Surface chemistry in photodissociation regions

Esplugues, G.; Cazaux, S.; Meijerink, R.; Spaans, Marco; Caselli, P.

doi:10.1051/0004-6361/201528001

Cited by 31 publications

(62 citation statements)

References 83 publications

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“…The modeling results obtained have shown that our gas-grain time-dependent model seems appropriate for modeling the chemistry of such an environement and provides some predictions for future observational campaigns, which might lead to a deeper understanding of the Horsehead nebula chemistry and elemental composition, such as for instance the elemental C/O ratio. In particular, our model supports the conclusion of Esplugues et al (2016) that the presence in the gas-phase of H 2 CO and CH 3 OH and other COMs is mainly due to chemical desorption and not as previously thought to photodesorption (Guzman et al 2014).…”

Section: Discussionsupporting

confidence: 89%

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Gal

Herbst

Dufour

et al. 2017

A&A

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A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the formation of organic molecules, and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR Code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH 2 CO, CH 3 CHO and CH 3 CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe.

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Section: Discussionsupporting

confidence: 89%

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Gal

Herbst

Dufour

et al. 2017

A&A

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show abstract

“…Moreover, increasing the number of molecules on grains will emphasize the need of considering a full surface chemistry balance. Some PDR models have introduced surface chemistry calculations (Hollenbach et al 2009;Esplugues et al 2016) and raised the importance of surface chemistry to explain observed gas-phase abundances. In these models, the main ice water formation pathway begins with oxygen adsorption on grains followed by two reactions with hydrogen atoms.…”

Section: Grain Surface Chemistrymentioning

confidence: 99%

The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region

Putaud

Michaut

Petit

et al. 2019

A&A

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Context. The ortho-to-para ratio (OPR) of water in the interstellar medium (ISM) is often assumed to be related to the formation temperature of water molecules, making it a potentially interesting tracer of the thermal history of interstellar gas. Aims. A very low OPR of 0.1-0.5 was previously reported in the Orion Bar photon-dominated region (PDR), based on observations of two optically thin H 18 2 O lines which were analyzed by using a single-slab large velocity gradient (LVG) model. The corresponding spin temperature does not coincide with the kinetic temperature of the molecular gas in this UV-illuminated region. This was interpreted as an indication of water molecules being formed on cold icy grains which were subsequently released by UV photodesorption. Methods. A more complete set of water observations in the Orion Bar, including seven H 16 2 O lines and one H 18 2 O line, carried out using Herschel/HIFI instrument, was reanalyzed using the Meudon PDR code to derive gas-phase water abundance and the OPR. The model takes into account the steep density and temperature gradients present in the region. Results. The model line intensities are in good agreement with the observations assuming that water molecules formed with an OPR corresponding to thermal equilibrium conditions at the local kinetic temperature of the gas and when solely considering gas-phase chemistry and water gas-grain exchanges through adsorption and desorption. Gas-phase water is predicted to arise from a region deep into the cloud, corresponding to a visual extinction of A V ∼ 9, with a H 16 2 O fractional abundance of ∼ 2 × 10 −7 and column density of (1.4 ± 0.8) × 10 15 cm −2 for a total cloud depth of A V = 15. A line-of-sight average ortho-to-para ratio of 2.8 ± 0.2 is derived. Conclusions. The observational data are consistent with a nuclear spin isomer repartition corresponding to the thermal equilibrium at a temperature of (36 ± 2) K, much higher than the spin temperature previously reported for this region and close to the gas kinetic temperature in the water-emitting gas.

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“…We also compare these new figures with those from the original paper. In spite of these variations, all the conclusions obtained in Esplugues et al (2016) remain the same.…”

Section: Introductionmentioning

confidence: 71%

“…In Table A.5 (page 17) of our original publication (Esplugues et al 2016), the rate coefficient considered for the CO ice photodesorption was 2.2 × 10 −15 s −1 , however we should have considered a coefficient of 3.67 × 10 −10 s −1 according to recent results (Fayolle et al 2011;Muñoz-Caro et al 2016). We also update here the values for the solid species H 2 O and H 2 CO considering a coefficient of 3.67 × 10 −11 s −1 for both of them (see Table 1) instead of 2.16 × 10 −11 s −1 .…”

Section: Introductionmentioning

confidence: 99%