Low sulfur depletion in the Horsehead PDR

20 pages, 17 figures, accepted in A&A, 2006 June 9 (Figs 1 & 4 are bitmaped)Aims.We present 3.65'' × 3.34'' angular-resolution IRAM Plateau de Bure Interferometer (PdBI) observations of the CS J = 2-1 line toward the Horsehead Photodissociation Region (PDR), complemented with IRAM-30m single-dish observations of several rotational lines of CS, C34S and HCS^+. We analyse the CS and HCS+ photochemistry, excitation and radiative transfer to obtain their abundances and the physical conditions prevailing in the cloud edge. Since the CS abundance scales to that of sulfur, we determine the gas phase sulfur abundance in the PDR, an interesting intermediate medium between translucent clouds (where sulfur remains in the gas phase) and dark clouds (where large depletions have been invoked). Methods: .A nonlocal non-LTE radiative transfer code including dust and cosmic background illumination adapted to the Horsehead geometry has been developed to carefuly analyse the CS, C34S, HCS+ and C18O rotational line emission. We use this model to consistently link the line observations with photochemical models to determine the CS/HCS^+/S/S+ structure of the PDR. Results: .Densities of n(H_2)?(0.5{-}1.0) × 105 cm-3 are required to reproduce the CS and C34S J = 2-1 and 3-2 line emission. CS J = 5-4 lines show narrower line widths than the CS low-J lines and require higher density gas components not resolved by the 10'' IRAM-30m beam. These values are larger than previous estimates based in CO observations. We found ?(CS) = (7 ± 3) × 10-9 and ?(HCS+) = (4 ± 2) × 10-11 as the averaged abundances in the PDR. According to photochemical models, the gas phase sulfur abundance required to reproduce these values is S/H = (3.5 ± 1.5) × 10-6, only a factor ?4 less abundant than the solar sulfur elemental abundance. Since only lower limits to the gas temperature are constrained, even lower sulfur depletion values are possible if the gas is significantly warmer. Conclusions: .The combination of CS, C34S and HCS+ observations together with the inclusion of the most recent CS collisional and chemical rates in our models implies that sulfur depletion invoked to account for CS and HCS+ abundances is much smaller than in previous studies