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

Goicoechea, J. R.; Joblin, C.; Contursi, A.; Berné, O.; Cernicharo, J.; Gérin, M.; Bourlot, J. Le; Bergin, Edwin A.; Bell, Thomas A.; Röllig, M.

doi:10.1051/0004-6361/201116977

Cited by 66 publications

(112 citation statements)

References 33 publications

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“…Even though neglecting dissociation introduces an additional ∼10% error, this calculation shows that for the radiation field in the Orion Bar (1−4 × 10 4 in Draine units), there is a large percentage of vibrationally excited H 2 to react with C + and form CH + . This has been observed by Van der Werf et al (1996) and Walmsley et al (2000) and has already been noted for the formation of OH through the O + H 2 → OH + H reaction by Goicoechea et al (2011). In the following section we test this idea with a more accurate approach, using PDR models.…”

Section: Estimate Based On An Analytic Approximationsupporting

confidence: 57%

“…The density of the clumps is in the range between 1.5 × 10 6 and 6 × 10 6 cm −3 (Lis & Schilke 2003). Apart from the large clumps detected in H 13 CN deep inside the Bar, small, warm (T kin ∼ 160−220 K), and dense (n H ∼ 10 6−7 cm −3 ) condensations have been suggested to explain the excited OH emission at the PDR surface (Goicoechea et al 2011).…”

Section: Introductionmentioning

confidence: 99%

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

Nagy

Tak

Ossenkopf

et al. 2013

A&A

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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: Estimate Based On An Analytic Approximationsupporting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

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

Nagy

Tak

Ossenkopf

et al. 2013

A&A

Self Cite

203

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“…Although PDR models with lower pressures predict qualitatively similar stratification, and a reactive ion abundance peak also at A V 1 mag (where C + , S + , and H * 2 coexist), a model with ten times lower P th underestimates N(HOC + ) and N(SO + ) by large factors ( 20). This is related to the lower predicted OH column densities (by a factor of ∼25), key in the formation of CO + , HOC + and SO + (Goicoechea et al 2011). Interestingly, SO + peaks deeper inside the cloud, while in the PDR models the main SO + peak is close to that of SH + .…”

Section: Discussionmentioning

confidence: 93%

“…Herschel allowed the detection of OH, CH + , and SH + emission toward dense PDRs (Goicoechea et al 2011;Nagy et al 2013Nagy et al , 2017Parikka et al 2017). Unfortunately, the limited size of the space telescope did not permit us to resolve the ∆A V 1 mag extent of the DF (a few arcsec for the closest PDRs).…”

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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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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“…The Orion Bar has been extensively observed, leading to a large dataset in different tracers of the gas component (e.g., White & Sandell 1995;van der Werf et al 1996;Walmsley et al 2000;Goicoechea et al 2011). Located between the Trapezium cluster and the Orion Molecular Cloud, the Bar is part of the Orion nebula and lies at 414 ± 7 pc away from A&A 541, A19 (2012) the Earth (Menten et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Evolution of dust in the Orion Bar withHerschel

Arab

Abergel

Habart

et al. 2012

A&A

104

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Context. Interstellar dust is a key element in our understanding of the interstellar medium and star formation. The manner in which dust populations evolve with the excitation and the physical conditions is a first step in comprehending the evolution of interstellar dust. Aims. Within the framework of the Evolution of interstellar dust Herschel key programme, we have acquired PACS and SPIRE spectrophotometric observations of various photodissociation regions, to characterise this evolution. The aim of this paper is to trace the evolution of dust grains in the Orion Bar photodissociation region. Methods. We used Herschel/PACS (70 and 160 μm) and SPIRE (250, 350 and 500 μm) together with Spitzer/IRAC observations to map the spatial distribution of the dust populations across the Bar. Brightness profiles were modelled using the DustEM model coupled with a radiative transfer code. Results. Thanks to Herschel, we are able to probe in great detail the dust emission of the densest parts of the Orion Bar with a resolution from 5.6 to 35.1 . These new observations allow us to infer the temperature of the biggest grains at different positions in the Bar, which reveals a gradient from ∼70 K to 35 K coupled with an increase of the spectral emissivity index from the ionization front to the densest regions. Combining Spitzer/IRAC observations, which are sensitive to the dust emission from the surface, with Herschel maps, we were able to measure the Orion Bar emission from 3.6 to 500 μm. We find a stratification in the different dust components that can be quantitatively reproduced by a simple radiative transfer model without dust evolution (diffuse interstellar medium abundances and optical properties). However, including dust evolution is needed to explain the brightness in each band. Polycyclic aromatic hydrocarbon (PAH) abundance variations, or a combination of PAH abundance variations with an enhancement of the biggest grain emissivity caused by coagulation give good results. Another hypothesis is to consider a length of the Bar along the line of sight different at the ionization front than in the densest parts.

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

Cited by 66 publications

References 33 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⁺

Spatially resolved images of reactive ions in the Orion Bar

Evolution of dust in the Orion Bar withHerschel

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

Cited by 66 publications

References 33 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+

Spatially resolved images of reactive ions in the Orion Bar

Evolution of dust in the Orion Bar withHerschel

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Product

Resources

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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⁺