Interstellar Hydrides

Gérin, M.; Neufeld, David A.; Goicoechea, J. R.

doi:10.1146/annurev-astro-081915-023409

Cited by 129 publications

(174 citation statements)

References 231 publications

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“…CH + is a light hydride (see Gerin et al 2016, for a review) meaning that its lowest rotational transitions lie at far-IR frequencies. Therefore, its critical densities are very high (n cr (CH + 4−3) 7 × 10 9 H 2 cm −3 , cf.…”

Section: Discussionmentioning

confidence: 99%

“…The formation of reactive ions such as CH + and SH + depends on the availability of C + and S + (i.e., of UV photons and thus high ionization fractions x e = n(e − )/n H ), and on the presence of excited H 2 (either UV-pumped or hot and thermally excited). This allows overcoming the high endothermicity (and sometimes energy barrier) of some of the key initiating chemical reactions (e.g., Gerin et al 2016). The reaction C + + H 2 (v) → CH + + H, for example, is endothermic by ∆E/k 4300 K if v = 0, but exothermic and fast for v ≥ 1 (Hierl et al 1997;Agúndez et al 2010).…”

Section: Introductionmentioning

confidence: 99%

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Spatially resolved images of reactive ions in the Orion Bar

Goicoechea

Cuadrado

Pety

et al. 2017

A&A

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We report high angular resolution (4.9 × 3.0 ) images of reactive ions SH + , HOC + , and SO + toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30 m telescope. The SH + and HOC + emission is restricted to a narrow layer of 2 -to 10 -width (≈800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H * 2 emission. Both ions efficiently form very close to the H/H 2 transition zone, at a depth of A V 1 mag into the neutral cloud, where abundant C + , S + , and H * 2 coexist. SO + peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T rot ≈ 10−30 K T k ) and narrow line-widths (∼2−3 km s −1 ), a factor of 2 narrower that those of the lighter reactive ion CH + . This is consistent with the higher reactivity and faster radiative pumping rates of CH + compared to the heavier ions, which are driven relatively more quickly toward smaller velocity dispersion by elastic collisions and toward lower T rot by inelastic collisions. We estimate column densities and average physical conditions from an excitation model (n(H 2 ) ≈ 10 5 −10 6 cm −3 , n(e − ) ≈ 10 cm −3 , and T k ≈ 200 K). Regardless of the excitation details, SH + and HOC + clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO + arises from slightly more shielded layers.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Spatially resolved images of reactive ions in the Orion Bar

Goicoechea

Cuadrado

Pety

et al. 2017

A&A

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“…In any case it appears that detection of H 2 in regions with very small < f H 2 > << 0.5 is best left to absorption line measurements of species like OH + which actually prefer such conditions and perhaps to observation of species like CH, OH and HCO + , whose abundances with respect to H 2 are not as sensitive to ambient conditions as that of CO (Gerin et al 2016). …”

Section: Summary and Discussionmentioning

confidence: 99%

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

Liszt

2017

ApJ

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We modelled H 2 and CO formation incorporating the fractionation and selective photodissociation affecting CO when A V 2mag. UV absorption measurements typically have N( 12 CO)/N( 13 CO) ≈ 65 that are reproduced with the standard UV radiation and little density dependence at n(H) ≈ 32 − 1024 cm −3 : Densities n(H) 256 cm −3 avoid overproducing CO. Sightlines observed in mm-wave absorption and a few in UV show enhanced 13 CO by factors of 2-4 and are explained by higher n(H) 256 cm −3

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“…In addition, we assumed that the destruction reactions for H 2 Cl + are not dependent on its spin state. To run our simulations, we coupled our network to the Nautilus code (Ruaud et al 2016), used in the gas-phase mode, to run pseudo-time-dependent simulations with the use of typical diffuse ISM gas-phase elemental abundances relative to total hydrogen (Gerin et al 2016), as listed in Table 2. Each pseudotime-dependent simulation was run with constant physical conditions.…”

Section: Model Descriptionmentioning

confidence: 99%

“…The electron abundance is computed internally in the code in order to have a neutral gas. (a) Elemental abundances from Gerin et al (2016).…”

Section: Model Descriptionmentioning

confidence: 99%

The ortho-to-para ratio of H₂Cl⁺: Quasi-classical trajectory calculations and new simulations in light of new observations

Gal

Xie

Herbst

et al. 2017

A&A

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Multi-hydrogenated species with proper symmetry properties can present different spin configurations, and thus exist under different spin symmetry forms, labeled as para and ortho for two-hydrogen molecules. We investigated here the ortho-to-para ratio (OPR) of H 2 Cl + in the light of new observations performed in the z=0.89 absorber toward the lensed quasar PKS 1830−211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Two independent lines of sight were observed, to the southwest (SW) and northeast (NE) images of the quasar, with OPR values found to be 3.15±0.13 and 3.1±0.5 in each region, respectively, in agreement with a spin statistical weight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogen nuclei can refer to either a statistical result or a high-temperature limit depending on the reaction mechanism leading to its formation. It is thus crucial to identify rigorously how OPRs are produced in order to constrain the information that these probes can provide. To understand the production of the H 2 Cl + OPR, we undertook a careful theoretical study of the reaction mechanisms involved with the aid of quasi-classical trajectory calculations on a new global potential energy surface fit to a large number of high-level ab initio data. Our study shows that the major formation reaction for H 2 Cl + produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Such a mechanism leads to a 3:1 OPR, which is not changed by destruction and possible thermalization reactions for H 2 Cl + and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.

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Interstellar Hydrides

Cited by 129 publications

References 231 publications

Spatially resolved images of reactive ions in the Orion Bar

Spatially resolved images of reactive ions in the Orion Bar

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

The ortho-to-para ratio of H₂Cl⁺: Quasi-classical trajectory calculations and new simulations in light of new observations

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Interstellar Hydrides

Cited by 129 publications

References 231 publications

Spatially resolved images of reactive ions in the Orion Bar

Spatially resolved images of reactive ions in the Orion Bar

Formation and Fractionation of CO (Carbon Monoxide) in Diffuse Clouds Observed at Optical and Radio Wavelengths

The ortho-to-para ratio of H2Cl+: Quasi-classical trajectory calculations and new simulations in light of new observations

Contact Info

Product

Resources

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The ortho-to-para ratio of H₂Cl⁺: Quasi-classical trajectory calculations and new simulations in light of new observations