| 1. |
- Lis, D. C., et al.
(författare)
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Herschel/HIFI discovery of interstellar chloronium (H2Cl+)
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521:1
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Tidskriftsartikel (refereegranskat)abstract
- We report the first detection of chloronium, H_2Cl^+, in the interstellar medium, using the HIFI instrument aboard the Herschel Space Observatory. The 2_12-1_01 lines of ortho-H\_2^35Cl^+ and ortho-H\_2^37Cl^+ are detected in absorption towards NGC 6334I, and the 1_11-0_00 transition of para-H\_2^35Cl^+ is detected in absorption towards NGC 6334I and Sgr B2(S). The H_2Cl^+ column densities are compared to those of the chemically-related species HCl. The derived HCl/H_2Cl^+ column density ratios, ~1-10, are within the range predicted by models of diffuse and dense photon dominated regions (PDRs). However, the observed H_2Cl^+ column densities, in excess of 10^13 cm^-2, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.
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| 2. |
- Liseau, René, 1949, et al.
(författare)
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Multi-line detection of O2 toward rho Ophiuchi A
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 541
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Tidskriftsartikel (refereegranskat)abstract
- Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O-2, and water, H2O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O-2 in the dense star-forming interstellar medium. Aims. Only toward rho OphA did Odin detect a very weak line of O-2 at 119 GHz in a beam of size 10 arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O-2 emission and pinpoint its origin. Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O-2 spectra toward selected positions in the rho Oph A core. These data are analysed using standard techniques for O2 excitation and compared to recent PDR-like chemical cloud models. Results. The N-J = 3(3)-1(2) line at 487.2 GHz is clearly detected toward all three observed positions in the rho Oph A core. In addition, an oversampled map of the 5(4)-3(4) transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O-2 emitting gas appears to vary quite substantially, with warm gas (greater than or similar to 50 K) being adjacent to a much colder region, of temperatures lower than 30 K. Conclusions. The exploited models predict that the O-2 column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O-2 column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O-2) = 3 to greater than or similar to 6 x 10(15) cm(-2). Beam-averaged O-2 abundances are about 5 x 10(-8) relative to H-2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire rho Oph A core. We speculate that one of the reasons for the generally very low detection rate of O-2 is the short period of time during which O-2 molecules are reasonably abundant in molecular clouds.
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| 3. |
- Goldsmith, Paul F., et al.
(författare)
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Herschel Measurements of Molecular Oxygen in Orion
- 2011
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 737:2, s. 96 (1-17)
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Nagy, Z., et al.
(författare)
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The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+ The effect of high electron density and vibrationally excited H-2 in a warm PDR surface
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 550, s. A96-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The abundances of interstellar CH+ and SH+ are not well understood as their most likely formation channels are highly endothermic. Several mechanisms have been proposed to overcome the high activation barriers, including shocks, turbulence, and H-2 vibrational excitation. Aims. Using data from the Herschel Space Observatory, we studied the formation of ions, in particular CH+ and SH+ in a typical high UV-illumination warm and dense photon-dominated region (PDR), the Orion Bar. Methods. The HIFI instrument on board Herschel provides velocity-resolved line profiles of CH+ 1-0 and 2-1 and three hyperfine transitions of SH+ 1(2)-0(1). The PACS instrument provides information on the excitation and spatial distribution of CH+ by extending the observed CH+ transitions up to J = 6-5. We compared the observed line intensities to the predictions of radiative transfer and PDR codes. Results. All CH+, SH+, and CF+ lines analyzed in this paper are seen in emission. The widths of the CH+ 2-1 and 1-0 transitions are of similar to 5 kms(-1), significantly broader than the typical width of dense gas tracers in the Orion Bar (similar to 2-3 km s(-1)) and are comparable to the width of species that trace the interclump medium such as C+ and HF. The detected SH+ transitions are narrower compared to CH+ and have line widths of similar to 3 kms(-1), indicating that SH+ emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH+ and SH+ and that reactive collisions need to be taken into account to calculate the excitation of CH+. Comparison to PDR models shows that CH+ and SH+ are tracers of the warm surface region (A(V)
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| 5. |
- Neufeld, David A., et al.
(författare)
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HERSCHEL OBSERVATIONS OF INTERSTELLAR CHLORONIUM
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 748:1, s. 37-
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Tidskriftsartikel (refereegranskat)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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