  Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m
Large Program designed to estimate the S, C, N, and O depletions and gas
ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus
and Orion. Our immediate goal is to build up a complete 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 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K
ranges in these clouds, avoiding protostars, HII regions, and outflows. A
statistical analysis is carried out to identify general trends between
different species and with physical parameters. Relations between molecules
reveal strong linear correlations which define three different families: (1)
13CO and C18O; (2) H13CO+, HC18O+, H13CN, 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 H13CO+, HC18O+,
H13CN, and HNC are well correlated with each other, and all of them decrease
with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The
abundances of S-bearing species also decrease with n(H2) at a rate of
(S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. 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
H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in
external galaxies.
