2019A&A...624A.105F

GEMS is an IRAM 30 m 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 Taurus molecular cloud (TMC) 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3 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 A_V∼3 to ∼20mag. Two phases with differentiated chemistry can be distinguished: (i) the translucent envelope with molecular hydrogen densities of 1-5x10³cm^–3; and (ii) the dense phase, located at A_V>10mag, with molecular hydrogen densities >10⁴cm^–3. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C⁺/C/CO transition zone (A_V∼3mag) where C/H∼8x10^–5 and C/O∼0.8-1, until the beginning of the dense phase at A_V∼10mag. 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 an 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 ζ_H2 to ~(0.5-1.8)x10^–16s^–1 in the translucent cloud.