Gas-phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking
  the CS chemistry: the CS+O reaction
  CS is among the most abundant gas-phase S-bearing molecules in cold dark
molecular clouds. It is easily observable with several transitions in the
millimeter wavelength range, and has been widely used as a tracer of the gas
density in the interstellar medium in our Galaxy and external galaxies.
Chemical models fail to account for the observed CS abundances when assuming
the cosmic value for the elemental abundance of sulfur. The CS+O -> CO + S
reaction has been proposed as a relevant CS destruction mechanism at low
temperatures, and could explain the discrepancy between models and
observations. Its reaction rate has been experimentally measured at
temperatures of 150-400 K, but the extrapolation to lower temperatures is
doubtful. Here we calculate the CS+O reaction rate at temperatures <150 K which
are prevailing in the interstellar medium. We performed ab initio calculations
to obtain the three lowest PES of the CS+O system. These PESs are used to study
the reaction dynamics, using several methods to eventually calculate the CS+O
thermal reaction rates. We compare the results of our theoretical calculations
for 150-400 K with those obtained in the laboratory. Our detailed theoretical
study on the CS+O reaction, which is in agreement with the experimental data
obtained at 150-400 K, demonstrates the reliability of our approach. After a
careful analysis at lower temperatures, we find that the rate constant at 10 K
is negligible, which is consistent with the extrapolation of experimental data
using the Arrhenius expression. We use the updated chemical network to model
the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS
project observations. In our model, we take into account the expected decrease
of the cosmic ray ionization rate along the cloud. The abundance of CS is still
overestimated when assuming the cosmic value for the sulfur abundance.
