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Inside-out planet formation. V. structure of the inner disk as implied by the MRI

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Title: Inside-out planet formation. V. structure of the inner disk as implied by the MRI
Authors: Mohanty, S
Jankovic, MR
Tan, JC
Owen, JE
Item Type: Journal Article
Abstract: The large population of Earth to super-Earth sized planets found very close to their host stars has motivated consideration of $in$ $situ$ formation models. In particular, Inside-Out Planet Formation is a scenario in which planets coalesce sequentially in the disk, at the local gas pressure maximum near the inner boundary of the dead zone. The pressure maximum arises from a decline in viscosity, going from the active innermost disk (where thermal ionization of alkalis yields high viscosities via the magneto-rotational instability (MRI)) to the adjacent dead zone (where the MRI is quenched). Previous studies of the pressure maximum, based on $\alpha$-disk models, have assumed ad hoc values for the viscosity parameter $\alpha$ in the active zone, ignoring the detailed physics of the MRI. Here we explicitly couple the MRI criteria to the $\alpha$-disk equations, to find steady-state (constant accretion rate) solutions for the disk structure. We consider the effects of both Ohmic and ambipolar resistivities, and find solutions for a range of disk accretion rates ($\dot{M}$ = $10^{-10}$ - $10^{-8}$ ${\rm M}_{\odot}$/yr), stellar masses ($M_{\ast}$ = 0.1 - 1 ${\rm M}_{\odot}$), and fiducial values of the $non$-MRI $\alpha$-viscosity in the dead zone ($\alpha_{\rm {DZ}} = 10^{-5}$ - $10^{-3}$). We find that: (1) A midplane pressure maximum forms radially $outside$ the inner boundary of the dead zone; (2) Hall resistivity dominates near the midplane in the inner disk, which may explain why close-in planets do $not$ form in $\sim$50% of systems; (3) X-ray ionization can be competitive with thermal ionization in the inner disk, because of the low surface density there in steady-state; and (4) our inner disk solutions are viscously unstable to surface density perturbations.
Issue Date: 13-Jul-2018
Date of Acceptance: 5-Apr-2018
URI: http://hdl.handle.net/10044/1/55670
DOI: 10.3847/1538-4357/aabcd0
ISSN: 0004-637X
Publisher: American Astronomical Society
Start Page: 1
End Page: 27
Journal / Book Title: Astrophysical Journal
Volume: 861
Issue: 2
Replaces: 10044/1/60199
http://hdl.handle.net/10044/1/60199
Copyright Statement: © 2017 The Authors
Sponsor/Funder: The Royal Society
Science and Technology Facilities Council
Science and Technology Facilities Council (STFC)
Funder's Grant Number: UF150412
ST-N000838
ST/N000838/1
Keywords: Science & Technology
Physical Sciences
Astronomy & Astrophysics
planets and satellites: formation
protoplanetary disks
MAGNETOROTATIONAL INSTABILITY
ACCRETION DISKS
NONLINEAR EVOLUTION
PROTOSTELLAR DISKS
SATURATION LEVEL
GIANT PLANET
TURBULENCE
DUST
IONIZATION
MIGRATION
astro-ph.SR
astro-ph.SR
astro-ph.SR
astro-ph.SR
0201 Astronomical and Space Sciences
0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics
0306 Physical Chemistry (incl. Structural)
Astronomy & Astrophysics
Notes: 34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical Journal
Publication Status: Published
Open Access location: https://arxiv.org/abs/1712.07049
Online Publication Date: 2018-07-13
Appears in Collections:Physics
Astrophysics
Faculty of Natural Sciences