Abundances of sulphur molecules in the Horsehead nebula. First NS+
  detection in a photodissociation region
  Aims. Our goal is to complete the inventory of S-bearing molecules and their
abundances in the prototypical photodissociation region (PDR) the Horsehead
nebula to gain insight into sulphur chemistry in UV irradiated regions. Based
on the WHISPER millimeter (mm) line survey, our goal is to provide an improved
and more accurate description of sulphur species and their abundances towards
the core and PDR positions in the Horsehead.
  Methods. The Monte Carlo Markov chain (MCMC) methodology and the molecular
excitation and radiative transfer code RADEX were used to explore the parameter
space and determine physical conditions and beam-averaged molecular abundances.
  Results. A total of 13 S-bearing species (CS, SO, SO2, OCS, H2CS - both ortho
and para - HDCS, C2S, HCS+, SO+, H2S, S2H, NS and NS+) have been detected in
the two targeted positions. This is the first detection of SO+ in the Horsehead
and the first detection of NS+ in any PDR. We find a differentiated chemical
behaviour between C-S and O-S bearing species within the nebula. The C-S
bearing species C2S and o-H2CS present fractional abundances a factor grater
than two higher in the core than in the PDR. In contrast, the O-S bearing
molecules SO, SO2, and OCS present similar abundances towards both positions. A
few molecules, SO+, NS, and NS+, are more abundant towards the PDR than towards
the core, and could be considered as PDR tracers.
  Conclusions. This is the first complete study of S-bearing species towards a
PDR. Our study shows that CS, SO, and H2S are the most abundant S-bearing
molecules 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. At
the spatial scale of our observations, the total abundance of S atoms locked in
the detected species is < 1E-8, only ~0.1% of the cosmic sulphur abundance.
