The Importance of Photoprocessing in Protoplanetary Disks

Willacy, Karen; Langer, W. D.

doi:10.1086/317236

Cited by 136 publications

(190 citation statements)

References 29 publications

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“…The column density of NH 3 has a radial profile similar to that of CH 4 , but with a value about one order of magnitude lower. Also, it is interesting to note that the abundance ratio of a closed-shell molecule and its related radical (e.g., H 2 O/OH and HCN/CN) is quite large (about 100-1000), in contrast with the values close to 1 predicted in the outer disk, r > 50 AU, (e.g., Willacy et al 2000). This is a consequence of the large gas densities prevailing in the inner disk which, in spite of the strong FUV field, tend to favor the formation of closed-shell species through reaction (1).…”

Section: Abundances In the Inner Region Of A T Tauri Diskmentioning

confidence: 99%

“…The extinction of stellar UV radiation is large in the midplane, but it rapidly decreases as z increases. Therefore, there exists a layer of low A V where the disk material is being photoprocessed (Aikawa & Herbst 1999;Willacy et al 2000). Here we focus on the chemistry that takes place in the photon-dominated region (PDR) of the inner disk (r < 10 AU), where typically temperatures are several hundreds of degrees Kelvin, and gas densities range from 10 6 to 10 11 cm −3 .…”

Section: Chemical Modelmentioning

confidence: 99%

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Formation of simple organic molecules in inner T Tauri disks

Agúndez¹,

Cernicharo²,

Goicoechea³

2008

A&A

155

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Aims. We present time dependent chemical models for a dense and warm O-rich gas exposed to a strong, far ultraviolet (FUV) field aimed at exploring the formation of simple organic molecules in the inner regions of protoplanetary disks around T Tauri stars.Methods. An up-to-date chemical network is used to compute the evolution of molecular abundances. Reactions of H 2 with small organic radicals such as C 2 and C 2 H, which are not included in current astrochemical databases, overcome their moderate activation energies at warm temperatures and become very important for the gas phase synthesis of C-bearing molecules.Results. The photodissociation of CO and release of C triggers the formation of simple organic species such as C 2 H 2 , HCN, and CH 4 . In timescales between 1 and 10 4 years, depending on the density and FUV field, a steady state is reached in the model in which molecules are continuously photodissociated, but also formed, mainly through gas phase chemical reactions involving H 2 . Conclusions. The application of the model to the upper layers of inner protoplanetary disks predicts large gas phase abundances of C 2 H 2 and HCN. The implied vertical column densities are as large as several 10 16 cm −2 in the very inner disk (<1 AU), in good agreement with the recent infrared observations of warm C 2 H 2 and HCN in the inner regions of IRS 46 and GV Tau disks. We also compare our results with previous chemical models studying the photoprocessing in the outer disk regions, and find that the gas phase chemical composition in the upper layers of the inner terrestrial zone (a few AU) is predicted to be substantially different from that in the upper layers of the outer disk (>50 AU).

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Section: Abundances In the Inner Region Of A T Tauri Diskmentioning

confidence: 99%

Section: Chemical Modelmentioning

confidence: 99%

Formation of simple organic molecules in inner T Tauri disks

Agúndez¹,

Cernicharo²,

Goicoechea³

2008

A&A

155

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“…If the UV radiation is energetic enough to dissociate CO, isotope selective processes may occur leading to fractionation of CO isotopes which can eventually be incorporated into meteorites [46]. Finally, UV photons can affect the chemistry through photodesorption of ices brought to the surface by vertical mixing in the disk [82]. The above discussion illustrates the need for accurate photodissociation cross sections to de-scribe the disk chemistry.…”

Section: Introductionmentioning

confidence: 99%

Photoprocesses in protoplanetary disks

2006

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Circumstellar disks are exposed to intense ultraviolet (UV) radiation from the young star. In the inner disks, the UV radiation can be enhanced by more than seven orders of magnitude compared with the average interstellar radiation field, resulting in a physical and chemical structure that resembles that of a dense photon-dominated region (PDR). This intense UV field affects the chemistry, the vertical structure of the disk, and the gas temperature, especially in the surface layers. The parameters which make disks different from more traditional PDRs are discussed, including the shape of the UV radiation field, grain growth, the absence of PAHs, the gas/dust ratio and the presence of inner holes. Illustrative infrared spectra from the Spitzer Space Telescope are shown. New photodissociation cross sections for selected species, including simple ions, are presented. Also, a summary of cross sections at the Lyman α 1216Å line, known to be strong for some T Tauri stars, is made. Photodissociation and ionization rates are computed for different radiation fields with color temperatures ranging from 30000 to 4000 K and grain sizes up to a few µm. The importance of a proper treatment of the photoprocesses is illustrated for the transitional disk toward HD 141569A which includes grain growth.

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“…This can lead to further chemical enrichment, especially in the warmer parts of the disk (Aikawa et al 1997;Aikawa & Herbst 1999;Willacy & Langer 2000;van Zadelhoff et al 2003;Rodgers & Charnley 2003;Aikawa et al 2008). The interior of the disk is shielded from direct irradiation by the star, so it is colder than the disk's surface and the remnant cloud.…”

Section: Introductionmentioning

confidence: 99%

The chemical history of molecules in circumstellar disks

Visser

Dishoeck

Doty

et al. 2009

A&A

226

319

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Context. Many chemical changes occur during the collapse of a molecular cloud to form a low-mass star and the surrounding disk. One-dimensional models have been used so far to analyse these chemical processes, but they cannot properly describe the incorporation of material into disks. Aims. The goal of this work is to understand how material changes chemically as it is transported from the cloud to the star and the disk. Of special interest is the chemical history of the material in the disk at the end of the collapse. Methods. A two-dimensional, semi-analytical model is presented that, for the first time, follows the chemical evolution from the pre-stellar core to the protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H 2 O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps towards illustrating the full chemistry. Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ∼18 K. Water remains solid almost everywhere during the infall and disk formation phases and evaporates within ∼10 AU of the star. Mixed CO-H 2 O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet-and comet-forming zones of the disk (∼5−30 AU from the star) is predicted to spend enough time in a warm zone (several 10 4 yr at a dust temperature of 20−40 K) during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.

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The Importance of Photoprocessing in Protoplanetary Disks

Cited by 136 publications

References 29 publications

Formation of simple organic molecules in inner T Tauri disks

Formation of simple organic molecules in inner T Tauri disks

Photoprocesses in protoplanetary disks

The chemical history of molecules in circumstellar disks

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