In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH + , H 2 O + , and H 3 O + begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H 2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζ H) and molecular hydrogen fraction (f H 2). We present observations targeting transitions of OH + , H 2 O + , and H 3 O + made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH + and H 2 O + are detected in absorption in multiple velocity components along every sight line, but H 3 O + is only detected along 7 sight lines. From the molecular abundances we compute f H 2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH + and H 2 O + primarily reside in gas with low H 2 fractions. We also infer ζ H throughout our sample, and find a lognormal distribution with mean log(ζ H) = −15.75 (ζ H = 1.78 × 10 −16 s −1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H + 3 observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.

Aims. Supersonic turbulence is a large reservoir of suprathermal energy in the interstellar medium. Its dissipation, because it is intermittent in space and time, can deeply modify the chemistry of the gas. This is clearly seen in the framework of shock chemistry. Intense turbulent dissipation also occurs in regions of large velocity shears, sharing with shocks the property of intermittency. Whether these bursts of dissipation, short-lived and localized, have a measurable impact on molecular abundances in the diffuse medium, and how the chemical enrichment they drive compares to observations, are the questions we address here. Methods. We further explore a hybrid method to compute the chemical and thermal evolution of a magnetized dissipative structure, under the energetic constraints provided by the observed properties of turbulence in the cold neutral medium. For the first time, we model a random line of sight by taking into account the relative duration of the bursts with respect to the thermal and chemical relaxation timescales of the gas. The key parameter is the turbulent rate of strain a due to the ambient turbulence. With the gas density, it controls the size of the dissipative structures, therefore the strength of the burst. It also sets the relative importance of viscous dissipation and ion-neutral friction in the gas heating and chemical enrichment. Results. For a large range of rates of strain and densities, the models of turbulent dissipation regions (TDR) reproduce the CH+ column densities observed in the diffuse medium and their correlation with highly excited H 2 . They do so without producing an excess of CH. As a natural consequence, they reproduce the abundance ratios of HCO + /OH and HCO + /H 2 O, and their dynamic range of about one order of magnitude observed in diffuse gas. Large C 2 H and CO abundances, also related to those of HCO + , are another outcome of the TDR models that compare well with observed values. Neutral carbon exceeds the abundance expected at ionization equilibrium, in agreement with fine-structure line observations. The abundances and column densities computed for CN, HCN and HNC are one order of magnitude above PDR model predictions, although still significantly smaller than observed values. The dependence of our results on the rate of strain and density reveals that the chemical enhancements are in better agreement with observations if the dissipation is dominated by ion-neutral friction, involving shear structures of thickness ∼100 AU.

Aims. The HIFI instrument onboard Herschel has allowed high spectral resolution and sensitive observations of ground-state transitions of three molecular ions: the methylidyne cation CH + , its isotopologue 13 CH + , 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 + , 13 CH + , 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 + , 13 CH + , 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 + , 13 CH + , 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 13 CH + . The linewidth distributions of the CH + , SH + , and 13 CH + Gaussian components are found to be similar. These distributions have the same mean ( Δυ ∼ 4.2 km s −1 ) and standard deviation (σ(Δυ) ∼ 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 + , 13 CH + , 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.

Context. Large-scale motions in galaxies (supernovae explosions, galaxy collisions, galactic shear etc.) generate turbulence, which allows a fraction of the available kinetic energy to cascade down to small scales before it is dissipated. Aims. We establish and quantify the diagnostics of turbulent dissipation in mildly irradiated diffuse gas in the specific context of shock structures. Methods. We incorporated the basic physics of photon-dominated regions into a state-of-the-art steady-state shock code. We examined the chemical and emission properties of mildly irradiated (G 0 = 1) magnetised shocks in diffuse media (n H = 10 2 to 10 4 cm −3 ) at lowto moderate velocities (from 3 to 40 km s −1 ). Results. The formation of some molecules relies on endoergic reactions. Their abundances in J-type shocks are enhanced by several orders of magnitude for shock velocities as low as 7 km s −1 . Otherwise most chemical properties of J-type shocks vary over less than an order of magnitude between velocities from about 7 to about 30 km s −1 , where H 2 dissociation sets in. C-type shocks display a more gradual molecular enhancement with increasing shock velocity. We quantified the energy flux budget (fluxes of kinetic, radiated and magnetic energies) with emphasis on the main cooling lines of the cold interstellar medium. Their sensitivity to shock velocity is such that it allows observations to constrain statistical distributions of shock velocities. We fitted various probability distribution functions (PDFs) of shock velocities to spectroscopic observations of the galaxy-wide shock in Stephan's Quintet and of a Galactic line of sight which samples diffuse molecular gas in Chamaeleon. In both cases, low velocities bear the greatest statistical weight and the PDF is consistent with a bimodal distribution. In the very low velocity shocks (below 5 km s −1 ), dissipation is due to ion-neutral friction and it powers H 2 low-energy transitions and atomic lines. In moderate velocity shocks (20 km s −1 and above), the dissipation is due to viscous heating and accounts for most of the molecular emission. In our interpretation a significant fraction of the gas in the line of sight is shocked (from 4% to 66%). For example, C + emission may trace shocks in UV irradiated gas where C + is the dominant carbon species. Conclusions. Low-and moderate velocity shocks are important in shaping the chemical composition and excitation state of the interstellar gas. This allows one to probe the statistical distribution of shock velocities in interstellar turbulence.

Multi-phase filamentary structures around Brightest Cluster Galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in 3 cool cluster cores: Centaurus, Abell S1101, and RXJ1539.5 and gathered ALMA (Atacama Large Millimeter/submillimeter Array) and MUSE (Multi Unit Spectroscopic Explorer) data for 12 other clusters. Those observations show clumpy, massive and long, 3-25 kpc, molecular filaments, preferentially located around the radio bubbles inflated by the AGN (Active Galactic Nucleus). Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the Hα/CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 to 7 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100-400 km s −1 , with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (as yet) unresolved filaments. The velocity fields may be stirred by turbulence induced by bubbles, jets or merger-induced sloshing. Velocity and dispersions are low, below the escape velocity. Cold clouds should eventually fall back and fuel the AGN. We compare the filament's radial extent, r fil , with the region where the X-ray gas can become thermally unstable. The filaments are always inside the low-entropy and short cooling time region, where t cool /t ff <20 (9 of 13 sources). The range t cool /t ff , 8-23 at r fil , is likely due to (i) a more complex gravitational potential affecting the free-fall time t ff (sloshing, mergers. . . ); (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time t cool . For some of the sources, r fil lies where the ratio of the cooling time to the eddy-turnover time, t cool /t eddy , is approximately unity.

Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH + and discuss its relevance in the context of hydride absorption in diffuse gas with a low H 2 fraction. The confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with different H 2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. , and HF column densities promises to be a faithful tracer of the distribution of the H 2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD) simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH + molecule is a better tracer of almost purely atomic hydrogen gas than H itself, since H can also be present in gas with a significant molecular content, but ArH + singles out gas that is >99.9% atomic.

Context. Tens of light hydrides and small molecules have now been detected over several hundreds sightlines sampling the diffuse interstellar medium (ISM) in both the solar neighbourhood and the inner Galactic disk. They provide unprecedented statistics on the first steps of chemistry in the diffuse gas. Aims. These new data confirm the limitations of the traditional chemical pathways driven by the UV photons and the cosmic rays (CR) and the need for additional energy sources, such as turbulent dissipation, to open highly endoenergetic formation routes. The goal of the present paper is to further investigate the link between specific species and the properties of the turbulent cascade in particular its space-time intermittency. Methods. We have analysed ten different atomic and molecular species in the framework of the updated model of turbulent dissipation regions (TDR). We study the influence on the abundances of these species of parameters specific to chemistry (density, UV field, and CR ionisation rate) and those linked to turbulence (the average turbulent dissipation rate, the dissipation timescale, and the ion-neutral velocity drift in the regions of dissipation). Results. The most sensitive tracers of turbulent dissipation are the abundances of CH + and SH + , and the column densities of the J = 3, 4, 5 rotational levels of H 2 . The abundances of CO, HCO + , and the intensity of the 158 μm [CII] emission line are significantly enhanced by turbulent dissipation. The vast diversity of chemical pathways allows the independent determinations of free parameters never estimated before: an upper limit to the average turbulent dissipation rate, ε 10 −23 erg cm −3 s −1 for n H = 20 cm −3 , from the CH + abundance; an upper limit to the ion-neutral velocity drift, υ in 3.5 km s −1 , from the SH + to CH + abundance ratio; and a range of dissipation timescales, 100 τ V 1000 yr, from the CO to HCO + abundance ratio. For the first time, we reproduce the large abundances of CO observed on diffuse lines of sight, and we show that CO may be abundant even in regions with UV-shieldings as low as 5 × 10 −3 mag. The best range of parameters also reproduces the abundance ratios of OH, C 2 H, and H 2 O to HCO + and are consistent with the known properties of the turbulent cascade in the Galactic diffuse ISM. Conclusions. Our results disclose an unexpected link between the dissipation of turbulence and the emergence of molecular richness in the diffuse ISM. Some species, such as CH + or SH + , turn out to be unique tracers of the energy trail in the ISM. In spite of some degeneracy, the properties of the turbulent cascade, down to dissipation, can be captured through specific molecular abundances.

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.