CS isotopes towards Galactic centre clouds

In the +50km/s 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 have also derived 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 +50km/s Cloud and Sgr B2(N), respectively.

We also obtain a ^34^S/^33^S ratio of 4.3+/-0.2 in the +50km/s Cloud. 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.