Electron-impact rotational excitation of CH<sup>+</sup>

Lim, Andrew Kean Seng; Rabadán, I.; Tennyson, Jonathan

doi:10.1046/j.1365-8711.1999.02533.x

Cited by 34 publications

(39 citation statements)

References 21 publications

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“…These rates have been scaled from CH + −He to CH + −H 2 based on Schöier et al (2005). For electron collisions, we use collision rates from Lim et al (1999) that are available for temperatures between 100 and 15 000 K.…”

Section: Physical Conditions Traced By Ch + and Sh +mentioning

confidence: 99%

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

Nagy

Tak

Ossenkopf

et al. 2013

A&A

203

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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 ∼5 km s −1 , significantly broader than the typical width of dense gas tracers in the Orion Bar (∼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 ∼3 km s −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 < 1.5) of the PDR with temperatures between 500 and 1000 K. We have also detected the 5-4 transition of CF + at a width of ∼1.9 km s −1 , consistent with the width of dense gas tracers. The intensity of the CF + 5-4 transition is consistent with previous observations of lower-J transitions toward the Orion Bar.Conclusions. An analytic approximation and a numerical comparison to PDR models indicate that the internal vibrational energy of H 2 can explain the formation of CH + for typical physical conditions in the Orion Bar near the ionization front. The formation of SH + is also likely to be explained by H 2 vibrational excitation. The abundance ratios of CH + and SH + trace the destruction paths of these ions, and indirectly, the ratios of H, H 2 , and electron abundances as a function of depth into the cloud.

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Section: Physical Conditions Traced By Ch + and Sh +mentioning

confidence: 99%

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

Nagy

Tak

Ossenkopf

et al. 2013

A&A

203

View full text Add to dashboard Cite

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“…The collisional rates for CH + -He collisions were taken from Turpin et al (2010) and were scaled to represent CH + -H 2 collisions based on Schöier et al (2005). Collisional rates for CH + -e − collisions are taken from Lim et al (1999); see also Hamilton et al (2016). As CH + is very reactive, inelastic collision rates with H 2 and electrons are similar to the chemical reaction rates with these species (e.g., Stäuber & Bruderer 2009).…”

Section: "Radex" Radiative Transfer Modelsmentioning

confidence: 99%

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

Morris

Gupta

Nagy

et al. 2016

ApJ

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The CH + ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH + J= 1−0, 2−1 rotational transitions, three sets of CH Λ-doubled triplet lines, 12 C + and 13 C + P P , and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH + in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H 2 S(1) emission tracing shocked gas in the outflow. We propose that C + is being consumed by rapid production of CO to explain the lack of both C + and CH + in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH + formation and excitation is incomplete without including the important-possibly dominant-role of UV irradiation.

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“…4 For CH + -H 2 and CH + -H, we scale the CH + -He rates by Hammami et al (2008) and Hammami et al (2009). As CH + is abundant in regions with electron fraction of order 10 −4 , we also consider excitation by electron impact (Lim et al 1999). A general discussion on the uncertainty of collision rates is given in Schöier et al (2005).…”

Section: Molecular Excitation and Line Fluxesmentioning

confidence: 99%

Multidimensional Chemical Modeling of Young Stellar Objects. Iii. The Influence of Geometry on the Abundance and Excitation of Diatomic Hydrides

Bruderer

Benz

Stäuber

et al. 2010

ApJ

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The Herschel Space Observatory enables observations in the far-infrared at high spectral and spatial resolution. A particular class of molecules will be directly observable: light diatomic hydrides and their ions (CH, OH, SH, NH, CH + , OH + , SH + , NH + ). These simple constituents are important both for the chemical evolution of the region and as tracers of high-energy radiation. If outflows of a forming star erode cavities in the envelope, protostellar far-UV (FUV; 6 < E γ < 13.6 eV) radiation may escape through such low-density regions. Depending on the shape of the cavity, the FUV radiation then irradiates the quiescent envelope in the walls along the outflow. The chemical composition in these outflow walls is altered by photoreactions and heating via FUV photons in a manner similar to photo-dominated regions. In this work, we study the effect of cavity shapes, outflow density, and of a disk with the two-dimensional chemical model of a high-mass young stellar object introduced in the second paper in this series. The model has been extended with a self-consistent calculation of the dust temperature and a multi-zone escape probability method for the calculation of the molecular excitation and the prediction of line fluxes. We find that the shape of the cavity is particularly important in the innermost part of the envelope, where the dust temperatures are high enough ( 100 K) for water ice to evaporate. If the cavity shape allows FUV radiation to penetrate this hot-core region, the abundance of FUV-destroyed species (e.g., water) is decreased. On larger scales, the shape of the cavity is less important for the chemistry in the outflow wall. In particular, diatomic hydrides and their ions CH + , OH + , and NH + are enhanced by many orders of magnitude in the outflow walls due to the combination of high gas temperatures and rapid photodissociation of more saturated species. The enhancement of these diatomic hydrides is sufficient for a detection using the HIFI and PACS instruments on board Herschel. The effect of X-ray ionization on the chemistry is found to be small, due to the much larger luminosity in FUV bands compared to X-rays.

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Electron-impact rotational excitation of CH⁺

Cited by 34 publications

References 21 publications

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION

Multidimensional Chemical Modeling of Young Stellar Objects. Iii. The Influence of Geometry on the Abundance and Excitation of Diatomic Hydrides

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Electron-impact rotational excitation of CH+

Cited by 34 publications

References 21 publications

The chemistry of ions in the Orion Bar I. – CH+, SH+, and CF+

The chemistry of ions in the Orion Bar I. – CH+, SH+, and CF+

HERSCHEL/HIFI SPECTRAL MAPPING OF C+, CH+, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH+ FORMATION

Multidimensional Chemical Modeling of Young Stellar Objects. Iii. The Influence of Geometry on the Abundance and Excitation of Diatomic Hydrides

Contact Info

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Resources

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Electron-impact rotational excitation of CH⁺

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

The chemistry of ions in the Orion Bar I. – CH⁺, SH⁺, and CF⁺

HERSCHEL/HIFI SPECTRAL MAPPING OF C⁺, CH⁺, AND CH IN ORION BN/KL: THE PREVAILING ROLE OF ULTRAVIOLET IRRADIATION IN CH⁺ FORMATION