The gas-phase abundance of sulfur in dense cores
2021
The abundance of volatile sulfur in dense clouds is an old problem in studies of the physics and chemistry of star forming regions. Sulfur is an important species because its low ionization potential makes it a potentially important charge carrier. 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. Yet, atomic sulfur, which could be the main gas-phase carrier, can not be observed directly in cold cores. We present measurements of the 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(S)/nH is close, or even equal, to the cosmic abundance of sulfur, 14x10^-6 , demonstrating that sulfur is not depleted and is atomic, 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 will also need to be explored from chemical and dynamical perspectives.
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