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

2023; EDP Sciences; Volume: 670; Linguagem: Inglês

10.1051/0004-6361/202244584

1432-0746

Y. T. Yan, C. Henkel, Chiaki Kobayashi, K. M. Menten, Y. Gong, J. S. Zhang, Hongzhi Yu, Kai Yang, Jinjin Xie, Y. X. Wang,

Astro and Planetary Science

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