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 +50 km 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 ¹²C³²S, ¹²C³⁴S, ¹³C³²S, ¹²C³³S, and ¹³C³⁴S, and the J = 3-2 transitions of ¹²C³²S and ¹²C³⁴S with the IRAM-30 m telescope, as well as the J = 6-5 transitions of ¹²C³⁴S and ¹³C³²S with the APEX 12 m telescope, all in emission. The J = 2-1 rotational transitions of ¹²C³²S, ¹²C³⁴S, ¹³C³²S, and ¹³C³⁴S were observed with ALMA in the envelope of Sgr B2(N), with those of ¹²C³²S and ¹²C³⁴S 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 ¹²C/¹³C isotopic ratio of 22.1_-2.4^+3.3, that leads, with the measured ¹³C³²S/¹²C³⁴S line intensity ratio, to a ³²S/³⁴S ratio of 16.3_-2.4^+3.0. We also derive the ³²S/³⁴S isotopic ratio more directly from the two isotopologues ¹³C³²S and ¹³C³⁴S, which leads to an independent ³²S/³⁴S estimation of 16.3_-1.7^+2.1 and 17.9 ± 5.0 for the +50 km s^-1 Cloud and Sgr B2(N), respectively. We also obtain a ³⁴S/³³S ratio of 4.3 ± 0.2 in the +50 km s^-1 Cloud.
Conclusions: Previous studies observed a decreasing trend in the ³²S/³⁴S isotopic ratios when approaching the Galactic centre. Our result indicates a termination of this tendency at least at a galactocentric distance of 130_-30⁺⁶⁰ pc. This is at variance with findings based on ¹²C/¹³C, ¹⁴N/¹⁵N, and ¹⁸O/¹⁷O 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.

The reduced spectra and datacubes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A222