Sulfur gas-phase abundance in dense cores
  The abundance of volatile sulfur in dense clouds is long-standing problem in
studies of the physics and chemistry of star-forming regions. Sulfur is an
important species because its low ionization potential may possibly make it an
important charge carrier. The observed sulfur-bearing species in the gas-phase
of dense clouds represent only a minor fraction of the cosmic sulfur abundance,
which has been interpreted as a signature of sulfur depletion into ices at the
surface of dust grains. However, atomic sulfur, which could be the main
gas-phase carrier, cannot be observed directly in cold cores. We present
measurements of the nitrogen sulfide (NS) radical toward four dense cores
performed with the IRAM-30m telescope. Analytical chemical considerations and
chemical models over a wide parameter space show that the NS:N2H+ abundance
ratio provides a direct constraint on the abundance of gas-phase atomic sulfur.
Toward early-type cores, we find that $n(\rm S)/n_{\rm H}$ is close, or even
equal, to the cosmic abundance of sulfur, 14$\times 10^{-6}$, demonstrating
that sulfur is not depleted and is atomic, which is in agreement with chemical
models. More chemically evolved cores show sulfur depletion by factors up to
100 in their densest parts. In L1544, atomic sulfur depletion is shown to
increase with increasing density. Future observations are needed to discover
the solid-phase carrier of sulfur. The initial steps of the collapse of
pre-stellar cores in the high sulfur abundance regime also need to be explored
from their chemical and dynamical perspectives.
