| 1. |
- Goldsmith, Paul F., et al.
(author)
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Herschel Measurements of Molecular Oxygen in Orion
- 2011
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In: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 737:2, s. 96 (1-17)
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Journal article (peer-reviewed)abstract
- We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using the Heterodyne Instrument for the Far Infrared on the Herschel Space Observatory, having velocities of 11 km s–1 to 12 km s–1 and widths of 3 km s–1. The beam-averaged column density is N(O2) = 6.5 × 1016 cm–2, and assuming that the source has an equal beam-filling factor for all transitions (beam widths 44, 28, and 19''), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is (0.3-7.3) × 10–6. The unusual velocity suggests an association with a ~5'' diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~10 Msun and the dust temperature is ≥150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T ≥ 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature ≥180 K. The inferred O2 column density sime5 × 1018 cm–2 can be produced in Peak A, having N(H2) sime 4 × 1024 cm–2. An alternative mechanism is a low-velocity (10-15 km s–1) C-shock, which can produce N(O2) up to 1017 cm–2.
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| 2. |
- Neufeld, David A., et al.
(author)
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HERSCHEL OBSERVATIONS OF INTERSTELLAR CHLORONIUM
- 2012
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 748:1, s. 37-
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Journal article (peer-reviewed)abstract
- Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared, we have observed parachloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight lines to the bright submillimeter continuum sources Sgr A (+50 km s(-1) cloud) and W31C. Both the para-(H2Cl+)-Cl-35 and para-(H2Cl+)-Cl-37 isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio (OPR) of 3, the observed optical depths imply that chloronium accounts for similar to 4%-12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed OPR of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of similar to 2 x 10(13) cm(-2) and similar to 1.2 x 10(13) cm(-2), respectively, for chloronium in these two sources. We obtained upper limits on the para-(H2Cl+)-Cl-35 line strengths toward H-2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor similar to 10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.
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| 3. |
- van der Tak, F. F. S., et al.
(author)
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Spatially extended OH+ emission from the Orion Bar and Ridge
- 2013
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 560, s. A95 (pp. 1-10)
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Journal article (peer-reviewed)abstract
- Context. The reactive HnO+ ions (OH+, H2O+ and H3O+) are widespread in the interstellar medium and act as precursors to the H2O molecule. While HnO+ absorption is seen on many Galactic lines of sight, active galactic nuclei often show the lines in emission. Aims: This paper shows the first example of a Galactic source of HnO+ line emission: the Orion Bar, a bright nearby photon-dominated region (PDR). Methods: We present line profiles and maps of OH+ line emission toward the Orion Bar, and upper limits to H2O+ and H3O+ lines. We analyze these HIFI data with non-local thermodynamic equilibrium radiative transfer and PDR chemical models, using newly calculated inelastic collision data for the e-OH+ system. Results: Line emission is detected over ~1' (0.12 pc), tracing the Bar itself as well as a perpendicular feature identified as the southern tip of the Orion Ridge, which borders the Orion Nebula on its western side. The line width of ≈ 4 km s-1 suggests an origin of the OH+ emission close to the PDR surface, at a depth of AV ~ 0.3-0.5 into the cloud where most hydrogen is in atomic form. Steady-state collisional and radiative excitation models for OH+ require unrealistically high column densities to match the observed line intensity, indicating that the formation of OH+ in the Bar is rapid enough to influence its excitation. Our best-fit OH+ column density of ~ 1.0 × 1014 cm-2 is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (≳ 40) and OH+/H3O+ (≳ 15) are somewhat higher than seen before. Conclusions: The column density of OH+ is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH+ than the reaction with H2, which limits the production of H2O+. The appearance of the OH+ lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH+ are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH+ excitation. Similar conditions may apply to extragalactic nuclei where HnO+ lines are seen in emission.
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