The internal energy available in vibrationally excited H2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this paper, we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H2 with C+, He+, O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H2 (v>0) and C+ has a profound impact on the abundances of some molecules, especially CH+, which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H2, relative to the ground state H2, in excess of ~10–6, independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds ζOph and HD 34078, the dense photon-dominated region (PDR) Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH+ is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 1013 cm–2 are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH+ with a column density close to the values derived from astronomical observations.

http://iopscience.iop.org/article/10.1088/0004-637X/713/1/662/pdf

The chemistry of vibrationally excited h2 in the interstellar medium

We report on Herschel/PACS observations of absorption lines of OH^+, H_2O^+ and H_3O^+ in NGC 4418 and Arp 220. Excited lines of OH^+ and H_2O^+ with E_(lower) of at least 285 and ~200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH^+/H_2O^+ of 1−2.5 are estimated in the nuclei of both sources. The inferred OH^+ column and abundance relative to H nuclei are (0.5−1) × 10^(16) cm^(-2) and ~ 2 × 10^(-8), respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH^+/H^2O^+ ~ 5−10. H_3O^+ is detected in both sources with N(H_3O^+) ~ (0.5−2) × 10^(16) cm^(-2), and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ~500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H^+ → O^+ → OH^+ → H_2O^+ → H_3O^+, and we also argue that the H^+ production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (≳10^4 cm^(-3)) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10^(-13) s^(-1), a lower limit that is several × 10^2 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production.

/pdf/excited-oh-h2o-and-h3o-in-ngc-4418-and-arp-220-57rfl5j0ap.pdf

Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220

A sensitive broadband molecular line survey of the Sagittarius B2(N) star-forming region has been obtained with the Heterodyne Instrument for the Far-Infrared (HIFI) instrument on the Herschel Space Observatory, offering the first high spectral resolution look at this well-studied source in a wavelength region largely inaccessible from the ground (625–157 μm). From the roughly 8000 spectral features in the survey, a total of 72 isotopologues arising from 44 different molecules have been identified, ranging from light hydrides to complex organics, and arising from a variety of environments from cold and diffuse to hot and dense gas. We present a local thermodynamic equilibrium (LTE) model to the spectral signatures of each molecule, constraining the source sizes for hot core species with complementary Submillimeter Array interferometric observations and assuming that molecules with related functional group composition are cospatial. For each molecule, a single model is given to fit all of the emission and absorption features of that species across the entire 480–1910 GHz spectral range, accounting for multiple temperature and velocity components when needed to describe the spectrum. As with other HIFI surveys toward massive star-forming regions, methanol is found to contribute more integrated line intensity to the spectrum than any other species. We discuss the molecular abundances derived for the hot core where the LTE approximation is generally found to describe the spectrum well, in comparison to abundances derived for the same molecules in the Orion KL region from a similar HIFI survey. Notably, we find significantly higher abundances of amine- and amide-bearing molecules (CH_3NH_2, CH_2NH, and NH_2CHO) toward Sgr B2(N) than Orion KL and lower abundances of some complex oxygen-bearing molecules (CH_3OCHO in particular). In addition to information on the chemical composition of the hot core, the strong far-infrared dust continuum allows a number of molecules to be detected in absorption in the Sgr B2(N) envelope for the first time at high spectral resolution, and we discuss the possible physical origin of the kinematic components observed in absorption. Additionally, from the detection of new HOCO^+ transitions in absorption compared to published HCO^+ isotopic observations, we discuss constraints on the gas-phase CO_2 abundance and compare this to observations of the ice composition in the Galactic center region, and to CO_2 abundance estimates toward other high-mass star-forming regions. The reduced HIFI spectral scan and LTE model are made available to the public as a resource for future investigations of star-forming regions in the submillimeter and far-infrared.

/pdf/herschel-observations-of-extraordinary-sources-analysis-of-3xm6o4a24h.pdf

Herschel observations of extraordinary sources: analysis of the full herschel/hifi molecular line survey of sagittarius b2(n) ∗

The internal energy available in vibrationally excited H2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this article we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H2 with C+, He+, O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H2 (v>0) and C+ has a profound impact on the abundances of some molecules, especially CH+, which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H2, relative to ground state H2, in excess of 10^(-6), independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds zeta Oph and HD 34078, the dense PDR Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH+ is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10^(13) cm^(-2) are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH+ with a column density close to the values derived from astronomical observations.

The chemistry of vibrationally excited H2 in the interstellar medium

The Atacama Pathfinder Experiment (APEX) 12m telescope was used to observe the N=1-0, J=0-1 ground state transitions of OH+ at 909.1588 GHz with the CHAMP+ heterodyne array receiver. Two blended hyperfine structure transitions were detected in absorption against the strong continuum source Sagittarius B2(M) and in several pixels offset by 18". Both, absorption from Galactic center gas as well as absorption from diffuse clouds in intervening spiral arms in a velocity range from -116 to 38.5 km/s is observed. The total OH+ column density of absorbing gas is 2.4 \times 10^15 cm-2. A column density local to Sgr B2(M) of 2.6 \times 10^14 cm-2 is found. On the intervening line-of-sight the column density per unit velocity interval are in the range from 1 to 40 \times 10^12 cm-2/(km/s). OH+ is found to be on average more abundant than other hydrides such as SH+ and CH+. Abundance ratios of OH and atomic oxygen to OH+ are found to be in the range of 10^1-2 and 10^3-4, respectively. The detected absorption of a continuous velocity range on the line-of-sight shows OH+ to be an abundant component of diffuse clouds.

First interstellar detection of OH

A full spectral survey was carried out towards the Giant Molecular Cloud complex, Sagittarius B2 (Sgr B2), using the ISO Long Wavelength Spectrometer Fabry-Perot mode. This provided complete wavelength coverage in the range 47-196 um (6.38-1.53 THz) with a spectral resolution of 30-40 km/s. This is an unique dataset covering wavelengths inaccessible from the ground. It is an extremely important region of the spectrum as it contains both the peak of the thermal emission from dust, and crucial spectral lines of key atomic (OI, CII, OIII, NII and NIII) and molecular species (NH3, NH2, NH, H2O, OH, H3O+, CH, CH2, C3, HF and H2D+). In total, 95 spectral lines have been identified and 11 features with absorption depth greater than 3 sigma remain unassigned. Most of the molecular lines are seen in absorption against the strong continuum, whereas the atomic and ionic lines appear in emission (except for absorption in the OI 63 um and CII 158 um lines). Sgr B2 is located close to the Galactic Centre and so many of the features also show a broad absorption profile due to material located along the line of sight. A full description of the survey dataset is given with an overview of each detected species and final line lists for both assigned and unassigned features.

The ISO LWS high resolution spectral survey towards Sagittarius B2

A full spectral survey was carried out towards the Giant Molecular Cloud complex, Sagittarius B2 (Sgr B2), using the ISO Long Wavelength Spectrometer Fabry-Perot mode. This provided complete wavelength coverage in the range 47-196 µm (6.38- 1.53 THz) with a spectral resolution of 30-40 km s 1 . This is an unique dataset covering wavelengths inaccessible from the ground. It is an extremely important region of the spectrum as it contains both the peak of the thermal emission from dust, and crucial spectral lines of key atomic (O i, C ii, O iii, N ii and N iii) and molecular species (NH3, NH2, NH, H2O, OH, H3O + , CH, CH2, C3, HF and H2D + ). In total, 95 spectral lines have been identified and 11 features with absorption depth greater than 3� remain unassigned. Most of the molecular lines are seen in absorption against the strong continuum, whereas the atomic and ionic lines appear in emission (except for absorption in the O i 63 µm and C ii 158 µm lines). Sgr B2 is located close to the Galactic Centre and so many of the features also show a broad absorption profile due to material located along the line of sight. A full description of the survey dataset is given with an overview of each detected species and final line lists for both assigned and unassigned features.

/pdf/the-iso-lws-high-resolution-spectral-survey-towards-34mmjd93dd.pdf

The ISO LWS high-resolution spectral survey towards Sagittarius B2

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The ISO LWS high resolution spectral survey towards Sagittarius B2

The ISO LWS high-resolution spectral survey towards Sagittarius B2