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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 (ζH) and molecular hydrogen fraction (). 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 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 ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –15.75 (ζH = 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 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. [ABSTRACT FROM AUTHOR]

Absorption lines from the molecules OH+, H2O+, and H+3 have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH+ and H2O+ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H+3 is used to infer the cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which we find to be ζ2 = (4.8 ± 3.4) × 10–16 s–1. Combining the results from all three species we find an efficiency factor—defined as the ratio of the formation rate of OH+ to the cosmic-ray ionization rate of H—of ε = 0.07 ± 0.04, much lower than predicted by chemical models. This is an important step in the future use of OH+ and H2O+ on their own as tracers of the cosmic-ray ionization rate. [ABSTRACT FROM AUTHOR]

We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady-state photodissociation region code that treats the freezeout of gas species, grain surface chemistry, and desorption of ices from grains. The code includes polycyclic aromatic hydrocarbons (PAHs), which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at AV ≲ 1 and one at AV ∼ 3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far-ultraviolet flux on the cloud (χ ≲ 1000; χ = 1 = local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic-ray primary ionization rate ζcrp divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ∼4 × 1013 cm–2 independent of ζcrp or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low-ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate ζcrp/n, χ/n and AV in them. We compare our models to Herschel observations of these two ions, and estimate ζcrp ∼4-6 × 10–16(n/100 cm–3) s–1 and χ/n = 0.03 cm3 for diffuse foreground clouds toward W49N. [ABSTRACT FROM AUTHOR]

We present results from a comprehensive submillimeter spectral survey toward the source Orion South, based on data obtained with the HIFI instrument aboard the Herschel Space Observatory , covering the frequency range 480 to 1900 GHz. We detect 685 spectral lines with S/N > 3 σ , originating from 52 different molecular and atomic species. We model each of the detected species assuming conditions of Local Thermodynamic Equilibrium. This analysis provides an estimate of the physical conditions of Orion South (column density, temperature, source size, & V LSR ). We find evidence for three different cloud components: a cool (T ex ~ 20 - 40 K), spatially extended (> 60″), and quiescent (Δ V FWHM ~ 4 km s -1 ) component; a warmer (T ex ~ 80 - 100 K), less spatially extended (~ 30″), and dynamic (Δ V FWHM ~ 8 km s -1 ) component, which is likely affected by embedded outflows; and a kinematically distinct region (T ex > 100 K; V LSR ~ 8 km s -1 ), dominated by emission from species which trace ultraviolet irradiation, likely at the surface of the cloud. We find little evidence for the existence of a chemically distinct "hot core" component, likely due to the small filling factor of the hot core or hot cores within the Herschel beam. We find that the chemical composition of the gas in the cooler, quiescent component of Orion South more closely resembles that of the quiescent ridge in Orion-KL. The gas in the warmer, dynamic component, however, more closely resembles that of the Compact Ridge and Plateau regions of Orion-KL, suggesting that higher temperatures and shocks also have an influence on the overall chemistry of Orion South.