The Gas‐Phase Deuterium Fractionation of Formaldehyde

Osamura, Yoshihiro; Roberts, H.; Herbst, Eric

doi:10.1086/427474

Cited by 24 publications

(25 citation statements)

References 36 publications

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“…Yet, except for ammonia and HNC, the direct destructive route is blocked for an assortment of reasons. First, consider H 2 CO. As shown by Osamura et al (2005), deuteration of formaldehyde leads to the ion H 2 COD + , with the deuterium on the opposite side of the molecule from the hydrogens. Dissociative recombination then leads just to H 2 CO + D rather than to HDCO + H, so that no deuteration is accomplished unless there is significant internal rearrangement during recombination, which is assumed not to be the case here.…”

Section: Secondary Speciesmentioning

confidence: 99%

“…Not only singly deuterated isotopologues are produced by secondary reactions involving deuterated ions; species such as NHD 2 , ND 3 , and D 2 CO are observed and understood at least partially in terms of gas-phase syntheses (Roberts & Millar 2000;Gerin et al 2006;Osamura et al 2005;Roueff et al 2005). Although H + 3 is not the only important molecular ion to exchange deuterons with HD exothermically and rapidly, it is the most abundant under most conditions.…”

Section: Introductionmentioning

confidence: 99%

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Deuterium fractionation in warm dense interstellar clumps

Roueff¹,

Parise

Herbst

2006

A&A

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Aims. In a recently published 1-millimeter line survey of the Orion Bar by Leurini et al., the confirmation of dense neutral clumps was reported based mainly on an analysis of methanol spectral lines. Within these clumps, a high abundance ratio was found between the deuterated isotopologue DCN and HCN. Methods. In this paper, we use steady-state chemical modelling to determine if the DCN/HCN abundance ratio can be understood in terms of gas-phase processes. Results. Our results show that, at temperatures (50−75 K) and densities (5−20 × 10 6 cm −3 ) in the vicinity of those measured, the calculated DCN/HCN abundance ratio can reach 0.01 or even higher, in good agreement with the measured range of 0.007−0.009. Moreover, the fractional abundance of HCN is also calculated to be in the vicinity of that reported. Other computed abundance ratios between deuterated and normal isotopologues are reported for the physical conditions of the clumps. The abundance ratios are analyzed as functions of temperature and density in terms of a simple formula. In general, calculated ratios can be somewhat dependent on the choice of two very different sets of elemental abundances, one of which is the "standard" low-metal set, while the other we term the "warm-core" set.

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Section: Secondary Speciesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Deuterium fractionation in warm dense interstellar clumps

Roueff¹,

Parise

Herbst

2006

A&A

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“…The hydrogenation of the unstable radicals HCO and CH 3 O into H 2 CO and CH 3 OH, respectively, is believed to take place quickly, whereas the formation of these radicals from CO and H 2 CO has a barrier (about 2500 K and 2200 K, respectively − Garrod et al 2008;Osamura et al 2005), according to both theoretical calculations and experiments (Peters et al 2013;Wang et al 1973) 3 . It is therefore likely that H atoms arriving on the grain surface will usually react with HCO or CH 3 O rather than with CO or H 2 CO, leading to the most stable products.…”

Section: Grain Surface Formation Routementioning

confidence: 99%

The origin of gas-phase HCO and CH₃O radicals in prestellar cores

Bacmann

Fauré

2016

A&A

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Context. The recent unexpected detection of terrestrial complex organic molecules in the cold (∼10 K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures. Aims. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10 K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH 3 O, as well as the stable species formaldehyde H 2 CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces. Methods. We present new observations of the HCO and CH 3 O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H 2 CO, CH 3 O, and CH 3 OH, which represent the various stages of grain-surface hydrogenation of CO to CH 3 OH. Results. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H 2 CO:CH 3 O:CH 3 OH ∼ 10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH 3 O are gas-phase products of the precursors H 2 CO and CH 3 OH, respectively. Various gas-phase pathways are considered, including neutral-neutral and ion-molecule reactions, and simple estimates of HCO and CH 3 O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified.

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“…We included the major gaseous D-exchange reactions of the UMIST database (McElroy et al 2013), the major surface reactions identified by Stantcheva & Herbst (2003), and gas-phase reactions related to HDO using the approach of Aikawa et al (2012). The initial chemical abundances were adopted from Garrod et al (2007), and include the initial abundance [HD] = 1.5 × 10 −5 × [H 2 ] (Osamura et al 2005).…”

Section: Chemical Modeling Of Abundancementioning

confidence: 99%

Detection of a dense clump in a filament interacting with W51e2

Mookerjea

Vastel

Hassel

et al. 2014

A&A

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In the framework of the Herschel/PRISMAS guaranteed time key program, the line of sight to the distant ultracompact H ii region W51e2 has been observed using several selected molecular species. Most of the detected absorption features are not associated with the background high-mass star-forming region and probe the diffuse matter along the line of sight. We present here the detection of an additional narrow absorption feature at ∼70 km s −1 in the observed spectra of HDO, NH 3 and C 3 . The 70 km s −1 feature is not uniquely identifiable with the dynamic components (the main cloud and the large-scale foreground filament) so-far identified toward this region. The narrow absorption feature is similar to the one found toward low-mass protostars, which is characteristic of the presence of a cold external envelope. The far-infrared spectroscopic data were combined with existing ground-based observations of 12 CO, 13 CO, CCH, CN, and C 3 H 2 to characterize the 70 km s −1 component. Using a non-LTE analysis of multiple transitions of NH 3 and CN, we estimated the density (n(H 2 ) ∼ (1-5) × 10 5 cm −3 ) and temperature (10-30 K) for this narrow feature. We used a gasgrain warm-up based chemical model with physical parameters derived from the NH 3 data to explain the observed abundances of the different chemical species. We propose that the 70 km s −1 narrow feature arises in a dense and cold clump that probably undergoes collapse to form a low-mass protostar, formed on the trailing side of the high-velocity filament, which is thought to be interacting with the W51 main cloud. While the fortuitous coincidence of the dense clump along the line of sight with the continuum-bright W51e2 compact H ii region has contributed to its nondetection in the continuum images, this same attribute makes it an appropriate source for absorption studies and in particular for ice studies of star-forming regions.

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The Gas‐Phase Deuterium Fractionation of Formaldehyde

Cited by 24 publications

References 36 publications

Deuterium fractionation in warm dense interstellar clumps

Deuterium fractionation in warm dense interstellar clumps

The origin of gas-phase HCO and CH₃O radicals in prestellar cores

Detection of a dense clump in a filament interacting with W51e2

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The Gas‐Phase Deuterium Fractionation of Formaldehyde

Cited by 24 publications

References 36 publications

Deuterium fractionation in warm dense interstellar clumps

Deuterium fractionation in warm dense interstellar clumps

The origin of gas-phase HCO and CH3O radicals in prestellar cores

Detection of a dense clump in a filament interacting with W51e2

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The origin of gas-phase HCO and CH₃O radicals in prestellar cores