Deep-down ionization of protoplanetary discs

Glassgold, A. E.; Lizano, Susana; Galli, Daniele

doi:10.1093/mnras/stx2145

Cited by 7 publications

(5 citation statements)

References 41 publications

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“…The dust grain size has a significant effect on the MRI, which has previously been noted in prior work (Sano et al 2000;Wardle 2007;Glassgold et al 2017). This result remains true in the presence of high CR fluxes.…”

Section: Cosmic-ray Mediated Mrisupporting

confidence: 67%

Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks

Offner

Gaches

Holdship

2019

ApJ

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We use the gas-grain chemistry code uclchem to explore the impact of cosmic-ray feedback on the chemistry of circumstellar disks. We model the attenuation and energy losses of the cosmic-rays as they propagate outwards from the star and also consider ionization due to stellar radiation and radionuclides. For accretion rates typical of young starsṀ * ∼ 10 −9 − 10 −6 M yr −1 , we show that cosmic rays accelerated by the stellar accretion shock produce a cosmic-ray ionization rate at the disk surface ζ 10 −15 s −1 , at least an order of magnitude higher than the ionization rate associated with the Galactic cosmic-ray background. The incident cosmic-ray flux enhances the disk ionization at intermediate to high surface densities (Σ > 10 g cm −2 ) particularly within 10 au of the star. We find the dominant ions are C + , S + and Mg + in the disk surface layers, while the H + 3 ion dominates at surface densities above 1.0 g cm −2 . We predict the radii and column densities at which the magnetorotational instability (MRI) is active in T Tauri disks and show that ionization by cosmic-ray feedback extends the MRI-active region towards the disk mid-plane. However, the MRI is only active at the mid-plane of a minimum mass solar nebula disk if cosmic-rays propagate diffusively (ζ ∝ r −1 ) away from the star. The relationship between accretion, which accelerates cosmic rays, the dense accretion columns, which attenuate cosmic rays, and the MRI, which facilitates accretion, create a cosmic-ray feedback loop that mediates accretion and may produce luminosity variability.

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Section: Cosmic-ray Mediated Mrisupporting

confidence: 67%

Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks

Offner

Gaches

Holdship

2019

ApJ

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“…The angular dependence f (θ) is chosen such that, at a given radius, the ion density increases rapidly in the tenuous disk atmosphere, to mimic the ionization by high energy photons (UV and X-rays) from the central young star in addition to cosmic rays (e.g., Umebayashi & Nakano 1981;Perez-Becker & Chiang 2011b;Glassgold et al 2017). In the simulations presented in this work, we take αAD = 0.5.…”

Section: Ambipolar Diffusionmentioning

confidence: 99%

The formation of rings and gaps in magnetically coupled disc-wind systems: ambipolar diffusion and reconnection

Suriano

Krasnopolsky

et al. 2018

Monthly Notices of the Royal Astronomical Society

125

120

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Radial substructures in circumstellar disks are now routinely observed by ALMA. There is also growing evidence that disk winds drive accretion in such disks. We show through 2D (axisymmetric) simulations that rings and gaps develop naturally in magnetically coupled disk-wind systems on the scale of tens of au, where ambipolar diffusion (AD) is the dominant non-ideal MHD effect. In simulations where the magnetic field and matter are moderately coupled, the disk remains relatively laminar with the radial electric current steepened by AD into a thin layer near the midplane. The toroidal magnetic field sharply reverses polarity in this layer, generating a large magnetic torque that drives fast accretion, which drags the poloidal field into a highly pinched radial configuration. The reconnection of this pinched field creates magnetic loops where the net poloidal magnetic flux (and thus the accretion rate) is reduced, yielding dense rings. Neighbouring regions with stronger poloidal magnetic fields accrete faster, forming gaps. In better magnetically coupled simulations, the so-called 'avalanche accretion streams' develop continuously near the disk surface, rendering the disk-wind system more chaotic. Nevertheless, prominent rings and gaps are still produced, at least in part, by reconnection, which again enables the segregation of the poloidal field and the disk material similar to the more diffusive disks. However, the reconnection is now driven by the non-linear growth of MRI channel flows. The formation of rings and gaps in rapidly accreting yet laminar disks has interesting implications for dust settling and trapping, grain growth, and planet formation.

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“…The function f (θ) is chosen to mimic the increase in ionization by high energy photons (UV and X-rays) from the central young star in addition to cosmic rays near the disk surface (e.g., Umebayashi & Nakano 1981;Perez-Becker & Chiang 2011;Glassgold et al 2017). For simplicity, we adopt αAD = 0.5, the value expected in the case where the volumetric cosmic ray ionization rate is balanced by the recombination rate of ions and electrons, under the constraint of charge neutrality (i.e., ζn ∝ neni ∝ n 2 i , where ζ is the cosmic ray ionization rate per hydrogen nucleus; see page 362 of Shu 1992).…”

Section: Ambipolar Diffusionmentioning

confidence: 99%

The formation of rings and gaps in wind-launching non-ideal MHD discs: three-dimensional simulations

Suriano

Krasnopolsky

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Previous axisymmetric investigations in two dimensions (2D) have shown that rings and gaps develop naturally in non-ideal magnetohydrodynamic (MHD) disk-wind systems, especially in the presence of ambipolar diffusion (AD). Here we extend these 2D simulations to three dimensions (3D) and find that rings and gaps are still formed in the presence of a moderately strong ambipolar diffusion. The rings and gaps form from the same basic mechanism that was identified in the 2D simulations, namely, the redistribution of the poloidal magnetic flux relative to the disk material as a result of the reconnection of a sharply pinched poloidal magnetic field lines. Thus, the less dense gaps are more strongly magnetized with a large poloidal magnetic field compared to the less magnetized (dense) rings. The rings and gaps start out rather smoothly in 3D simulations that have axisymmetric initial conditions. Non-axisymmetric variations arise spontaneously at later times, but they do not grow to such an extent as to disrupt the rings and gaps. These disk substructures persist to the end of the simulations, lasting up to 3000 orbital periods at the inner edge of the simulated disk. The longevity of the perturbed yet still coherent rings make them attractive sites for trapping large grains that would otherwise be lost to rapid radial migration due to gas drag. As the ambipolar diffusivity decreases, both the disk and the wind become increasingly turbulent, driven by the magnetorotational instability, with tightly-wound spiral arms becoming more prominent in the disk.

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Deep-down ionization of protoplanetary discs

Cited by 7 publications

References 41 publications

Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks

Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks

The formation of rings and gaps in magnetically coupled disc-wind systems: ambipolar diffusion and reconnection

The formation of rings and gaps in wind-launching non-ideal MHD discs: three-dimensional simulations

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