Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30 m Large Program designed to provide estimates of the S, C, N, and O depletions and gas ionization degree, X(e-), in a selected set of star-forming filaments of Taurus, Perseus, and Orion. Our immediate goal is to build up a complete and large database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV ~3 to ~100 mag, n(H2) ~ a few 103 to 106 cm-3, and Tk ~10 to ~30 K ranges in these clouds, and avoiding protostars, HII regions, and bipolar outflows. A statistical analysis is carried out in order to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families of species: (1) 13CO and C18O isotopologs; (2) H13CO+, HC18O+, H13 CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK ~ 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of ~3. The abundances of H13 CO+, HC18 O+, H13 CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law ∝ n(H2)-0.8  ±  0.2. The abundances of S-bearing species also decrease with molecular hydrogen density at a rate of (S-bearing/H)gas ∝ n(H2)-0.6  ±  0.1. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13 CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.@en

Context. Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30-m 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. Aims. This paper aims to constrain the sulfur elemental abundance in Taurus, Perseus, and Orion A based on the GEMS molecular database. The selected regions are prototypes of low-mass, intermediate-mass, and high-mass star-forming regions, respectively, providing useful templates for the study of interstellar chemistry. Methods. We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO+, HCN, HNC, CS, SO, H2S, OCS, and HCS+ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from AV ∼ 3 mag to >50 mag, molecular hydrogen densities ranging from a few × 103 cm3 to 3 × 106 cm3, and Tk ∼ 10-35 K. We investigate the possible relationship between sulfur depletion and the grain charge distribution in different environments. Results. Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t = 0.1 Myr, ζH2 ∼ (0.51) × 1016 s1, and [S/H] ∼ 1.5 × 106. On the contrary, most of the positions in Orion are fitted with t = 1 Myr and ζH2 ∼ 1017 s1. Moreover, ∼40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H] ∼ 1.5 × 105. We find a tentative trend of sulfur depletion increasing with density. Conclusions. 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 ∼20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution might explain these variations. Grains become negatively charged at a visual extinction of AV ∼ 3.5 mag in Taurus and Perseus. At this low visual extinction, the S+ abundance is high, X(S+) > 106, and the electrostatic attraction between S+ and negatively charged grains could contribute to enhance sulfur depletion. In Orion, the net charge of grains remains approximately zero until higher visual extinctions (AV ∼ 5.5 mag), where the abundance of S+ is already low because of the higher densities, thus reducing sulfur accretion. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H].@en