Chemistry in Dense Photon-dominated Regions

Sternberg, A.; Dalgarno, A.

doi:10.1086/192198

Cited by 477 publications

(486 citation statements)

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“…However, with the set of assumed SH + collisional rates, fitting the SH + lines requires denser gas, ∼10 6 cm −3 (see also Nagy et al 2013). As T k is expected to sharply vary along the DF (e.g., Sternberg & Dalgarno 1995), the result of these single-component models should be taken as average conditions (over the ACA resolution). We note that at the distance to Orion, 4 is equivalent to 1 mag of visual extinction for n H = 10 5 cm −3 .…”

Section: Results: Morphology and Emission Propertiesmentioning

confidence: 99%

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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: Results: Morphology and Emission Propertiesmentioning

confidence: 99%

“…They are predicted to form close to the H/H 2 transition zone, the dissociation front (DF), by hightemperature reactions of C + with OH and H 2 O, respectively (e.g., Sternberg & Dalgarno 1995). HOC + also forms by the reaction CO + + H 2 → HOC + + H; thus, CO + and HOC + abundances are likely related (Smith et al 2002).…”

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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“…They are the high I UV (> 10 3 ), high n H (> 10 4 cm −3 ) versions of the diffuse clouds discussed above. At the edge of the cloud, the gas temperature becomes so high (∼ 1000 K) that endothermic processes and reactions with energy barriers can proceed, most notably the C + + H 2 → CH + + H and the O + H 2 → OH + H reactions 170 . Subsequent reactions with C + lead to other characteristic PDR tracers such as CO + .…”

Section: Photon-dominated Regionsmentioning

confidence: 99%

Astrochemistry of dust, ice and gas: introduction and overview

Dishoeck

Ewine²

2014

Faraday Discuss.

138

102

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A brief introduction and overview of the astrochemistry of dust, ice and gas and their interplay is presented, aimed at non-specialists. The importance of basic chemical physics studies of critical reactions is illustrated through a number of recent examples. Such studies have also triggered new insight into chemistry, illustrating how astronomy and chemistry can enhance each other. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas-grain chemistry rather than pure gas-phase chemistry, and a critical discussion of the state of such models is given. Recent developments in studies of diffuse clouds and PDRs, cold dense clouds, hot cores, protoplanetary disks and exoplanetary atmospheres are summarized, both for simple and more complex molecules, with links to papers presented in this volume. In spite of many lingering uncertainties, the future of astrochemistry is bright: new observational facilities promise major advances in our understanding of the journey of gas, ice and dust from clouds to planets.Comment: Introductory paper for Faraday Discussions 168 conference, April 201

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“…In KOSMA-τ we solve pure gas-phase steady-state chemistry with the exception of H 2 forming on grains (Sternberg & Dalgarno 1995). It relies on the availability of comprehensive databases of chemical reaction rate coefficients.…”

Section: Model Chemistrymentioning

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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Chemistry in Dense Photon-dominated Regions

Cited by 477 publications

References 0 publications

Spatially resolved images of reactive ions in the Orion Bar

Spatially resolved images of reactive ions in the Orion Bar

Astrochemistry of dust, ice and gas: introduction and overview

Carbon fractionation in photo-dissociation regions

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