Gas-Phase CO in Protoplanetary Disks: A Challenge for Turbulent Mixing

Semenov, D.; Wiebe, D. S.; Henning, Th.

doi:10.1086/507096

Cited by 84 publications

(94 citation statements)

References 34 publications

(45 reference statements)

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“…In our model, the vertical column densities of C 2 H 2 and HCN are a few 10 16 cm −2 in the inner 1 AU, but noticeably decrease as r increases following the decrease in temperature. The column densities of these two species could be enhanced at radii larger than 1 AU if the gas temperatures are higher than in our disk model or if vertical and radial mixing brings warm material toward the cooler midplane regions, as has been suggested for CO in the outer regions of PPDs (Semenov et al 2006). Our model predicts that CH 4 has a large abundance in the very inner disk (<0.5 AU).…”

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

confidence: 57%

Formation of simple organic molecules in inner T Tauri disks

Agúndez¹,

Cernicharo²,

Goicoechea³

2008

A&A

154

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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 Disksupporting

confidence: 57%

Formation of simple organic molecules in inner T Tauri disks

Agúndez¹,

Cernicharo²,

Goicoechea³

2008

A&A

154

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“…Advection and/or stirring by magnetically-driven turbulence affects disk chemistry (e.g. Semenov, Wiebe & Henning 2006;Ilgner & Nelson 2006), the aggregation and settling of small particles that are the preliminary stages of planet building (Johansen & Klahr 2005;Turner et al 2006;Fromang & Papaloizou 2006;Ciesla 2007), and magnetic activity at the disk surface may produce a corona (e.g. Fleming & Stone 2003) and affect observational signatures of protoplanetary disks.…”

Section: Introductionmentioning

confidence: 99%

Magnetic fields in protoplanetary disks

Wardle

2007

Astrophys Space Sci

246

336

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Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines.For a standard population of 0.1µm grains the active surface layers have a combined column Σ active ≈ 2 g cm −2 at 1 AU; by the time grains have aggregated to 3 µm, Σ active ≈ 80 g cm −2 . Ionisation in the active layers is dominated by stellar x-rays. In the absence of grains, x-rays maintain magnetic coupling to 10% of

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“…Finally, models of gas-phase chemistry including surface chemistry on grains are sensitive to the level of turbulent mixing (e.g. Ilgner et al 2004;Ilgner & Nelson 2006;Semenov & Wiebe 2011) which may partially explain the presence of cold CO in disks (Dartois et al 2003;Semenov et al 2006;Aikawa 2007;Hersant et al 2009).…”

Section: Introductionmentioning

confidence: 99%

Chemistry in disks

Guilloteau

Dutrey

Wakelam

et al. 2012

A&A

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Context. Turbulence is thought to be a key driver of the evolution of protoplanetary disks, regulating the mass accretion process, the transport of angular momentum, and the growth of dust particles. Aims. We intend to determine the magnitude of the turbulent motions in the outer parts (>100 AU) of the disk surrounding DM Tau. Methods. Turbulent motions can be constrained by measuring the nonthermal broadening of line emission from heavy molecules. We used the IRAM Plateau de Bure interferometer to study emission from the CS molecule in the disk of DM Tau. High spatial (1.4 × 1. ) and spectral resolution (0.126 km s −1 ) CS J = 3-2 images provide constraints on the molecule distribution and velocity structure of the disk. A low sensitivity CS J = 5-4 image was used in conjunction to evaluate the excitation conditions. We analyzed the data in terms of two parametric disk models, and compared the results with detailed time-dependent chemical simulations.Results. The CS data confirm the relatively low temperature suggested by observations of other simple molecules. The intrinsic linewidth derived from the CS J = 3-2 data is much larger than expected from pure thermal broadening. The magnitude of the derived nonthermal component depends only weakly on assumptions about the location of the CS molecules with respect to the disk plane. Our results indicate turbulence with a Mach number around 0.4-0.5 in the molecular layer. Geometrical constraints suggest that this layer is located near one scale height, in reasonable agreement with chemical model predictions.

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Gas-Phase CO in Protoplanetary Disks: A Challenge for Turbulent Mixing

Cited by 84 publications

References 34 publications

Formation of simple organic molecules in inner T Tauri disks

Formation of simple organic molecules in inner T Tauri disks

Magnetic fields in protoplanetary disks

Chemistry in disks

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