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Context: Atomic and molecular lines in galaxies offer insights into energy budgets and feedback mechanisms. Aims: This study establishes a new framework for interpreting these lines and deducing energy budgets from observations. Methods: Atomic and molecular lines detected in a given object are assumed to result from the combination of distributions of shocks and photo-dissociation regions (PDR). Using the Paris-Durham shock code and the Meudon PDR code, emissions are computed over a wide range of parameters. Total emissions are calculated using probability distribution functions, with a defined distance metric based on observed and predicted intensity ratios. Results: We analyze the radio galaxy 3C 326 N, finding both shocks and PDRs necessary to explain the line fluxes. Viable solutions occur only at low densities ($\rm n_H < 100 cm^{-3}$), indicating emission from diffuse interstellar matter. The optimal solution involves low-velocity shocks (5-20 km/s) in PDRs illuminated by UV radiation ten times stronger than in the solar neighborhood. The H$2$ 0-0 S(0) $28 \mu$m, [CII] $158 \mu$m, and [OI] $63 \mu$m lines originate from PDRs, while other H$2$ lines mostly come from shocks. The reprocessed radiative and mechanical energies are $\rm {L_{UV} = 6.3\times10^9 L\odot}$ and $\rm {L_K = 3.9\times10^8 L_\odot}$, respectively, in agreement with 3C 326 N's infrared luminosity, and consistent with 1% of the AGN jet kinetic power dissipated in the interstellar medium. Conclusions: This study demonstrates that the radiative and mechanical energy budgets of galaxies can be derived from observations of atomic and molecular lines alone. It highlights the unexpected importance of the diffuse medium for 3C 326 N. Comparison with new JWST data for 3C 326 N shows striking agreement, opening new prospects for predicting and interpreting extragalactic observations., Comment: Accepted for publication in A&A, 22 pages, 15 figures

(Abridged) H2 is the most abundant molecule in the Universe. Thanks to its widely spaced energy levels, it predominantly lights up in warm gas, T > 100 K, such as shocked regions, and it is one of the key targets of JWST observations. These include shocks from protostellar outflows, all the way up to starburst galaxies and AGN. Shock models are able to simulate H2 emission. We aim to explore H2 excitation using such models, and to test over which parameter space distinct signatures are produced in H2 emission. We present simulated H2 emission using the Paris-Durham shock code over an extensive grid of 14,000 plane-parallel stationary shock models, a large subset of which are exposed to an external UV radiation field. The grid samples 6 input parameters: preshock density, shock velocity, transverse magnetic field strength, UV radiation field strength, cosmic-ray-ionization rate, and PAH abundance. Physical quantities, such as temperature, density, and width, have been extracted along with H2 integrated line intensities. The strength of the transverse magnetic field, set by the scaling factor, b, plays a key role in the excitation of H2. At low values of b (<~ 0.3, J-type shocks), H2 excitation is dominated by vibrationally excited lines; at higher values (b >~ 1, C-type shocks), rotational lines dominate the spectrum for shocks with an external radiation field comparable to (or lower than) the solar neighborhood. Shocks with b >= 1 can be spatially resolved with JWST for nearby objects. When the input kinetic energy flux increases, the excitation and integrated intensity of H2 increases similarly. An external UV field mainly serves to increase the excitation, particularly for shocks where the input radiation energy is comparable to the input kinetic energy flux. These results provide an overview of the energetic reprocessing of input kinetic energy flux and the resulting H2 line emission., Comment: Published in A&A

As a complement to the HyGAL Stratospheric Observatory for Infrared Astronomy Legacy Program, we report the results of a ground-based absorption line survey of simple molecules in diffuse and translucent Galactic clouds. Using the Institut de Radioastronomie Millim\'etrique (IRAM) 30 m telescope, we surveyed molecular lines in the 2 mm and 3 mm wavelength ranges toward 15 millimeter continuum sources. These sources, which are all massive star-forming regions located mainly in the first and second quadrants of the Milky Way, form the subset of the HyGAL sample that can be observed by the IRAM 30 m telescope. We detected HCO$^+$ absorption lines toward 14 sightlines, toward which we identified 78 foreground cloud components, as well as lines from HCN, HNC, C$_2$H, and c-C$_3$H$_2$ toward most sightlines. In addition, CS and H$_2$S absorption lines are found toward at least half of the continuum sources. Static Meudon photodissociation region (PDR) isobaric models that consider ultraviolet-dominated chemistry were unable to reproduce the column densities of all seven molecular species by just a factor of a few, except for H$_2$S. The inclusion of other formation routes driven by turbulent dissipation could possibly explain the observed high column densities of these species in diffuse clouds. There is a tentative trend for H$_2$S and CS abundances relative to H$_2$ to be larger in diffuse clouds ($X$(H$_2$S) and $X$(CS) $\sim 10^{-8} - 10^{-7}$) than in translucent clouds ($X$(H$_2$S) and $X$(CS) $\sim 10^{-9} - 10^{-8}$) toward a small sample; however, a larger sample is required in order to confirm this trend. The derived H$_2$S column densities are higher than the values predicted from the isobaric PDR models, suggesting that chemical desorption of H$_2$S from sulfur-containing ice mantles may play a role in increasing the H$_2$S abundance., Comment: 41 pages, 25 figures, 10 tables, and 5 appendices. Accepted for publication in A&A

The HyGAL SOFIA legacy program surveys six hydride molecules -- ArH+, OH+, H2O+, SH, OH, and CH -- and two atomic constituents -- C+ and O -- within the diffuse interstellar medium (ISM) by means of absorption-line spectroscopy toward 25 bright Galactic background continuum sources. This detailed spectroscopic study is designed to exploit the unique value of specific hydrides as tracers and probes of different phases of the ISM, as demonstrated by recent studies with the Herschel Space Observatory. The observations performed under the HyGAL program will allow us to address several questions related to the lifecycle of molecular material in the ISM and the physical processes that impact its phase transition, such as: (1) What is the distribution function of the H2 fraction in the ISM? (2) How does the ionization rate due to low-energy cosmic-rays vary within the Galaxy? (3) What is the nature of interstellar turbulence, and what mechanisms lead to its dissipation? This overview discusses the observing strategy, synergies with ancillary and archival observations, the data reduction and analysis schemes adopted; and presents the first results obtained toward three of the survey targets, W3(OH), W3IRS5 and NGC7538IRS1. Robust measurements of the column densities of these hydrides -- obtained through widespread observations of absorption lines-- help address the questions raised, and there is a timely synergy between these observations and the development of theoretical models, particularly pertaining to the formation of H2 within the turbulent ISM. The provision of enhanced HyGAL data products will therefore serve as a legacy for future ISM studies., Comment: 33 pages, 17 figures, 7 tables. Accepted for publication in the Astrophysical Journal

The origin of the cold gas in central galaxies in groups is still a matter of debate. We present Multi-Unit Spectroscopic Explorer (MUSE) observations of 18 optically selected local Brightest Group Galaxies (BGGs) to study the kinematics and distribution of the optical emission-line gas. MUSE observations reveal a distribution of gas morphologies including ten complex networks of filaments extending up to 10 kpc to two compact (<3 kpc) and five extended (>5 kpc) disk-dominated structures. Some rotating disks show rings and elongated structures arising from the central disk. The kinematics of the stellar component is mainly rotation-dominated, which is very different from the disturbed kinematics and distribution found in the filamentary sources. The ionized gas is kinematically decoupled from the stellar component for most systems, suggesting an external origin for the gas. We also find that the Halpha luminosity correlates with the cold molecular mass. By exploring the thermodynamical properties of the hot atmospheres, we find that the filamentary sources and compact disks are found in systems with small central entropy values and tcool/teddy ratios. This suggests that, like for Brightest Cluster Galaxies in cool core clusters, the ionized gas are likely formed from hot halo gas condensations, consistently with the Chaotic Cold Accretion simulations (as shown via the C-ratio, Tat, and k-plot). We note that gaseous rotating disks are more frequent than in BCGs. An explanation for the origin of the gas in those objects is a contribution to gas fueling by mergers or group satellites, as qualitatively hinted by some sources of the present sample. Nonetheless, we discuss the possibility that some extended disks could also be a transition stage in an evolutionary sequence including filaments, extended disks and compact disks, as described by hot gas condensation models of cooling flows., Comment: 38 pages, and 16 figures. Resubmitted to A&A after referee report

As material from an infalling protostellar envelope hits the forming disk, an accretion shock may develop which could (partially) alter the envelope material entering the disk. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that emission originating from warm SO and SO$_2$ might be good tracers of such accretion shocks. The goal of this work is to test under what shock conditions the abundances of gas-phase SO and SO$_2$ increase in an accretion shock at the disk-envelope interface. Detailed shock models including gas dynamics are computed using the Paris-Durham shock code for non-magnetized J-type accretion shocks in typical inner envelope conditions. The effect of pre-shock density, shock velocity, and strength of the ultraviolet (UV) radiation field on the abundance of warm SO and SO$_2$ is explored. Warm gas-phase chemistry is efficient in forming SO under most J-type shock conditions considered. In lower-velocity (~3 km/s) shocks, the abundance of SO is increased through subsequent reactions starting from thermally desorbed CH$_4$ toward H$_2$CO and finally SO. In higher velocity (>4 km/s) shocks, both SO and SO$_2$ are formed through reactions of OH and atomic S. The strength of the UV radiation field is crucial for SO and in particular SO$_2$ formation through the photodissociation of H$_2$O. Thermal desorption of SO and SO$_2$ ice is only relevant in high-velocity (>5 km/s) shocks at high densities ($>10^7$ cm$^{-3}$). Warm emission from SO and SO$_2$ is a possible tracer of accretion shocks at the disk-envelope interface as long as a local UV field is present. Additional observations with ALMA at high-angular resolution could provide further constraints. Moreover, the James Webb Space Telescope will give access to other possible slow, dense shock tracers such as H$_2$, H$_2$O, and [S I] 25$\mu$m., Comment: Accepted to A&A, 18 pages, 17 figures

We present a comparative analysis of the $\rm CH^+$(1-0) and $\rm Ly \alpha$ lines, observed with the Atacama Large Millimeter Array (ALMA) and Keck telescope respectively, in the field of the submillimetre-selected galaxy (SMG) SMM\,J02399$-$0136 at $z\sim2.8$, which comprises a heavily obscured starburst galaxy and a broad absorption line quasar, immersed in a large $\rm Ly \alpha$ nebula. This comparison highlights the critical role played by turbulence in channeling the energy across gas phases and scales, splitting the energy trail between hot/thermal and cool/turbulent phases in the circum-galactic medium (CGM). The unique chemical and spectroscopic properties of $\rm CH^+$ are used to infer the existence of a massive ($\sim 3.5 \times 10^{10}$ ${\rm M}_\odot$), highly turbulent reservoir of diffuse molecular gas of radius $\sim 20\,$kpc coinciding with the core of the $\rm Ly \alpha$ nebula. The whole cool and cold CGM is shown to be inflowing towards the galaxies at a velocity $\sim$ 400 km$\,s^{-1}$. Several kpc-scale shocks are detected tentatively in $\rm CH^+$ emission. Their specific location in space and velocity with respect to the high-velocity $\rm Ly \alpha$ emission suggests that they lie at the interface of the inflowing CGM and the high-velocity $\rm Ly \alpha$ emission, and signpost the feeding of CGM turbulence by AGN- and stellar-driven outflows. The mass and energy budgets of the CGM require net mass accretion at a rate commensurate with the star formation rate (SFR). From this similarity, we infer that the merger-driven burst of star formation and black-hole growth are ultimately fuelled by large-scale gas accretion., Comment: 24 pages, 17 figures. Accepted for publication in MNRAS

The excitation of the filamentary gas structures surrounding giant elliptical galaxies at the center of cool-core clusters, a.k.a BCGs (brightest cluster galaxies), is key to our understanding of active galactic nucleus feedback, and of the impact of environmental and local effects on star formation. We investigate the contribution of the thermal radiation from the cooling flow surrounding BCGs to the excitation of the filaments. We explore the effects of small levels of extra-heating (turbulence), and of metallicity, on the optical and infrared lines. Using the Cloudy code, we model the photoionization and photodissociation of a slab of gas of optical depth AV{\leq}30mag at constant pressure, in order to calculate self-consistently all of the gas phases, from ionized gas to molecular gas. The ionizing source is the EUV and soft X-ray radiation emitted by the cooling gas. We test these models comparing their predictions to the rich multi-wavelength observations, from optical to submillimeter. These models reproduce most of the multi-wavelength spectra observed in the nebulae surrounding the BCGs, not only the LINER-like optical diagnostics: [O iii]{\lambda} 5007 {\AA}/H\b{eta}, [N ii]{\lambda} 6583 {\AA}/H{\alpha} and ([S ii]{\lambda} 6716 {\AA}+[S ii]{\lambda} 6731 {\AA})/H{\alpha} but also the infrared emission lines from the atomic gas. The modeled ro-vib H2 lines also match observations, which indicates that near and mid-IR H2 lines are mostly excited by collisions between H2 molecules and secondary electrons produced naturally inside the cloud by the interaction between the X-rays and the cold gas in the filament. However, there is still some tension between ionized and molecular line tracers (i.e. CO), which requires to optimize the cloud structure and the density of the molecular zone., Comment: 16 pages, 9 figures, 3 tables. Accepted for publication in A&A

Shocks are ubiquitous in the interstellar and intergalactic media, where their chemical and radiative signatures reveal the physical conditions in which they arise. Detailed astrochemical models of shocks at all velocities are necessary to understand the physics of many environments including protostellar outflows, supernova remnants, and galactic outflows. We present an accurate treatment of the self-generated UV radiation in intermediate velocity, stationary, weakly magnetised, J-type, molecular shocks. Shock solutions computed with the Paris-Durham shock code are post-processed using a multi-level accelerated $\Lambda$-iteration radiative transfer algorithm to compute Ly$\alpha$, Ly$\beta$, and 2-photon continuum emission. The subsequent impacts on the ionisation and dissociation of key atomic and molecular species as well as on the heating by the photoelectric effect take the wavelength dependent cross-sections and the fluid velocity profile into account. We analyse shock models with velocities $V=25-60$ km/s, propagating in dense ($n \geq 10^4$ ${\rm cm}^{-3}$), shielded gas. Self-absorption traps Ly$\alpha$ photons in a small region in the shock, though a large fraction escapes into the line wings. We find a critical velocity $V\sim 30$ km/s above which shocks produce a Ly$\alpha$ photon flux exceeding that of the standard ISRF. The escaping photons generate a warm slab (T~100 K) ahead of the shock as well as pre-ionise C and S. These shocks are traced by bright atomic fine structure (e.g. O and S) and metastable (e.g. O and C) lines, substantive molecular emission (e.g. H2, OH, and CO), enhanced column densities of several species (e.g. CH+ and HCO+), as well as a severe destruction of H2O. As much as 13-21% of the initial kinetic energy of the shock escapes in Ly$\alpha$ and Ly$\beta$ photons if the dust opacity in the radiative precursor allows it., Comment: Accepted for publication in A&A, 16 main body pages, 8 appendices pages

Recent ALMA observations suggest that the highest velocity part of molecular protostellar jets are launched from the dust-sublimation regions of the accretion disks (<0.3 au). However, formation and survival of molecules in inner protostellar disk winds, in the presence of a harsh FUV radiation field and the absence of dust, remain unexplored. We aim at determining if simple molecules can be synthesized and spared in fast and collimated dust-free disk winds or if a fraction of dust is necessary to explain the observed molecular abundances. This work is based on the Paris-Durham shock code designed to model irradiated environments. Fundamental properties of the dust-free chemistry are investigated from single point models. A laminar 1D disk wind model is then built using a parametric flow geometry. This model includes time-dependent chemistry and the attenuation of the radiation field by gas-phase photoprocesses. We show that a small fraction of H2 (< 1e-2), primarily formed through the H- route, can efficiently initiate molecule synthesis such as CO and SiO above TK ~ 800 K. The attenuation of the radiation field by atomic species (eg. C, Si, S) proceeds through continuum self-shielding. This process ensures efficient formation of CO, OH, SiO, H2O through neutral-neutral reactions, and the survival of these molecules. Class 0 dust-free winds with high mass-loss rates ($\dot{M}_w >$ 2e-6 Msun/yr) are predicted to be rich in molecules if warm (TK > 800 K). The molecular content of disk winds is very sensitive to the presence of dust and a mass-fraction of surviving dust as small as 1e-5 significantly increases the H2O and SiO abundances. Chemistry of high-velocity jets is a powerful tool to probe their content in dust and uncover their launching point. Models of internal shocks are required to fully exploit the current (sub-)millimeter observations and prepare future JWST observations., Comment: accepted for publication in A&A

(abridged) The far-UV wavelength range (912-2000A) provides access to atomic and molecular transitions of many species the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium, within phases spanning a wide range of ionization, density, temperature, and molecular gas fraction. Far-UV space telescopes have enabled detailed studies of the ISM in the Milky Way thanks to absorption features appearing in the UV spectra of hot stars and yielding fundamental insights into the composition and physical characteristics of all phases of the ISM along with the processes that influence them. However, we have yet to design a spectrometer able to observe the full UV domain at resolving power R>10^5 with a signal-to-noise ratio SNR>500. Such a resolution is necessary to resolve lines from both the cold molecular hydrogen and the warm metal ions with a turbulent velocity of about 1 km s-1, and to differentiate distinct velocity components. Future UV spectroscopic studies of the Milky Way ISM must revolutionize our understanding of the ISM as a dynamical, unstable, and magnetized medium, and rise to the challenge brought forward by current theories. Another interesting prospect is to transpose the same level of details that has been reached for the Milky Way to the ISM in external galaxies, in particular in metal-poor galaxies, where the ISM chemical composition, physical conditions, and topology change dramatically, with significant consequences on the star-formation properties. Finally, we need to be able to perform statistical analyses of background quasar lines of sight intersecting the CGM of galaxies at various redshifts and to comprehend the role of gas exchanges and flows for galaxy evolution., Comment: This ESA white paper draws from and expands on the following white papers previously submitted for the Astro 2020 Decadal Survey, Gry et al. (2019), Lebouteiller et al. (2019), and Yan et al. (2019). The original ESA white paper was reformatted as a 2-column paper for the present document. (arXiv admin note: substantial text overlap with arXiv:1903.07295, arXiv:1903.06250, arXiv:1903.11065)

Multi-phase filamentary structures around Brightest Cluster Galaxies 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 and MUSE 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 H$\alpha$/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 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 (e.g., sloshing, mergers); (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time. 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., Comment: 33 Pages, 17 Figures, submitted to A&A

The transition between atomic and molecular hydrogen is associated with important changes in the structure of interstellar clouds, and marks the beginning of interstellar chemistry. Because of the relatively simple networks controlling their abundances, molecular ions are usually good probes of the underlying physical conditions including for instance the fraction of gas in molecular form or the fractional ionization. In this paper we focus on three possible probes of the molecular hydrogen column density, HCO+, HOC+, and CF+. We presented high sensitivity ALMA absorption data toward a sample of compact HII regions and bright QSOs with prominent foreground absorption, in the ground state transitions of the molecular ions HCO+, HOC+, and CF+ and the neutral species HCN and HNC, and from the excited state transitions of C3H+(4-3) and 13CS(2-1). These data are compared with Herschel absorption spectra of the ground state transition of HF and p-H2O. We show that the HCO+, HOC+, and CF+ column densities are well correlated with each other. HCO+ and HOC+ are tightly correlated with p-H2O, while they exhibit a different correlation pattern with HF depending on whether the absorbing matter is located in the Galactic disk or in the central molecular zone. We report new detections of C3H+ confirming that this ion is ubiquitous in the diffuse matter, with an abundance relative to H2 of ~7E-11. We confirm that the CF+ abundance is lower than predicted by simple chemical models and propose that the rate of the main formation reaction is lower by a factor of about 3 than usually assumed. In the absence of CH or HF data, we recommend to use the ground state transitions of HCO+, CCH, and HOC+ to trace diffuse molecular hydrogen, with mean abundances relative to H2 of 3E-9, 4E-8 and 4E-11., Comment: 15 pages, 9 Figures, accepted by Astronomy and Astrophysics. slightly modified abstract

Cosmic rays are predominantly accelerated in shocks associated with star formation such as supernova remnants and stellar wind bubbles, so the cosmic-ray flux and thus cosmic-ray ionization rate, $\zeta_{\rm H}$, should correlate with the star-formation rate in a galaxy. Submillimeter bright galaxies (SMGs) are some of the most prolific star forming galaxies in the Universe, and gravitationally lensed SMGs provide bright continuum sources suitable for absorption line studies. Abundances of OH$^+$ and H$_2$O$^+$ are useful for inferring $\zeta_{\rm H}$ when combined with chemical models, and have been used for this purpose within the Milky Way. At redshifts $z\gtrsim2$ transitions out of the ground rotational states of OH$^+$ and H$_2$O$^+$ are observable with ALMA, and we present observations of both molecules in absorption toward the lensed SMGs SMM J2135$-$0102 and SDP 17b. These detections enable an exploration of $\zeta_{\rm H}$ in galaxies with extreme star formation and high supernova rates, both of which should significantly enhance cosmic-ray production. The observed OH$^+$ and H$_2$O$^+$ absorption is thought to arise in massive, extended halos of cool, diffuse gas that surround these galaxies. Using a chemical model designed to focus on the reaction network important to both species, we infer cosmic-ray ionization rates of $\zeta_{\rm H}\sim10^{-16}$-$10^{-14}$ s$^{-1}$ in these extended gaseous halos. As our estimates come from gas that is far away from the sites of cosmic-ray acceleration, they imply that cosmic-ray ionization rates in the compact regions where star formation occurs in these galaxies are orders of magnitude higher., Comment: 16 pages, 5 figures, 3 tables, accepted for publication in ApJ

Starburst galaxies at the peak of cosmic star formation are among the most extreme starforming engines in the universe, producing stars over ~100 Myr. The star formation rates of these galaxies, which exceed 100 $M_\odot$ per year, require large reservoirs of cold molecular gas to be delivered to their cores, despite strong feedback from stars or active galactic nuclei. Starburst galaxies are therefore ideal targets to unravel the critical interplay between this feedback and the growth of a galaxy. The methylidyne cation, CH$^+$, is a most useful molecule for such studies because it cannot form in cold gas without supra-thermal energy input, so its presence highlights dissipation of mechanical energy or strong UV irradiation. Here, we report the detection of CH$^+$(J=1-0) emission and absorption lines in the spectra of six lensed starburst galaxies at redshifts z~2.5. This line has such a high critical density for excitation that it is emitted only in very dense ($>10^5$ cm$^{-3}$) gas, and is absorbed in low-density gas. We find that the CH$^+$ emission lines, which are broader than 1000 km s$^{-1}$, originate in dense shock waves powered by hot galactic winds. The CH$^+$ absorption lines reveal highly turbulent reservoirs of cool ($T\sim 100$K), low-density gas, extending far outside (>10 kpc) the starburst cores (radii <1 kpc). We show that the galactic winds sustain turbulence in the 10 kpc-scale environments of the starburst cores, processing these environments into multi-phase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star formation rates. Another mass input is therefore required for these reservoirs, which could be provided by on-going mergers or cold stream accretion. Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase instead of quenching it., Comment: Published as a Nature Letter

Protostellar jets and outflows are key features of the star-formation process, and primary processes of the feedback of young stars on the interstellar medium. Understanding the underlying shocks is necessary to explain how jets and outflows are launched, and to quantify their chemical and energetic impacts on the surrounding medium. We performed a high-spectral resolution study of the [OI]$_{\rm 63 \mu m}$ emission in the outflow of the intermediate-mass Class 0 protostar Cep E-mm. We present observations of the OI $^3$P$_1 \rightarrow$ $^3$P$_2$, OH between $^2\Pi_{1/2}$ $J = 3/2$ and $J = 1/2$ at 1837.8 GHz, and CO (16-15) lines with SOFIA-GREAT at three positions in the Cep E outflow: mm (the driving protostar), BI (in the southern lobe), and BII (the terminal position in the southern lobe). The CO line is detected at all three positions. The OI line is detected in BI and BII, whereas the OH line is not detected. In BII, we identify three kinematical components in OI and CO, already detected in CO: the jet, the HH377 terminal bow-shock, and the outflow cavity. The OI column density is higher in the outflow cavity than in the jet, which itself is higher than in the terminal shock. The terminal shock is where the abundance ratio of OI to CO is the lowest (about 0.2), whereas the jet component is atomic (ratio $\sim$2.7). In the jet, we compare the OI observations with shock models that successfully fit the integrated intensity of 10 CO lines: these models do not fit the OI data. The high intensity of OI emission points towards the propagation of additional dissociative or alternative FUV-irradiated shocks, where the illumination comes from the shock itself. From the sample of low-to-high mass protostellar outflows where similar observations have been performed, the effects of illumination seem to increase with the mass of the protostar., Comment: 9 pages, 4 figures, 6 tables, Astronomy and astrophysics, in press

The abundance of CH+ and OH and excitation are predicted to be enhanced by the presence of vibrationally excited H2 or hot gas (~500-1000 K) in PDRs with high incident FUV radiation field. The excitation may also originate in dense gas (>10^5 cm-3) followed by nonreactive collisions. Previous observations suggest that the CH+ and OH correlate with dense and warm gas, and formation pumping contributes to CH+ excitation. We examine the spatial distribution of the CH+ and OH emission in the Orion Bar to establish their physical origin and main formation and excitation mechanisms. We present spatially sampled maps of the CH+ J=3-2 transition at 119.8 {\mu}m and the OH {\Lambda}-doublet at 84 {\mu}m in the Orion Bar over an area of 110"x110" with Herschel (PACS). We compare the spatial distribution of these molecules with those of their chemical precursors, C+, O and H2, and tracers of warm and dense gas. We assess the spatial variation of CH+ J=2-1 velocity-resolved line profile observed with Herschel (HIFI). The OH and CH+ lines correlate well with the high-J CO emission and delineate the warm and dense molecular region. While similar, the differences in the CH+ and OH morphologies indicate that CH+ formation and excitation are related to the observed vibrationally excited H2. This indicates that formation pumping contributes to the excitation of CH+. Interestingly, the peak of the rotationally excited OH 84 {\mu}m emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas. The spatial distribution of CH+ and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH+ J=3-2 excitation processes. The excitation of the OH {\Lambda}-doublet at 84 {\mu}m is mainly sensitive to the temperature and density., Comment: Accepted to A&A

More complete knowledge of galaxy evolution requires understanding the process of star formation and interaction between the interstellar radiation field and the interstellar medium in galactic environments traversing a wide range of physical parameter space. Here we focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the early universe, results in less ultra-violet shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photo-dissociation regions where the chemistry and thermal balance are regulated by far-ultraviolet photons (6 eV< h\nu <13.6 eV). We present Herschel observations of far-infrared fine-structure lines obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and the structure of the gas by modeling it with the Meudon PDR code. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to 3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three dimensional structure of the gas. (Abridged), Comment: 20 pages, 23 figures, accepted in A&A

We have observed five sulphur-bearing molecules in foreground diffuse molecular clouds lying along the sight-lines to five bright continuum sources. We have used the GREAT instrument on SOFIA to observe the 1383 GHz $^2\Pi_{3/2} J=5/2-3/2$ transitions of SH towards the star-forming regions W31C, G29.96-0.02, G34.3+0.1, W49N and W51, detecting foreground absorption towards all five sources; and the EMIR receivers on the IRAM 30m telescope at Pico Veleta to detect the H$_2$S 1(10)-1(01), CS J=2-1 and SO 3(2)-2(1) transitions. In nine foreground absorption components detected towards these sources, the inferred column densities of the four detected molecules showed relatively constant ratios, with N(SH)/N(H$_2$S) in the range 1.1 - 3.0, N(CS)/N(H$_2$S) in the range 0.32 - 0.61, and N(SO)/N(H$_2$S) in the range 0.08 - 0.30. The observed SH/H$_2$ ratios - in the range (0.5-2.6) $\times 10^{-8}$ - indicate that SH (and other sulphur-bearing molecules) account for << 1% of the gas-phase sulphur nuclei. The observed abundances of sulphur-bearing molecules, however, greatly exceed those predicted by standard models of cold diffuse molecular clouds, providing further evidence for the enhancement of endothermic reaction rates by elevated temperatures or ion-neutral drift. We have considered the observed abundance ratios in the context of shock and turbulent dissipation region (TDR) models. Using the TDR model, we find that the turbulent energy available at large scale in the diffuse ISM is sufficient to explain the observed column densities of SH and CS. Standard shock and TDR models, however, fail to reproduce the column densities of H$_2$S and SO by a factor of about 10; more elaborate shock models - in which account is taken of the velocity drift, relative to H$_2$, of SH molecules produced by the dissociative recombination of H$_3$S$^+$ - reduce this discrepancy to a factor ~ 3., Comment: 30 pages, accepted for publication in A&A

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (zeta_H) and molecular hydrogen fraction, f(H2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f(H2) 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 H2O+ primarily reside in gas with low H2 fractions. We also infer zeta_H throughout our sample, and find a log-normal distribution with mean log(zeta_H)=-15.75, (zeta_H=1.78x10^-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 H3+ 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., Comment: 76 pages, 25 figures, 6 tables; accepted for publication in ApJ

Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium and its 158 micron fine structure transition [CII] is the most important cooling line of the diffuse interstellar medium (ISM). We combine [CII] absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. We present high resolution [CII] spectra obtained with the Herschel/HIFI instrument towards bright dust continuum sources regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by observations of the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 microns with Herschel/PACS. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution data and that high spectral resolution spectra are needed to interpret the [CII] and [OI] emission and the [CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seen in external luminous infrared galaxies. The C+ and C excitation in the diffuse gas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight: the median value is 2.4 %, in good agreement with the CNM properties. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 x 10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}., Comment: 25 pages

The rate constants required to model the OH$^+$ observations in different regions of the interstellar medium have been determined using state of the art quantum methods. First, state-to-state rate constants for the H$_2(v=0,J=0,1)$+ O$^+$($^4S$) $\rightarrow$ H + OH$^+(X ^3\Sigma^-, v', N)$ reaction have been obtained using a quantum wave packet method. The calculations have been compared with time-independent results to asses the accuracy of reaction probabilities at collision energies of about 1 meV. The good agreement between the simulations and the existing experimental cross sections in the $0.01-$1 eV energy range shows the quality of the results. The calculated state-to-state rate constants have been fitted to an analytical form. Second, the Einstein coefficients of OH$^+$ have been obtained for all astronomically significant ro-vibrational bands involving the $X^3\Sigma^-$ and/or $A^3\Pi$ electronic states. For this purpose the potential energy curves and electric dipole transition moments for seven electronic states of OH$^+$ are calculated with {\it ab initio} methods at the highest level and including spin-orbit terms, and the rovibrational levels have been calculated including the empirical spin-rotation and spin-spin terms. Third, the state-to-state rate constants for inelastic collisions between He and OH$^+(X ^3\Sigma^-)$ have been calculated using a time-independent close coupling method on a new potential energy surface. All these rates have been implemented in detailed chemical and radiative transfer models. Applications of these models to various astronomical sources show that inelastic collisions dominate the excitation of the rotational levels of OH$^+$. In the models considered the excitation resulting from the chemical formation of OH$^+$ increases the line fluxes by about 10 % or less depending on the density of the gas.

We present new observations of the (6,6) and (9,9) inversion transitions of the hydronium ion toward Sagittarius B2(N) and W31C. Sensitive observations toward Sagittarius B2(N) show that the high, ~ 500 K, rotational temperatures characterizing the population of the highly-excited metastable H3O+ rotational levels are present over a wide range of velocities corresponding to the Sagittarius B2 envelope, as well as the foreground gas clouds between the Sun and the source. Observations of the same lines toward W31C, a line of sight that does not intersect the Central Molecular Zone, but instead traces quiescent gas in the Galactic disk, also imply a high rotational temperature of ~ 380 K, well in excess of the kinetic temperature of the diffuse Galactic interstellar medium. While it is plausible that some fraction of the molecular gas may be heated to such high temperatures in the active environment of the Galactic center, characterized by high X-ray and cosmic ray fluxes, shocks and high degree of turbulence, this is unlikely in the largely quiescent environment of the Galactic disk clouds. We suggest instead that the highly-excited states of the hydronium ion are populated mainly by exoergic chemical formation processes and temperature describing the rotational level population does not represent the physical temperature of the medium. The same arguments may be applicable to other symmetric top rotors, such as ammonia. This offers a simple explanation to the long-standing puzzle of the presence of a pervasive, hot molecular gas component in the central region of the Milky Way. Moreover, our observations suggest that this is a universal process, not limited to the active environments associated with galactic nuclei., Comment: Accepted for publication in The Astrophysical Journal, 10 pages, 4 figures, 3 tables

We report the detection of hydrogen chloride, HCl, in diffuse molecular clouds on the line of sight towards the star-forming region W31C (G10.6-0.4). The J = 1-0 lines of the two stable HCl isotopologues, H35Cl and H37Cl, are observed using the 1b receiver of the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory. The HCl line is detected in absorption, over a wide range of velocities associated with diffuse clouds along the line of sight to W31C. The analysis of the absorption strength yields a total HCl column density of few 10^13 cm^-2, implying that HCl accounts for ~0.6 % of the total gasphase chlorine, which exceeds by a factor of ~6 the theoretical model predictions. This result is comparable to those obtained from the chemically-related species H2Cl+ and HCl+, for which large column densities have also been reported on the same line of sight. The source of discrepancy between models and observations is still unknown; however, the detection of these Cl-bearing molecules, provides key constraints for the chlorine chemistry in the diffuse gas.

We present Herschel/HIFI observations of nine transitions of \hho and \hheo towards six high-mass star-forming regions, obtained as part of the PRISMAS Key Program. Water vapor in translucent clouds is detected in absorption along every sightline. We derive the column density of \hho or \hheo for the lower energy level of each transition observed. The total water column density is about a few $10^{13} \rm{cm^{-2}}$. We find that the abundance of water relative to hydrogen nuclei is $1\times10^{-8}$ in agreement with models for oxygen chemistry with high cosmic ray ionization rates. Relative to \hh, the abundance of water is remarkably constant at $5\times10^{-8}$. The abundance of water in excited levels is at most 15%, implying that the excitation temperature $T_{ex}$ in the ground state transitions is below 10 K. The column densities derived from the two ortho ground state transitions indicates that $T_{ex}\simeq5$ K and that the density $n($\hh$)$ in the clouds is $\le10^4 \rm{cm^{-3}}$. For most clouds we derive a water ortho-to-para ratio consistent with the value of 3 expected in thermodynamic equilibrium in the high temperature limit. Two clouds with large column densities exhibit a ratio significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains, or when they later become at least partially thermalized with the cold gas ($\sim25$ K) in the shielded, low temperature regions of the clouds; evidently, they have not yet fully thermalized with the warmer ($\sim50$ K) translucent portions of the clouds., Comment: 32 pages, 86 figures, accepted for publication in ApJ

We have used the Herschel-HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards W49N and G10.6-0.4 in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2x10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities < 2 - 14 % of N(NH). Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1. This result cannot be explained by current models as we had expected to find a value of unity or higher., Comment: 35 pages, 74 figures

We report the first detection of interstellar mercapto radicals, obtained along the sight-line to the submillimeter continuum source W49N. We have used the GREAT instrument on SOFIA to observe the 1383 GHz Doublet Pi 3/2 J = 5/2 - 3/2 lambda doublet in the upper sideband of the L1 receiver. The resultant spectrum reveals SH absorption in material local to W49N, as well as in foreground gas, unassociated with W49N, that is located along the sight-line. For the foreground material at velocities in the range 37 - 44 km/s with respect to the local standard of rest, we infer a total SH column density ~ 2.6 E+12 cm-2, corresponding to an abundance of ~ 7 E-9 relative to H2, and yielding an SH/H2S abundance ratio ~ 0.13. The observed SH/H2S abundance ratio is much smaller than that predicted by standard models for the production of SH and H2S in turbulent dissipation regions and shocks, and suggests that the endothermic neutral-neutral reaction SH + H2 -> H2S + H must be enhanced along with the ion-neutral reactions believed to produce CH+ and SH+ in diffuse molecular clouds., Comment: Accepted for publication in Astronomy and Astrophysics (SOFIA/GREAT special issue)

The HIFI instrument on board the Herschel Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards G10.6-0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH N=1-0, J=2-1 and ortho-NH2 1_1,1-0_0,0. We also observed ortho-NH3 1_0-0_0, and 2_0-1_0, para-NH3 2_1-1_1, and searched unsuccessfully for NH+. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total column densities over the velocity range 11-54 km/s are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately 6*10^-9, 3*10^-9, and 3*10^-9 for NH, NH2, and NH3, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry., Comment: 5 pages, 3 figures, 2 online pages with 2 figures. Accepted for publication in A&A July 6 (Herschel/HIFI special issue)

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1 - 0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure - as opposed to obtain a lower limit on - the column density of HF for the first time. As in previous observations of HF toward the source G10.6-0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH)-N(H2) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H2 is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H2 column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ~ 24kms-1, that had not been identified in molecular absorption line studies prior to the launch of Herschel., Comment: 4 pages, 3 figures, A&A Letter special issue, accepted on 07/13/2010

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$O in absorption towards Sagittarius~B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$27~K, towards Sagittarius~B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust., Comment: Accepted for publication in A&A, HIFI Special Issue; 5 pages, 3 figures

We report the first detection of the ground-state rotational transition of the methylidyne cation CH+ towards the massive star-forming region DR21 with the HIFI instrument onboard the Herschel satellite. The line profile exhibits a broad emission line, in addition to two deep and broad absorption features associated with the DR21 molecular ridge and foreground gas. These observations allow us to determine a CH+ J=1-0 line frequency of 835137 +/- 3 MHz, in good agreement with a recent experimental determination. We estimate the CH+ column density to be a few 1e13 cm^-2 in the gas seen in emission, and > 1e14 cm^-2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance [CH+]/[H] > 1.2x10^-8. We show that the CH+ column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line, and with those of turbulent dissipation models in diffuse gas for the absorption lines., Comment: Accepted for publication in A&A

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1 - 0 transition of OH+ and the 1115 GHz 1(11) - 0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km/s with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from ~ 3 to ~ 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2 - 8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few E-8 suggests a cosmic ray ionization rate for atomic hydrogen of (0.6 - 2.4) E-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species., Comment: Accepted for publication in A&A Letters

We present spectrally resolved observations of triatomic carbon (C3) in several ro-vibrational transitions between the vibrational ground state and the low-energy nu2 bending mode at frequencies between 1654-1897 GHz along the sight-lines to the submillimeter continuum sources W31C and W49N, using Herschel's HIFI instrument. We detect C3 in absorption arising from the warm envelope surrounding the hot core, as indicated by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C3 absorption due to diffuse foreground clouds. From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_rot) of ~50--70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on the derived T_rot, we get column densities N(C3) ~7-9x10^{14} cm^{-2} and abundance x(C3)~10^{-8} with respect to H2. For W31C, using a radiative transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find that a model with x(C3)=10^{-8}, T_kin=30-50 K, N(C3)=1.5 10^{15} cm^{-2} fits the observations reasonably well and provides parameters in very good agreement with the simple excitation analysis., Comment: Accepted for publication in Astronomy and Astrophysics (HIFI first results issue)

We report the detection of strong absorption by interstellar hydrogen fluoride along the sight-line to the submillimeter continuum source G10.6-0.4 (W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz J=1-0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 at LSR velocities in the range -10 to -3 km/s, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km/s. The spectrum is similar to that of the 1113.3430 GHz 1(11)-0(00) transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H2O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6E+14 cm-2 on the HF column density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance, 6E-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ~ 30 and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that - because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen - HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions., Comment: Accepted for publication in Astronomy and Astrophysics (Herschel special issue). This revised version corrects a typographic error in the HTML abstract, in which the lower limit on the HF abundance (should be 6E-9) was previously misstated. The abstract in the PDF version is correct and the latter has not been modified

Since its launch in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) has made over 4600 observations of some 2500 individual targets. The data are reduced by the Principal Investigator team at the Johns Hopkins University and archived at the Multimission Archive at Space Telescope (MAST). The data-reduction software package, called CalFUSE, has evolved considerably over the lifetime of the mission. The entire FUSE data set has recently been reprocessed with CalFUSE v3.2, the latest version of this software. This paper describes CalFUSE v3.2, the instrument calibrations upon which it is based, and the format of the resulting calibrated data files., Comment: To appear in PASP; 29 pages, 13 figures, uses aastex, emulateapj