H<sub>2</sub>Pure Rotational Lines in the Orion Bar

Allers, Katelyn; Jaffe, D. T.; Lacy, J. H.; Draine, B. T.; Richter, Matthew J.

doi:10.1086/431919

Cited by 83 publications

(133 citation statements)

References 54 publications

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“…At these higher temperatures, the rate of H 2 formation drops to 10 −17 nH n(H) cm −3 s −1 . This value is similar to the one observed in PDRs where warm dust grains are present as in NGC 2023 and the Orion Bar (Habart et al 2004;Allers et al 2005). For the case of D 2 , for standard D/H, its formation comes from the big grains.…”

Section: Discussionsupporting

confidence: 87%

“…In molecular clouds and PDRs, the physical conditions at which H 2 has been observed can cover a wide range of gas and grain temperatures (100 K ≤ T gas ≤ 1000 K; 10 K ≤ T dust ≤ 100 K). The H 2 formation rate varies from 3 × 10 −17 to 1.5 × 10 −16 cm −3 s −1 for the PDRs associated with Orion Bar, NGC 2023, Chamaeleon, S140, IC 63, and Oph W (Habart et al 2004;Allers et al 2005). In active galaxies, i.e.…”

Section: Introductionmentioning

confidence: 99%

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The role of carbon grains in the deuteration of H₂

Cazaux

Caselli

Cobut

et al. 2008

A&A

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Aims. The production of molecular hydrogen and its deuterated forms onto carbonaceous dust grains is investigated in detail. The goal of this study is to estimate the importance of the chemistry occuring on grain surfaces for the deuteration of H 2 . Furthermore, we aim to find a robust and general surface chemical model that can be used in different astrophysical environments. Methods. Surface processes are described for the cases of graphitic and amorphous-carbon grains, where laboratory work is available. Langmuir-Hinshelwood, as well as Eley-Rideal surface chemistries are included in the model and their relative contributions highlighted. Analytic expressions are derived for H 2 , HD, and D 2 formation efficiencies for both types of grains. Rate equations are tested against stochastic methods. Results. As expected, rate equations and stochastic methods diverge for grain sizes below a critical value a crit . For these sizes, D 2 formation decreases to favour HD formation. The formation efficiencies of H 2 and D 2 can be calculated by adding a correction factor to the rate equations methods (this factor is a simple exponential factor that becomes unity when a > a crit ). We find that, because of the presence of chemisorbed sites, which can store atoms to form molecules up to high grain temperatures, the formation efficiency of HD and D 2 is very high compared to models where only physisorption sites are taken into account. When considering a realistic distribution of dust grains, we find that the formation rates of H 2 and HD are enhanced by an order of magnitude if small grains are taken into account. The formation of D 2 , on the other hand, comes from the contribution of small (≤100 Å) and big (≥100 Å) grains, depending on the D/H ratio, the grain temperature, and the volume density. The processes described in this paper, which allow a strong enhancement of the deuterated forms of molecular hydrogen, could explain the high degree of deuterium fractionation observed in protostellar environments.

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Section: Discussionsupporting

confidence: 87%

Section: Introductionmentioning

confidence: 99%

The role of carbon grains in the deuteration of H₂

Cazaux

Caselli

Cobut

et al. 2008

A&A

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“…The C91α carbon recombination line was observed with the VLA with a width of 2−2.5 km s −1 (Wyrowski et al 1997). It was found to match the H 2 [1-0 S(1)] distribution (Van der Werf et al 1996) and its radial velocity was found to be consistent with that of H 2 pure rotational lines H 2 v = 0-0 S(1), S(2), and S(4) (Allers et al 2005). The 13 C + lines have a slightly larger width of 2.5−2.8 km s −1 , compared to that of the C91α line.…”

Section: Ch + and Sh + As Tracers Of The Warm Pdr Surfacesupporting

confidence: 55%

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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“…All in all, the best model is found for a source of high density (n H 10 7 cm −3 ) and warm gas temperatures (T k = 160−220 K). This temperature lies in between the ∼600 K derived in the H 2 emitting regions (Allers et al 2005) and the ∼150 K derived from NH 3 lines (Batrla & Wilson 2003) near the ridge of dense and cooler H 13 CN clumps (T k 50 K; Lis & Schilke 2003). In our simple model, the best fit is obtained for a source of small filling factor (η 10%) with a source-averaged OH column density of 10 15 cm −2 (see Appendix A).…”

Section: Oh Column Density Determinationmentioning

confidence: 54%

OH emission from warm and dense gas in the Orion Bar PDR

Goicoechea

Joblin²,

Contursi³

et al. 2011

A&A

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As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present the first detection of the hydroxyl radical (OH) towards the Orion Bar photodissociation region (PDR). Five OH (X 2 Π; ν = 0) rotational Λ-doublets involving energy levels out to E u /k ∼ 511 K have been detected (at ∼65, ∼79, ∼84, ∼119 and ∼163 μm). The total intensity of the OH lines is I(OH) 5 × 10 −4 erg s −1 cm −2 sr −1 . The observed emission of rotationally excited OH lines is extended and correlates well with the high-J CO and CH + J = 3−2 line emission (but apparently not with water vapour), pointing towards a common origin. Nonlocal, non-LTE radiative transfer models including excitation by the ambient FIR radiation field suggest that OH arises in a small filling factor component of warm (T k 160-220 K) and dense (n H 10 6−7 cm −3 ) gas with source-averaged OH column densities of 10 15 cm −2 . High density and temperature photochemical models predict such enhanced OH columns at low depths (A V 1) and small spatial scales (∼10 15 cm), where OH formation is driven by gas-phase endothermic reactions of atomic oxygen with molecular hydrogen. We interpret the extended OH emission as coming from unresolved structures exposed to far-ultraviolet (FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and not from the lower density "interclump" medium. Photodissociation leads to OH/H 2 O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.

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H₂Pure Rotational Lines in the Orion Bar

Cited by 83 publications

References 54 publications

The role of carbon grains in the deuteration of H₂

The role of carbon grains in the deuteration of H₂

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

OH emission from warm and dense gas in the Orion Bar PDR

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H2Pure Rotational Lines in the Orion Bar

Cited by 83 publications

References 54 publications

The role of carbon grains in the deuteration of H2

The role of carbon grains in the deuteration of H2

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

OH emission from warm and dense gas in the Orion Bar PDR

Contact Info

Product

Resources

About

H₂Pure Rotational Lines in the Orion Bar

The role of carbon grains in the deuteration of H₂

The role of carbon grains in the deuteration of H₂

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