  Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30m
large program aimed at determining the elemental abundances of carbon (C),
oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical
star-forming filaments. In particular, the elemental abundance of S remains
uncertain by several orders of magnitude and its determination is one of the
most challenging goals of this program. We have carried out an extensive
chemical modeling of the fractional abundances of CO, HCO$^+$, HCN, HNC, CS,
SO, H$_2$S, OCS, and HCS$^+$ to determine the sulfur depletion toward the 244
positions in the GEMS database. These positions sample visual extinctions from
A$_V$ $\sim$ 3 mag to $>$50 mag, molecular hydrogen densities ranging from a
few 10$^3$~cm$^{-3}$ to 3$\times$10$^6$~cm$^{-3}$, and T$_k$ $\sim$ 10$-$35 K.
Most of the positions in Taurus and Perseus are best fitted assuming early-time
chemistry, t=0.1 Myr, $\zeta_{H_2}$$\sim$ (0.5$-$1)$\times$10$^{-16}$ s$^{-1}$,
and [S/H]$\sim$1.5$\times$10$^{-6}$. On the contrary, most of the positions in
Orion are fitted with t=1~Myr and $\zeta_{H_2}$$\sim$ 10$^{-17}$ s$^{-1}$.
Moreover, $\sim$40% of the positions in Orion are best fitted assuming the
undepleted sulfur abundance, [S/H]$\sim$1.5$\times$10$^{-5}$. Our results
suggest that sulfur depletion depends on the environment. While the abundances
of sulfur-bearing species are consistent with undepleted sulfur in Orion, a
depletion factor of $\sim$20 is required to explain those observed in Taurus
and Perseus. We propose that differences in the grain charge distribution in
the envelopes of the studied clouds might explain these variations. The shocks
associated with past and ongoing star formation could also contribute to
enhance [S/H] in Orion.
