10:45 AM - 12:15 PM
[PPS07-P21] Effects of magnetic flux transport on the evolution of magnetohydrodynamically accreting protoplanetary disks
Keywords:protoplanetary disk, planet formation, accretion disk
Protoplanetary disks are the birthplace of planets, where dust particles grow into kilometer-sized planetesimals. The structure and evolution of protoplanetary disks are primarily governed by disk angular momentum transport via magnetic field. Previous magnetohydrodynamical simulations of protoplanetary disks have shown that the strength of the net vertical magnetic field (the large-scale magnetic field threading the disk) determines the efficiency of disk accretion. However, how the distribution of the net vertical magnetic fields in the disks evolves are highly uncertain both observationally and theoretically.
The goal of this thesis is to study the coupled evolution of the mass and net vertical magnetic field in protoplanetary disks. To this end, we construct a one-dimensional disk model that treats the radial advection and diffusion of the gas surface density and net vertical magnetic field simultaneously. We treat the advection speed and diffusion coefficient for the net vertical field as free parameters. We consider accretion by angular momentum transport driven by the magnetic wind. From the result of numerical simulations, we give the mass accretion rate as a function of the magnetic field and the distance from the central star.
We find that when magnetic diffusion outside the disk is inefficient and net flux inside the disk is conserved, the mass accretion rate increases with time and the disk surface density decreases rapidly. The disk size decreases with time because of wind-driven accretion. As the disk shrinks in radius, the strength of the net vertical strength increases due to magnetic flux conservation, the increase in the net field strength causes an increase in the accretion rate. We also find that the late stage of the disk evolution is self-similar, with the surface density and net field strength approaching a power law of orbital radius and time. We analytically derive a self-similar solution that describes the late-stage accretion. Our results suggest that magnetically driven accretion can explain the observed lifetimes of protoplanetary disks (~ million years) even if magnetic diffusion dominates over magnetic field advection within the disks. Our results also suggest that the accretion rate in the disk is not necessarily non-uniform in the disk (especially if magnetic diffusion is more dominant than inward accretion, accretion rate is a function of distance from the star). The stellar accretion rate predicted from our model is more than an order of magnitude smaller than the observed stellar accretion rate. This indicates that mechanisms other than magnetic disk winds are also important to disk accretion around the central star.
The goal of this thesis is to study the coupled evolution of the mass and net vertical magnetic field in protoplanetary disks. To this end, we construct a one-dimensional disk model that treats the radial advection and diffusion of the gas surface density and net vertical magnetic field simultaneously. We treat the advection speed and diffusion coefficient for the net vertical field as free parameters. We consider accretion by angular momentum transport driven by the magnetic wind. From the result of numerical simulations, we give the mass accretion rate as a function of the magnetic field and the distance from the central star.
We find that when magnetic diffusion outside the disk is inefficient and net flux inside the disk is conserved, the mass accretion rate increases with time and the disk surface density decreases rapidly. The disk size decreases with time because of wind-driven accretion. As the disk shrinks in radius, the strength of the net vertical strength increases due to magnetic flux conservation, the increase in the net field strength causes an increase in the accretion rate. We also find that the late stage of the disk evolution is self-similar, with the surface density and net field strength approaching a power law of orbital radius and time. We analytically derive a self-similar solution that describes the late-stage accretion. Our results suggest that magnetically driven accretion can explain the observed lifetimes of protoplanetary disks (~ million years) even if magnetic diffusion dominates over magnetic field advection within the disks. Our results also suggest that the accretion rate in the disk is not necessarily non-uniform in the disk (especially if magnetic diffusion is more dominant than inward accretion, accretion rate is a function of distance from the star). The stellar accretion rate predicted from our model is more than an order of magnitude smaller than the observed stellar accretion rate. This indicates that mechanisms other than magnetic disk winds are also important to disk accretion around the central star.