Magnetic Fields and Turbulence in Protoplanetary Disks
Protoplanetary disks (PPDs) are disks of gas and dust that form around young stars and evolve into planetary systems over the course of a few million years. PPDs are poorly ionized, with only the innermost radii being hot enough to be thermally ionized, and radiation only able to ionize the surface layers of the optically thick disk. As a result, the disk will have turbulence in it’s surface layers driven by the magnetorotational instability (MRI), but the mid-plane will be dominated by non-ideal magnetohydrodynamic (MHD) eﬀects that damp the MRI, leading to a “dead zone” (DZ). This thesis explores several topics relating to magnetic ﬁelds, non-ideal MHD eﬀects, and turbulence in PPDs.
We quantify how forced turbulence in Ohmic DZs varies with the ratio between the column of magnetically active and inactive gas. We ﬁnd that the Reynolds stress near the mid-plane drops rapidly with increasing DZ thickness, becoming negligible for DZs with an active to dead mass ratio less than a few percent. The DZ’s ﬂuid motions are dominated by “r-modes”, a oscillatory circulation pattern that is ineﬃcient at transporting angular momentum.
We present a 1D model that couples the net-magnetic-ﬂux and surface density evolution of the disk. We use this model to investigate the interaction between the stellar magnetic cycle and the disk, showing that some disks may exhibit cyclical or outburst-like accretion behaviour due to the interaction between the net-ﬂux in the disk and a Hall-eﬀect dominated DZ.
We study turbulence in the outer regions of protoplanetary disks, where ambipolar diﬀusion is the dominant non-ideal eﬀect. We show that the properties of the MRI are not determined purely locally, with transport of toroidal magnetic ﬂux from the active layers to the mid-plane playing a signiﬁcant role.
We investigate the streaming instability’s ability to form planetesimals in a turbulent environment. We ﬁnd that turbulence does not greatly change the shape of the mass-spectrum of planetesimals that form. However, no planetesimals are able to form at all in the presence of tur-bulence with α ∼ 10−3, implying that DZs are essential if planetesimals are to be formed by this mechanism.
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Department of Physics
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University of Colorado Boulder
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