Sulphur and carbon isotopes towards Galactic centre clouds
  Measuring isotopic ratios is a sensitive technique used to obtain information
on stellar nucleosynthesis and chemical evolution. 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 Cloud and several
l.o.s. clouds towards Sgr B2(N). Towards the +50km/s Cloud, we observed the
J=2-1 rotational transitions of CS, C34S, 13CS, C33S, and 13C34S, and the J=3-2
transitions of CS and C34S with the IRAM-30m telescope, as well as the J=6-5
transitions of C34S and 13CS with the APEX 12m telescope, all in emission. The
J=2-1 rotational transitions of CS, C34S, 13CS, and 13C34S were observed with
ALMA in the envelope of Sgr B2(N), with those of CS and C34S also observed in
the l.o.s. clouds towards Sgr B2(N), all in absorption. In the +50km/s Cloud we
derive a 12C13C isotopic ratio of ~22.1, that leads, with the measured
13CS/C34S line intensity ratio, to a 32S/34S ratio of 16.3+3.0-2.4. We also
derive the 32S/34S isotopic ratio more directly from the two isotopologues 13CS
and 13C34S, which leads to an independent 32S/34S 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 34S/33S ratio of ~4.3 in the +50 km/s Cloud. 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 studies, and opens the work
towards a comparison with Galactic and stellar evolution models.
