X‐Ray Ionization of Protoplanetary Disks

Glassgold, A. E.; Najita, J.; Igea, J.

doi:10.1086/303952

Cited by 260 publications

(288 citation statements)

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“…51 approximating the X-ray spectrum by an exponential; however, the stellar X-ray spectrum is continuously being absorbed along the pathway from the star to the disk surface by a disk wind (which is perhaps driven by the X-ray irradiation itself -see below). We follow Glassgold et al (1997) and separate the dependence of the photon flux spectrum on energy and position (r = radius of the disk segment) for a geometrically thin, flat disk,…”

Section: X-ray and Cr Ionization And The Magnetorotational Instabilitymentioning

confidence: 99%

“…For photon energy ε, integration is over the cross section σ k starting at the ionization edge ε k for the considered species k. The sum is over all possible species. Numerical evaluation of these expressions for a realistic disk profile were performed by Glassgold et al (1997); Igea & Glassgold (1999) improved these calculations for more realistic disk shapes and consideration of Compton scattering of energetic X-rays. The results can be summarized as follows (see Fig.…”

Section: X-ray and Cr Ionization And The Magnetorotational Instabilitymentioning

confidence: 99%

“…Various assumptions can be made to solve this equation for n(e). For a purely molecular gas, we may neglect terms with M and keep only the first and the third term, from which we find for the electron fraction x e = n e /n x e = ζ X nα 1/2 (63) where, in a realistic situation for protoplanetary disks, α ≈ 2 × 10 −6 T −1/2 cm 3 s −1 and n is the hydrogen number density (Glassgold et al 1997;Igea & Glassgold 1999). Calculations by Glassgold et al (1997) and Igea & Glassgold (1999) show (accounting for sedimentation of condensible heavy elements) that n(e) decreases rapidly toward the disk midplane; for a given vertical column density N ⊥ , n(e) decreases with distance r; together, the disk ionization structure is stratified in such a way that a given ionization fraction defines a wedge-shaped region around the inner disk mid-plane with its apex at the mid-plane at some outer radius (Fig.…”

Section: Summer School "Protoplanetary Disks: Theory and Modeling Meementioning

confidence: 99%

“…Computing the loci of the critical ionization fraction for the operation of the magnetorotational instability (Balbus & Hawley 1991), Glassgold et al (1997) and Igea & Glassgold (1999) find that a significant part of the disk surface will couple sufficiently to magnetic fields to drive the instability, while the central layer around the midplane of the inner (few AU) disk is insufficiently ionized and is therefore not subject to the instability ("dead zone" not participating in MRI-driven accretion; Fig. 7b; see Turner et al 2014 for a discussion of non-ideal MHD effects working against MRI).…”

Section: Summer School "Protoplanetary Disks: Theory and Modeling Meementioning

confidence: 99%

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Ionization and heating by X-rays and cosmic rays

Güdel

2015

EPJ Web of Conferences

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Abstract. High-energy radiation from the central T Tauri and protostars plays an important role in shaping protoplanetary disks and influences their evolution. Such radiation, in particular X-rays and extreme-ultraviolet (EUV) radiation, is predominantly generated in unstable stellar magnetic fields (e.g., the stellar corona), but also in accretion hot spots. Even jets may produce X-ray emission. Cosmic rays, i.e., high-energy particles either from the interstellar space or from the star itself, are of crucial importance. Both highenergy photons and particles ionize disk gas and lead to heating. Ionization and heating subsequently drive chemical networks, and the products of these processes are accessible through observations of molecular line emission. Furthermore, ionization supports the magnetorotational instability and therefore drives disk accretion, while heating of the disk surface layers induces photoevaporative flows. Both processes are crucial for the dispersal of protoplanetary disks and therefore critical for the time scales of planet formation. This chapter introduces the basic physics of ionization and heating starting from a quantum mechanical viewpoint, then discusses relevant processes in astrophysical gases and their applications to protoplanetary disks, and finally summarizes some properties of the most important high-energy sources for protoplanetary disks.

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Section: X-ray and Cr Ionization And The Magnetorotational Instabilitymentioning

confidence: 99%

Section: X-ray and Cr Ionization And The Magnetorotational Instabilitymentioning

confidence: 99%

Section: Summer School "Protoplanetary Disks: Theory and Modeling Meementioning

confidence: 99%

Section: Summer School "Protoplanetary Disks: Theory and Modeling Meementioning

confidence: 99%

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Ionization and heating by X-rays and cosmic rays

Güdel

2015

EPJ Web of Conferences

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“…At any given radius in the disk the density declines rapidly with height because of tidal squeezing by the gravitational field of the central star. The corresponding rapid decline in diffusivity towards the disk surface is aided by external ionisation by cosmic rays and by stellar x-rays which dominate cosmic rays by five orders of magnitude within a few g cm −2 of the disk surface (Glassgold, Najita & Igea 1997;Igea & Glassgold 1999), so that at least the surface layers may be magnetically active (Gammie 1996;Wardle 1997). In addition, the Hall effect provides a dissipationless diffusion pathway that can maintain field gradients under a much broader range of conditions than would otherwise occur (Wardle & Ng 1999;Wardle 1999;Balbus & Terquem 2001).…”

Section: Introductionmentioning

confidence: 99%

Magnetic fields in protoplanetary disks

Wardle

2007

Astrophys Space Sci

256

341

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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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