Inside-out Planet Formation. V. Structure of the Inner Disk as Implied by the MRI
Abstract
The ubiquity of Earth- to super-Earth-sized planets found very close to their host stars has motivated 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 yields high viscosities via the magnetorotational instability [MRI]) to the adjacent dead zone (where the MRI is quenched). Previous studies of the pressure maximum, based on α-disk models, have assumed ad hoc values for the viscosity parameter α in the active zone, ignoring the detailed MRI physics. Here we explicitly couple the MRI criteria to the α-disk equations, to find steady-state solutions for the disk structure. We consider both Ohmic and ambipolar resistivities, a range of disk accretion rates (10-10-10-8 M ⊙ yr-1), stellar masses (0.1-1 M ⊙), and fiducial values of the non-MRI α-viscosity in the dead zone (α DZ = 10-5 to 10-3). We find that (1) a midplane pressure maximum forms radially outside the dead zone inner boundary; (2) Hall resistivity dominates near the inner disk midplane, perhaps explaining why close-in planets do not form in ∼50% of systems; (3) X-ray ionization can compete with thermal ionization in the inner disk, because of the low steady-state surface density there; and (4) our inner disks are viscously unstable to surface density perturbations.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2018
- DOI:
- 10.3847/1538-4357/aabcd0
- arXiv:
- arXiv:1712.07049
- Bibcode:
- 2018ApJ...861..144M
- Keywords:
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- planets and satellites: formation;
- protoplanetary disks;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical Journal