The gas dynamics of weakly ionized protoplanetary disks (PPDs) are largely governed by the coupling between gas and magnetic fields, described by three non-ideal magnetohydrodynamical (MHD) effects ...(Ohmic, Hall, ambipolar). Previous local simulations incorporating these processes have revealed that the inner regions of PPDs are largely laminar and accompanied by wind-driven accretion. We conduct 2D axisymmetric, fully global MHD simulations of these regions (∼1-20 au), taking into account all non-ideal MHD effects, with tabulated diffusion coefficients and approximate treatment of external ionization and heating. With the net vertical field aligned with disk rotation, the Hall-shear instability strongly amplifies horizontal magnetic field, making the overall dynamics dependent on initial field configuration. Following disk formation, the disk likely relaxes into an inner zone characterized by asymmetric field configuration across the midplane, which smoothly transitions to a more symmetric outer zone. Angular momentum transport is driven by both MHD winds and laminar Maxwell stress, with both accretion and decretion flows present at different heights, and modestly asymmetric winds from the two disk sides. With anti-aligned field polarity, weakly magnetized disks settle into an asymmetric field configuration with supersonic accretion flow concentrated at one side of the disk surface, and highly asymmetric winds between the two disk sides. In all cases, the wind is magneto-thermal in nature, characterized by a mass loss rate exceeding the accretion rate. More strongly magnetized disks give more symmetric field configuration and flow structures. Deeper far-UV penetration leads to stronger and less stable outflows. Implications for observations and planet formation are also discussed.
ABSTRACT A global picture of the evolution of protoplanetary disks (PPDs) is key to understanding almost every aspect of planet formation, where standard -disk models have been continually employed ...for their simplicity. In the meantime, disk mass loss has been conventionally attributed to photoevaporation, which controls disk dispersal. However, a paradigm shift toward accretion driven by magnetized disk winds has taken place in recent years, thanks to studies of non-ideal magnetohydrodynamic effects in PPDs. I present a framework of global PPD evolution aiming to incorporate these advances, highlighting the role of wind-driven accretion and wind mass loss. Disk evolution is found to be largely dominated by wind-driven processes, and viscous spreading is suppressed. The timescale of disk evolution is controlled primarily by the amount of external magnetic flux threading the disks, and how rapidly the disk loses the flux. Rapid disk dispersal can be achieved if the disk is able to hold most of its magnetic flux during the evolution. In addition, because wind launching requires a sufficient level of ionization at the disk surface (mainly via external far-UV (FUV) radiation), wind kinematics is also affected by the FUV penetration depth and disk geometry. For a typical disk lifetime of a few million years, the disk loses approximately the same amount of mass through the wind as through accretion onto the protostar, and most of the wind mass loss proceeds from the outer disk via a slow wind. Fractional wind mass loss increases with increasing disk lifetime. Significant wind mass loss likely substantially enhances the dust-to-gas mass ratio and promotes planet formation.
We perform three-dimensional stratified shearing-box magneto-hydrodynamic (MHD) simulations on the gas dynamics of protoplanetary disks with a net vertical magnetic flux of Bz0. All three non-ideal ...MHD effects, Ohmic resistivity, the Hall effect, and ambipolar diffusion, are included in a self-consistent manner based on equilibrium chemistry. We focus on regions toward outer disk radii, from 5 to 60 AU, where Ohmic resistivity tends to become negligible, ambipolar diffusion dominates over an extended region across the disk height, and the Hall effect largely controls the dynamics near the disk mid-plane. Overall, the basic picture is analogous to the conventional layered accretion scenario applied to the outer disk. In addition, we find that the vertical magnetic flux is strongly concentrated into thin, azimuthally extended shells in most of our simulations beyond 15 AU, leading to enhanced radial density variations know as zonal flows. Theoretical implications and observational consequences are briefly discussed.
The gas dynamics of protoplanetary disks (PPDs) is largely controlled by non-ideal magnetohydrodynamic (MHD) effects including Ohmic resistivity, the Hall effect, and ambipolar diffusion. In this ...series, we have included, for the first time, all three non-ideal MHD effects in a self-consistent manner to investigate the role of the Hall effect on PPD gas dynamics using local shearing-box simulations. In this first paper, we focus on the inner region of PPDs, where previous studies (Bai & Stone 2013; Bai 2013) excluding the Hall effect have revealed that the inner disk up to ~10 AU is largely laminar, with accretion driven by a magnetocentrifugal wind. We confirm this basic picture and show that the Hall effect modifies the wind solutions depending on the polarity of the large-scale poloidal magnetic field B sub(0) threading the disk. Scaling relations for the wind properties, especially the wind-driven accretion rate, are provided for aligned and anti-aligned field geometries.
Abstract
A major uncertainty in understanding the transport and feedback of cosmic rays (CRs) within and beyond our Galaxy lies in the unknown CR scattering rates, which are primarily determined by ...wave–particle interaction at microscopic gyroresonant scales. The source of the waves for the bulk CR population is believed to be self-driven by the CR streaming instability (CRSI), resulting from the streaming of CRs downward a CR pressure gradient. While a balance between driving by the CRSI and wave damping is expected to determine wave amplitudes and hence the CR scattering rates, the problem involves significant scale separation with substantial ambiguities based on quasi-linear theory (QLT). Here we propose a novel “streaming box” framework to study the CRSI with an imposed CR pressure gradient, enabling first-principles measurement of the CR scattering rates as a function of environmental parameters. By employing the magnetohydrodynamic particle-in-cell method with ion–neutral damping, we conduct a series of simulations with different resolutions and CR pressure gradients and precisely measure the resulting CR scattering rates in steady state. The measured rates show scalings consistent with QLT, but with a normalization smaller by a factor of several than typical estimates based on the single-fluid treatment of CRs. A momentum-by-momentum treatment provides better estimates when integrated over momentum but is also subject to substantial deviations, especially at small momentum. Our framework thus opens up the path toward providing comprehensive subgrid physics for macroscopic studies of CR transport and feedback in broad astrophysical contexts.
Magnetized winds may be important in dispersing protoplanetary disks and influencing planet formation. We carry out global magnetohydrodynamic simulations in axisymmetry, coupled with ray-tracing ...radiative transfer, consistent thermochemistry, and non-ideal MHD diffusivities. Magnetized models lacking EUV photons ( ) feature warm molecular outflows that have typical poloidal speeds . When the magnetization is sufficient to drive accretion rates , the wind mass-loss rate is comparable. Such outflows are driven not centrifugally but by the pressure of toroidal magnetic fields produced by bending the poloidal field. Both the accretion and outflow rates increase with the poloidal field energy density, the former almost linearly. The mass-loss rate is also strongly affected by ionization due to UV and X-ray radiation near the wind base. Adding EUV irradiation to the system heats, ionizes, and accelerates the part of the outflow nearest the symmetry axis, but reduces the overall mass-loss rate by exerting pressure on the wind base. Most of our models are non-turbulent, but some with reduced dust abundance and therefore higher ionization fractions exhibit magnetorotational instabilities near the base of the wind.
ABSTRACT
Non-ideal MHD effects play an important role in the gas dynamics in protoplanetary disks (PPDs). This paper addresses the influence of non-ideal MHD effects on the magnetorotational ...instability (MRI) and angular momentum transport in PPDs using the most up-to-date results from numerical simulations. We perform chemistry calculations using a complex reaction network with standard prescriptions for X-ray and cosmic-ray ionizations. We first show that whether or not grains are included, the recombination time is at least one order of magnitude less than the orbital time within five disk scale heights, justifying the validity of local ionization equilibrium and strong coupling limit in PPDs. The full conductivity tensor at different disk radii and heights is evaluated, with the MRI active region determined by requiring that (1) the Ohmic Elsasser number Λ be greater than 1 and (2) the ratio of gas to magnetic pressure β be greater than β
min
(
Am
) as identified in the recent study by Bai & Stone, where
Am
is the Elsasser number for ambipolar diffusion. With full flexibility as to the magnetic field strength, we provide a general framework for estimating the MRI-driven accretion rate
and the magnetic field strength in the MRI active layer. We find that the MRI active layer always exists at any disk radius as long as the magnetic field in PPDs is sufficiently weak. However, the optimistically predicted
in the inner disk (
r
= 1–10 AU) appears insufficient to account for the observed range of accretion rates in PPDs (around 10
−8
M
☉
yr
−1
) even in the grain-free calculation, and the presence of solar abundance sub-micron grains further reduces
by one to two orders of magnitude. Moreover, we find that the predicted
increases with radius in the inner disk where accretion is layered, which would lead to runaway mass accumulation if disk accretion is solely driven by the MRI. Our results suggest that stronger sources of ionization and/or additional mechanisms such as magnetized wind are needed to explain the observed accretion rates in PPDs. In contrast, our predicted
is on the order of 10
−9
M
☉
yr
−1
in the outer disk, consistent with the observed accretion rates in transitional disks.
Abstract
Planetesimal formation is a crucial yet poorly understood process in planet formation. It is widely believed that planetesimal formation is the outcome of dust clumping by the streaming ...instability (SI). However, recent analytical and numerical studies have shown that the SI can be damped or suppressed by external turbulence, and at least the outer regions of protoplanetary disks are likely weakly turbulent due to magneto-rotational instability (MRI). We conduct high-resolution local shearing-box simulations of hybrid particle-gas magnetohydrodynamics (MHD), incorporating ambipolar diffusion as the dominant nonideal MHD effect, applicable to outer disk regions. We first show that dust backreaction enhances dust settling toward the midplane by reducing turbulence correlation time. Under modest level of MRI turbulence, we find that dust clumping is in fact easier than the conventional SI case, in the sense that the threshold of solid abundance for clumping is lower. The key to dust clumping includes dust backreaction and the presence of local pressure maxima, which in our work is formed by the MRI zonal flows overcoming background pressure gradient. Overall, our results support planetesimal formation in the MRI-turbulent outer protoplanetary disks, especially in ring-like substructures.
Abstract
Giant planets embedded in protoplanetary disks (PPDs) can create annulus density gaps around their orbits in the type-II regime, potentially responsible for the ubiquity of annular ...substructures observed in PPDs. Although a substantial amount of works studying type-II planetary migration and gap properties have been published, they have almost exclusively all been conducted under the viscous accretion disk framework. However, recent studies have established magnetized disk winds as the primary mechanism driving disk accretion and evolution, which can coexist with turbulence from the magnetorotational instability (MRI) in the outer PPDs. We conduct a series of 3D global nonideal magnetohydrodynamic (MHD) simulations of type-II planet–disk interactions applicable to the outer PPDs. Our simulations properly resolve the MRI turbulence and accommodate the MHD disk wind. We found that the planet triggers the poloidal magnetic flux concentration around its orbit. The concentrated magnetic flux strongly enhances angular momentum removal in the gap, which is along the inclined poloidal field through a strong outflow emanating from the disk surface outward to the planet gap. The resulting planet-induced gap shape is more similar to an inviscid disk, while being much deeper, which can be understood from a simple inhomogeneous wind torque prescription. The corotation region is characterized by a fast trans-sonic accretion flow that is asymmetric in azimuth about the planet and lacking the horseshoe turns, and the meridional flow is weakened. The torque acting on the planet generally drives inward migration, though the migration rate can be affected by the presence of neighboring gaps through stochastic, planet-free magnetic flux concentration.
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