Galactic interstellar sulfur isotopes: A radial $^{32}$S$/$$^{34}$S
  gradient?
  We present observations of $^{12}$C$^{32}$S, $^{12}$C$^{34}$S,
$^{13}$C$^{32}$S and $^{12}$C$^{33}$S J=2$-$1 lines toward a large sample of
massive star forming regions by using the Arizona Radio Observatory 12-m
telescope and the IRAM\,30-m. Taking new measurements of the carbon
$^{12}$C/$^{13}$C ratio, the $^{32}$S$/$$^{34}$S isotope ratio was determined
from the integrated $^{13}$C$^{32}$S/$^{12}$C$^{34}$S line intensity ratios for
our sample. Our analysis shows a $^{32}$S$/$$^{34}$S gradient from the inner
Galaxy out to a galactocentric distance of 12\,kpc. An unweighted least-squares
fit to our data yields $^{32}$S$/$$^{34}$S = (1.56 $\pm$ 0.17)$\rm D_{\rm GC}$
+ (6.75 $\pm$ 1.22) with a correlation coefficient of 0.77. Errors represent
1$\sigma$ standard deviations. Testing this result by (a) excluding the
Galactic center region, (b) excluding all sources with C$^{34}$S opacities $>$
0.25, (c) combining our data and old data from previous study, and (d) using
different sets of carbon isotope ratios leads to the conclusion that the
observed $^{32}$S$/$$^{34}$S gradient is not an artefact but persists
irrespective of the choice of the sample and carbon isotope data. A gradient
with rising $^{32}$S$/$$^{34}$S values as a function of galactocentric radius
implies that the solar system ratio should be larger than that of the local
interstellar medium. With the new carbon isotope ratios we obtain indeed a
local $^{32}$S$/$$^{34}$S isotope ratio about 10$\%$ below the solar system
one, as expected in case of decreasing $^{32}$S$/$$^{34}$S ratios with time and
increased amounts of stellar processing. However, taking older carbon isotope
ratios based on a lesser amount of data, such a decrease is not seen. No
systematic variation of $^{34}$S$/$$^{33}$S ratios along galactocentric
distance was found.
