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Aims. Our goal is to complete the inventory of S-bearing molecules and theirabundances in the prototypical photodissociation region (PDR) the Horseheadnebula to gain insight into sulphur chemistry in UV irradiated regions. Basedon the WHISPER millimeter (mm) line survey, our goal is to provide an improvedand more accurate description of sulphur species and their abundances towardsthe core and PDR positions in the Horsehead.  Methods. The Monte Carlo Markov chain (MCMC) methodology and the molecularexcitation and radiative transfer code RADEX were used to explore the parameterspace and determine physical conditions and beam-averaged molecular abundances.  Results. A total of 13 S-bearing species (CS, SO, SO2, OCS, H2CS - both orthoand para - HDCS, C2S, HCS+, SO+, H2S, S2H, NS and NS+) have been detected inthe two targeted positions. This is the first detection of SO+ in the Horseheadand the first detection of NS+ in any PDR. We find a differentiated chemicalbehaviour between C-S and O-S bearing species within the nebula. The C-Sbearing species C2S and o-H2CS present fractional abundances a factor graterthan two higher in the core than in the PDR. In contrast, the O-S bearingmolecules SO, SO2, and OCS present similar abundances towards both positions. Afew molecules, SO+, NS, and NS+, are more abundant towards the PDR than towardsthe core, and could be considered as PDR tracers.  Conclusions. This is the first complete study of S-bearing species towards aPDR. Our study shows that CS, SO, and H2S are the most abundant S-bearingmolecules in the PDR with abundances of a few 1E-9. We recall that SH, SH+, S,and S+ are not observable at the wavelengths covered by the WHISPER survey. Atthe spatial scale of our observations, the total abundance of S atoms locked inthe detected species is < 1E-8, only ~0.1% of the cosmic sulphur abundance.

Abundances of sulphur molecules in the Horsehead nebula. First NS+ detection in a photodissociation region