Inside-out planet formation. V. structure of the inner disk as implied
by the MRI
by the MRI
OA Location
Author(s)
Mohanty, Subhanjoy
Jankovic, Marija R
Tan, Jonathan C
Owen, James E
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.
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.
Date Issued
2018-07-13
Date Acceptance
2018-04-05
Citation
Astrophysical Journal, 2018, 861 (2), pp.1-27
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
Copyright Statement
© 2017 The Authors
Sponsor
The Royal Society
Science and Technology Facilities Council
Science and Technology Facilities Council (STFC)
Identifier
http://arxiv.org/abs/1712.07049v2
Grant Number
UF150412
ST-N000838
ST/N000838/1
Subjects
astro-ph.SR
astro-ph.SR
Notes
34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical Journal
Publication Status
Published
Date Publish Online
2018-07-13