Gas phase Elemental abundances in Molecular cloudS (GEMS) I. The
  prototypical dark cloud TMC 1
  GEMS is an IRAM 30m Large Program whose aim is determining the elemental
depletions and the ionization fraction in a set of prototypical star-forming
regions. This paper presents the first results from the prototypical dark cloud
TMC 1. Extensive millimeter observations have been carried out with the IRAM
30m telescope (3mm and 2mm) and the 40m Yebes telescope (1.3cm and 7mm) to
determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in
three cuts which intersect the dense filament at the well-known positions TMC
1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from Av~3 to
~20mag. Two phases with differentiated chemistry can be distinguished: i) the
translucent envelope with molecular hydrogen densities of (1-5)x10$^3$
cm$^{-3}$; and ii) the dense phase, located at Av>10mag, with molecular
hydrogen densities >10$^4$ cm$^{-3}$. Observations and modeling show that the
gas phase abundances of C and O progressively decrease along the C+/C/CO
transition zone where C/H~8x10$^{-5}$ and C/O~0.8-1, until the beginning of the
dense phase at Av~10 mag. This is consistent with the grain temperatures being
below the CO evaporation temperature in this region. In the case of sulfur, a
strong depletion should occur before the translucent phase where we estimate a
S/H~(0.4 - 2.2) x10$^{-6}$, an abundance ~7-40 times lower than the solar
value. A second strong depletion must be present during the formation of the
thick icy mantles to achieve the values of S/H measured in the dense cold cores
(S/H~8x10$^{-8}$). Based on our chemical modeling, we constrain the value of
$\zeta_{\rm H_2}$ to ~(0.5 - 1.8) x10$^{-16}$ s$^{-1}$ in the translucent
cloud.
