Comparative study of CH+ and SH+ absorption lines observed towards
  distant star-forming regions
  Aims. The HIFI instrument onboard Herschel has allowed high spectral
resolution and sensitive observations of ground-state transi- tions of three
molecular ions: the methylidyne cation CH+, its isotopologue 13CH+, and
sulfanylium SH+. Because of their unique chemical properties, a comparative
analysis of these cations provides essential clues to the link between the
chemistry and dynamics of the diffuse interstellar medium. Methods. The CH+,
13CH+, and SH+ lines are observed in absorption towards the distant high-mass
star-forming regions (SFRs) DR21(OH), G34.3+0.1, W31C, W33A, W49N, and W51, and
towards two sources close to the Galactic centre, SgrB2(N) and SgrA*+50. All
sight lines sample the diffuse interstellar matter along pathlengths of several
kiloparsecs across the Galactic Plane. In order to compare the velocity
structure of each species, the observed line profiles were deconvolved from the
hyperfine structure of the SH+ transition and the CH+, 13CH+, and SH+ spectra
were independently decomposed into Gaussian velocity components. To analyse the
chemical composition of the foreground gas, all spectra were divided, in a
second step, into velocity intervals over which the CH+, 13CH+, and SH+ column
densities and abundances were derived. Results. SH+ is detected along all
observed lines of sight, with a velocity structure close to that of CH+ and
13CH+. The linewidth distributions of the CH+, SH+, and 13CH+ Gaussian
components are found to be similar. These distributions have the same mean
(<\delta\u{psion}> ~ 4.2 km s-1) and standard deviation
(\sigma(\delta\u{psion}) ~ 1.5 km s-1). This mean value is also close to that
of the linewidth distribution of the CH+ visible transitions detected in the
solar neighbourhood. We show that the lack of absorption components narrower
than 2 km s-1 is not an artefact caused by noise: the CH+, 13CH+, and SH+ line
profiles are therefore statistically broader than those of most species
detected in absorption in diffuse interstellar gas (e. g. HCO+, CH, or CN). The
SH+/CH+ column density ratio observed in the components located away from the
Galactic centre spans two orders of magnitude and correlates with the CH+
abundance. Conversely, the ratio observed in the components close to the
Galactic centre varies over less than one order of magnitude with no apparent
correlation with the CH+ abundance. The observed dynamical and chemical
properties of SH+ and CH+ are proposed to trace the ubiquitous process of
turbulent dissipation, in shocks or shears, in the diffuse ISM and the specific
environment of the Galactic centre regions.
