10:45 AM - 12:15 PM
[PPS07-P26] Snow line migration and rocky planetesimal formation in magnetically accreting protoplanetary disks
Keywords:Protoplanetary disks, Magnetohydrodynamics, Planet formation, Solar system terrestrial planets
The terrestrial planets in our solar system are thought to be significantly depleted in water compared to outer solar system bodies. The mass of the present Earth's ocean comprises only 0.02% of the Earth's total mass. The Earth's initial water content may be higher but is unlikely to well exceed 1 wt%. In contrast, icy planets or comets, which would have formed in the outer part of the solar system, contain more than 10 wt% water. To constrain when, where, and how the terrestrial planets formed in the solar nebula, it is important to understand how the temperature structure of protoplanetary disks evolves. The temperature structure determines the location of the snow line, where water ice sublimates. Rocky planets and planetesimals are widely believed to form inside the snow line.
Magnetohydrodynamic simulations have shown that in protoplanetary disks where magnetic fields and gas interact (magnetically accreting protoplanetary disks), internal heating by Joule dissipation is inefficient. The interior of protoplanetary disks is a low ionization fraction due to high optical depths. The current layer, which is the heating source of the disk, is then formed on the surface layer of the disk where the ionization fraction is relatively high.
Therefore, the heat generated at low optical depths easily escapes to the outside of the disk, making internal heating inefficient. Recent studies have shown that the temperature structure of magnetically accreting disks varies with the size and spatial distribution of small dust grains controlling the disk's ionization fraction and opacity. However, the growth and motion of the grains depend on the background temperature, implying that the coevolution of the dust and temperature should determine how the snow line migrates.
In this study, we aim to clarify the coevolution of dust and temperature structure in magnetically accreting protoplanetary disks. We construct a numerical model that solves the evolution of the temperature and the gas and dust surface densities. The temperature is determined by the balance between Joule heating and radiative cooling, which depend on the disk's ionization fraction and opacity. We compute the evolution of the disk's ionization structure and opacity by taking into account the grain’s growth, fragmentation, and radial drift. The stickiness of silicate and icy grains inside and outside the snow line are taken as free parameters.
We find that the migration timescale of the snow line is mainly controlled by the stickiness of icy grains. When icy grains can stick up to 1 and 10 m/s, the snow line moves inside the current Earth’s orbit within 2 and 1 Myr after star formation, respectively. As the stickiness of ice is increased, the grains in the outer disk region grow to larger sizes and drift inward more rapidly, leading to faster depletion of the solids in the disk. The faster depletion of solids causes a faster decrease in the disk opacity and results in faster inward migration of the snow line. However, the higher inward dust flux also causes an enhancement of solids interior to the snow line if silicates are less sticky than ice. In this case, rocky planetesimals may form at 1 au through the gravitational and/or streaming instabilities in the first 1 Myr of planet formation.
Magnetohydrodynamic simulations have shown that in protoplanetary disks where magnetic fields and gas interact (magnetically accreting protoplanetary disks), internal heating by Joule dissipation is inefficient. The interior of protoplanetary disks is a low ionization fraction due to high optical depths. The current layer, which is the heating source of the disk, is then formed on the surface layer of the disk where the ionization fraction is relatively high.
Therefore, the heat generated at low optical depths easily escapes to the outside of the disk, making internal heating inefficient. Recent studies have shown that the temperature structure of magnetically accreting disks varies with the size and spatial distribution of small dust grains controlling the disk's ionization fraction and opacity. However, the growth and motion of the grains depend on the background temperature, implying that the coevolution of the dust and temperature should determine how the snow line migrates.
In this study, we aim to clarify the coevolution of dust and temperature structure in magnetically accreting protoplanetary disks. We construct a numerical model that solves the evolution of the temperature and the gas and dust surface densities. The temperature is determined by the balance between Joule heating and radiative cooling, which depend on the disk's ionization fraction and opacity. We compute the evolution of the disk's ionization structure and opacity by taking into account the grain’s growth, fragmentation, and radial drift. The stickiness of silicate and icy grains inside and outside the snow line are taken as free parameters.
We find that the migration timescale of the snow line is mainly controlled by the stickiness of icy grains. When icy grains can stick up to 1 and 10 m/s, the snow line moves inside the current Earth’s orbit within 2 and 1 Myr after star formation, respectively. As the stickiness of ice is increased, the grains in the outer disk region grow to larger sizes and drift inward more rapidly, leading to faster depletion of the solids in the disk. The faster depletion of solids causes a faster decrease in the disk opacity and results in faster inward migration of the snow line. However, the higher inward dust flux also causes an enhancement of solids interior to the snow line if silicates are less sticky than ice. In this case, rocky planetesimals may form at 1 au through the gravitational and/or streaming instabilities in the first 1 Myr of planet formation.