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.
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.
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.
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.
Lead halide perovskite nanocrystals (NCs) have demonstrated great potential as appealing candidates for advanced optoelectronic applications. However, the toxicity of lead and the intrinsic ...instability toward moisture hinder their mass production and commercialization. Herein, to solve such thorny problems, novel lead‐free Cs2AgBiBr6 double perovskite NCs fabricated via a simple hot‐injection method are reported, which exhibit impressive stability in moisture, light, and temperature. Such materials are then applied into photocatalytic CO2 reduction, achieving a total electron consumption of 105 µmol g−1 under AM 1.5G illumination for 6 h. This study offers a reliable avenue for Cs2AgBiBr6 perovskite nanocrystals preparation, which holds a great potential in the further photochemical applications.
Stable lead‐free Cs2AgBiBr6 double perovskite nanocrystals with a cubic shape and an average size of 9.5 nm are successfully synthesized via the hot‐injection route, and are employed as photocatalysts to convert CO2 into solar fuels (CO and CH4). This work offers a reliable avenue for Cs2AgBiBr6 perovskite nanocrystals preparation, which holds a great potential in the further photochemical applications.
ABSTRACT
The structure and evolution of protoplanetary discs (PPDs) are largely governed by disc angular momentum transport, mediated by magnetic fields. In the most observable outer disc, ambipolar ...diffusion is the primary non-ideal magnetohydrodynamic (MHD) effect. In this work, we study the gas dynamics in outer PPDs by conducting a series of global three-dimensional non-ideal MHD simulations with ambipolar diffusion and net poloidal magnetic flux, using the Athena++ MHD code, with resolution comparable to local simulations. Our simulations demonstrate the coexistence of magnetized disc winds and turbulence driven by the magneto-rotational instability (MRI). While MHD winds dominate disc angular momentum transport, the MRI turbulence also contributes significantly. We observe that magnetic flux spontaneously concentrates into axisymmetric flux sheets, leading to radial variations in turbulence levels, stresses, and accretion rates. Annular substructures arise as a natural consequence of magnetic flux concentration. The flux concentration phenomena show diverse properties with different levels of disc magnetization and ambipolar diffusion. The disc generally loses magnetic flux over time, though flux sheets could prevent the leak of magnetic flux in some cases. Our results demonstrate the ubiquity of disc annular substructures in weakly MRI turbulent outer PPDs and imply a stochastic nature of disc evolution.
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.
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.
We perform local, vertically stratified shearing-box MHD simulations of protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account the effects of both Ohmic resistivity and ...ambipolar diffusion (AD). The magnetic diffusion coefficients are evaluated self-consistently from a look-up table based on equilibrium chemistry. We first show that the inclusion of AD dramatically changes the conventional picture of layered accretion. Without net vertical magnetic field, the system evolves into a toroidal field dominated configuration with extremely weak turbulence in the far-UV ionization layer that is far too inefficient to drive rapid accretion. In the presence of a weak net vertical field (plasma beta ~ 10 super(5) at midplane), we find that the magnetorotational instability (MRI) is completely suppressed, resulting in a fully laminar flow throughout the vertical extent of the disk. A strong magnetocentrifugal wind is launched that efficiently carries away disk angular momentum and easily accounts for the observed accretion rate in PPDs. Moreover, under a physical disk wind geometry, all the accretion flow proceeds through a strong current layer with a thickness of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4 times the sound speed. Both Ohmic resistivity and AD are essential for the suppression of the MRI and wind launching. The efficiency of wind transport increases with increasing net vertical magnetic flux and the penetration depth of the FUV ionization. Our laminar wind solution has important implications on planet formation and global evolution of PPDs.
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