⁶S abundance ratios with rare isotopologs of CS, thus avoiding significant saturation effects. With accurate distances obtained from parallax data, we confirm previously identified ¹²C/¹³C and ³²S/³⁴S gradients as a function of galactocentric distance. In the central molecular zone, ¹²C/¹³C ratios are higher than suggested by a linear fit to the disk values as a function of galactocentric radius. While ³²S/³⁴S ratios near the Galactic center and in the inner disk are similar, this is not the case for ¹²C/¹³C, when comparing central values with those near galactocentric radii of 5 kpc. As was already known, there is no ³⁴S/³³S gradient but the average ratio of 4.35 ± 0.44 derived from the J = 2−1 transition lines of C³⁴S and C³³S is well below previously reported values. A comparison between solar and local interstellar ³²S/³⁴S and ³⁴S/³³S ratios suggests that the Solar System may have been formed from gas with a particularly high ³⁴S abundance. For the first time, we report positive gradients of ³²S/³³S, ³⁴S/³⁶S, ³³S/³⁶S, and ³²S/³⁶S in our Galaxy. The predicted ¹²C/¹³C ratios from the latest Galactic chemical-evolution models are in good agreement with our results. While ³²S/³⁴S and ³²S/³⁶S ratios show larger differences at larger galactocentric distances, ³²S/³³S ratios show an offset across the entire inner 12 kpc of the Milky Way.