Sulphur and carbon isotopes towards Galactic centre clouds

Context. Measuring isotopic ratios is a sensitive technique used to obtain information on stellar nucleosynthesis and chemical evolution. Aims. We present measurements of the carbon and sulphur abundances in the interstellar medium of the central region of our Galaxy. The selected targets are the +50km s −1 Cloud and several line-of-sight clouds towards Sgr B2(N). Methods. Towards the +50 km s −1 Cloud, we observed the J =2-1 rotational transitions of 12 C 32 S, 12 C 34 S, 13 C 32 S, 12 C 33 S, and 13 C 34 S, and the J =3-2 transitions of 12 C 32 S and 12 C 34 S with the IRAM-30 m telescope, as well as the J =6-5 transitions of 12 C 34 S and 13 C 32 S with the APEX 12 m telescope, all in emission. The J =2-1 rotational transitions of 12 C 32 S, 12 C 34 S, 13 C 32 S, and 13 C 34 S were observed with ALMA in the envelope of Sgr B2(N), with those of 12 C 32 S and 12 C 34 S also observed in the line-of-sight clouds towards Sgr B2(N), all in absorption. Results. In the +50 km s −1 Cloud we derive a 12 C/ 13 C isotopic ratio of 22.1 +3.3 −2.4 , that leads, with the measured 13 C 32 S/ 12 C 34 S line intensity ratio, to a 32 S/ 34 S ratio of 16.3 +3.0 −2.4. We also derive the 32 S/ 34 S isotopic ratio more directly from the two isotopologues 13 C 32 S and 13 C 34 S, which leads to an independent 32 S/ 34 S estimation of 16.3 +2.1 −1.7 and 17.9±5.0 for the +50 km s −1 Cloud and Sgr B2(N), respectively. We also obtain a 34 S/ 33 S ratio of 4.3±0.2 in the +50 km s −1 Cloud. Conclusions. Previous studies observed a decreasing trend in the 32 S/ 34 S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130 +60 −30 pc. This is at variance with findings based on 12 C/ 13 C, 14 N/ 15 N and 18 O/ 17 O isotope ratios, where the above-mentioned trend is observed to continue right to the central molecular zone. This can indicate a drop in the production of massive stars at the Galactic centre, in the same line as recent metallicity gradient ([Fe/H]) studies, and opens the work towards a comparison with Galactic and stellar evolution models.