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, C 34 S 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, C 34 S, HCS + and C 18 O 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) × 10 5 cm −3 are required to reproduce the CS and C 34 S 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 χ(CS) = (7 ± 3) × 10 −9 and χ(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, C 34 S 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.Key words. astrochemistry -ISM: clouds -ISM: molecules -ISM: individual objects: Horsehead nebula -radio lines: ISMradiative transfer IntroductionSulfur is an abundant element (the solar photosphere abundance is S/H = 1.38 × 10 −5 ; Asplund et al. 2005), which remains undepleted in diffuse interstellar gas (e.g. Howk et al. 2006) and HII regions (e.g. Martín-Hernández et al. 2002;García-Rojas et al. 2006, and references therein) but it is historically assumed to deplete on grains in higher density molecular clouds by factors as large as ∼10 3 (Tieftrunk et al. 1994). This conclusion is simply reached by adding up the observed gas phase abundances of S-bearing molecules in well known dark clouds such as TMC1 (e.g. Irvine et al. 1985;Ohishi & Kaifu 1998). As Appendix A is only available in electronic form at http://www.edpsciences.org sulfur is easily ionized (ioniz...

Aims. The comparative study of several molecular species at the origin of the gas phase chemistry in the diffuse interstellar medium (ISM) is a key input in unraveling the coupled chemical and dynamical evolution of the ISM. Methods. The lowest rotational lines of HCO + , HCN, HNC, and CN were observed at the IRAM-30m telescope in absorption against the λ3 mm and λ1.3 mm continuum emission of massive star-forming regions in the Galactic plane. The absorption lines probe the gas over kiloparsecs along these lines of sight. The excitation temperatures of HCO + are inferred from the comparison of the absorptions in the two lowest transitions. The spectra of all molecular species on the same line of sight are decomposed into Gaussian velocity components. Most appear in all the spectra of a given line of sight. For each component, we derived the central opacity, the velocity dispersion, and computed the molecular column density. We compared our results to the predictions of UV-dominated chemical models of photodissociation regions (PDR models) and to those of non-equilibrium models in which the chemistry is driven by the dissipation of turbulent energy (TDR models). )= 18 ± 9. These ratios are similar to those inferred from observations of high Galactic latitude lines of sight, suggesting that the gas sampled by absorption lines in the Galactic plane has the same chemical properties as that in the Solar neighbourhood. The FWHM of the Gaussian velocity components span the range 0.3 to 3 km s −1 and those of the HCO + lines are found to be 30% broader than those of CN-bearing molecules. The PDR models fail to reproduce simultaneously the observed abundances of the CN-bearing species and HCO + , even for high-density material (100 cm −3 < n H < 10 4 cm −3 ). The TDR models, in turn, are able to reproduce the observed abundances and abundance ratios of all the analysed molecules for the moderate gas densities (30 cm −3 < n H < 200 cm −3 ) and the turbulent energy observed in the diffuse interstellar medium. Conclusions. Intermittent turbulent dissipation appears to be a promising driver of the gas phase chemistry of the diffuse and translucent gas throughout the Galaxy. The details of the dissipation mechanisms still need to be investigated.

Context. Isolated starless cores within molecular clouds can be used as a testbed to investigate the conditions prior to the onset of fragmentation and gravitational proto-stellar collapse. Aims. We aim to determine the distribution of the dust temperature and the density of the starless core B68. Methods. In the framework of the Herschel guaranteed-time key programme "The Earliest Phases of Star formation" (EPoS), we have imaged B68 between 100 and 500 μm. Ancillary data at (sub)millimetre wavelengths, spectral line maps of the 12 CO (2-1), and 13 CO (2-1) transitions, as well as an NIR extinction map were added to the analysis. We employed a ray-tracing algorithm to derive the 2D mid-plane dust temperature and volume density distribution without suffering from the line-of-sight averaging effects of simple SED fitting procedures. Additional 3D radiative transfer calculations were employed to investigate the connection between the external irradiation and the peculiar crescent-shaped morphology found in the FIR maps. Results. For the first time, we spatially resolve the dust temperature and density distribution of B68, convolved to a beam size of 36. 4. We find a temperature gradient dropping from (16.7 ) × 10 5 cm −3 . B68 has a mass of 3.1 M of material with A K > 0.2 mag for an assumed distance of 150 pc. We detect a compact source in the southeastern trunk, which is also seen in extinction and CO. At 100 and 160 μm, we observe a crescent of enhanced emission to the south. Conclusions. The dust temperature profile of B68 agrees well with previous estimates. We find the radial density distribution from the edge of the inner plateau outward to be n H ∝ r −3.5 . Such a steep profile can arise from either or both of the following: external irradiation with a significant UV contribution or the fragmentation of filamentary structures. Our 3D radiative transfer model of an externally irradiated core by an anisotropic ISRF reproduces the crescent morphology seen at 100 and 160 μm. Our CO observations show that B68 is part of a chain of globules in both space and velocity, which may indicate that it was once part of a filament that dispersed. We also resolve a new compact source in the southeastern trunk and find that it is slightly shifted in centroid velocity from B68, lending qualitative support to core collision scenarios.