A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code

Petit, Franck; Nehmé, Cyrine; Bourlot, J. Le; Roueff, E.

doi:10.1086/503252

Cited by 456 publications

(609 citation statements)

References 84 publications

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“…We used version 1.5.2 of the Meudon PDR code (Le Petit et al 2006) to model the formation and destruction of reactive ions in a constant thermal-pressure slab of FUVirradiated gas (P th /k from 2 × 10 7 to 2 × 10 8 K cm −3 ). Figure 2b shows abundance profiles (with respect to H nuclei) predicted by the high-pressure model.…”

Section: Discussionmentioning

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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Section: Discussionmentioning

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 particular, the number of lowenergy FUV photons (e.g., below 5 eV) relative to the highenergy photons (e.g., those above 11 eV dissociating molecules such as CO and ionising atoms such as carbon) increases with A V . In this work we used a FUV radiative transfer and thermochemical model (Le Petit et al 2006;Goicoechea & Le Bourlot 2007) to estimate N ph (λ) at different positions of the Orion Bar. The well-known "2175 Å bump" of the dust extinction curve (absorption of λ = 1700−2500 Å photons by PAHs and small carbonaceous grains, Cardelli et al 1989;Joblin et al 1992) greatly reduces the number of HCOOH dissociating photons relative to those producing HCOOH fluorescence (Fig.…”

Section: Photoisomerization Rates and Discussionmentioning

confidence: 99%

“…N ph, λ i (photon cm −2 s −1 ) is the flux of photons at each wavelength producing absorption. In order to estimate the most realistic ξ tc and ξ ct rates for the FUV-irradiation conditions in the Orion Bar, we used the Meudon PDR code (Le Petit et al 2006) and calculate N ph (λ) at different cloud depth A V values. Following our previous studies of the Bar (Cuadrado et al 2015;Goicoechea et al 2016) we run a model of an isobaric PDR (P th /k = 10 8 K cm −3 ) illuminated by χ = 4 × 10 4 times the mean interstellar radiation field (Draine 1978).…”

Section: Appendix B: Estimation Of the Photoisomerization Rate In Thementioning

confidence: 99%

Trans-cismolecular photoswitching in interstellar space

Cuadrado

Goicoechea

Roncero

et al. 2016

A&A

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As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8 ± 1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.

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“…A similar automatic procedure was used by Le Bourlot et al (1993); Le Petit et al (2006). We applied our routine to the UDfA06 reaction set, but it can be applied to any given set of chemical reactions.…”

Section: Isotopizationmentioning

confidence: 99%

Carbon fractionation in photo-dissociation regions

Röllig

Ossenkopf

2013

A&A

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We upgraded the chemical network from the UMIST Database for Astrochemistry 2006 to include isotopes such as 13 C and 18 O. This includes all corresponding isotopologues, their chemical reactions and the properly scaled reaction rate coefficients. We study the fractionation behavior of astrochemically relevant species over a wide range of model parameters, relevant for modelling of photo-dissociation regions (PDRs). We separately analyze the fractionation of the local abundances, fractionation of the total column densities, and fractionation visible in the emission line ratios. We find that strong C + fractionation is possible in cool C + gas. Optical thickness as well as excitation effects produce intensity ratios between 40 and 400. The fractionation of CO in PDRs is significantly different from the diffuse interstellar medium. PDR model results never show a fractionation ratio of the CO column density larger than the elemental ratio. Isotope-selective photo-dissociation is always dominated by the isotope-selective chemistry in dense PDR gas. The fractionation of C, CH, CH + and HCO + is studied in detail, showing that the fractionation of C, CH and CH + is dominated by the fractionation of their parental species. The light hydrides chemically derive from C + , and, consequently, their fractionation state is coupled to that of C + . The fractionation of C is a mixed case depending on whether formation from CO or HCO + dominates. Ratios of the emission lines of [C ii], [C i], 13 CO, and H 13 CO + provide individual diagnostics to the fractionation status of C + , C, and CO.

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A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code

Cited by 456 publications

References 84 publications

Spatially resolved images of reactive ions in the Orion Bar

Spatially resolved images of reactive ions in the Orion Bar

Trans-cismolecular photoswitching in interstellar space

Carbon fractionation in photo-dissociation regions

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