2017/12/31 by Subhanjoy Mohanty, Marija R. Jankovic, Jonathan C. Tan +1 · 1 citation
Physics and Astronomy · #astro-ph.SR
paper · pdf · doi:10.3847/1538-4357/aabcd0
34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical Journal
arxiv created 2018/06/15 · arxiv updated 2018/07/25
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 α-disk models, have assumed ad hoc values for the viscosity parameter α in the active zone, ignoring the detailed physics of the MRI. Here we explicitly couple the MRI criteria to the α-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 (M = 10-10 - 10-8 \rm M\odot/yr), stellar masses (M∗ = 0.1 - 1 \rm M\odot), and fiducial values of the non-MRI α-viscosity in the dead zone (α_\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 ∼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.