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