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.

Context. Under cold conditions in dense cores, gas-phase molecules and atoms are depleted from the gas-phase to the surface of interstellar grains. Considering the time scales and physical conditions within these cores, a portion of these molecules has to be brought back into the gas-phase to explain their observation by milimeter telescopes. Aims. We tested the respective efficiencies of the different mechanisms commonly included in the models (photo-desorption, chemical desorption, and cosmic-ray-induced whole-grain heating). We also tested the addition of sputtering of ice grain mantles via a collision with cosmic rays in the electronic stopping power regime, leading to a localized thermal spike desorption that was measured in the laboratory. Methods. The ice sputtering induced by cosmic rays has been added to the Nautilus gas-grain model while the other processes were already present. Each of these processes were tested on a 1D physical structure determined by observations in TMC1 cold cores. We focused the discussion on the main ice components, simple molecules usually observed in cold cores (CO, CN, CS, SO, HCN, HC3N, and HCO+), and complex organic molecules (COMs such as CH3OH, CH3CHO, CH3OCH3, and HCOOCH3). The resulting 1D chemical structure was also compared to methanol gas-phase abundances observed in these cores. Results. We found that all species are not sensitive in the same way to the non-thermal desorption mechanisms, and the sensitivity also depends on the physical conditions. Thus, it is mandatory to include all of them. Chemical desorption seems to be essential in reproducing the observations for H densities smaller than 4 × 104 cm−3, whereas sputtering is essential above this density. The models are, however, systematically below the observed methanol abundances. A more efficient chemical desorption and a more efficient sputtering could better reproduce the observations. Conclusions. In conclusion, the sputtering of ices by cosmic-rays collisions may be the most efficient desorption mechanism at high density (a few 104 cm−3 under the conditions studied here) in cold cores, whereas chemical desorption is still required at smaller densities. Additional works are needed on both mechanisms to assess their efficiency with respect to the main ice composition.

Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question. Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance. Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when nH > 2 × 104. This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5−10. Along the three cores, atomic S is predicted to be the main sulphur reservoir. Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.