Xuening Bai

Research Overview

I am broadly interested in theoretical topics on planet formation and plasma astrophysics. My main research expertise lies in computational magnetohydrodynamics (MHD), and for most of my research, I utilize and develop advanced computational tools to study the microphysical processes of astrophysical systems, which generally involve magnetic field, instabilities and turbulence. Upon getting better understandings, I further explore their outcomes on macroscopic phenomona, which are observables. Some of the topics I have been working on recently are listed below.

Protoplanetary Disks: Gas Dynamics and Planet Formation

Planets form in gaseous protoplanetary disks (PPDs). The gas dynamics in PPDs largely determines the level of turbulence, as well as the structure and evolution of PPDs. Both aspects play a crucial role in many stages of planet formation. The primary goal of my research is to
  • Depict the most realistic picture on the structure and evolution of PPDs.

  • Study planet formation under realistic gas dynamics.
Being extremely weakly ionized, the gas dynamics of PPDs is largely controlled by the so-called non-ideal MHD effects including Ohmic resistivity, the Hall effect and ambipolar diffusion. The three effects dominate in different regions of PPDs, and affect the gas dynamics in different ways. A significant fraction of my research effort has been to explore how strong these microphysical effects are in PPDs and how they influence the large-scale gas dynamics.

Global Picture of PPDs Based on detailed chemistry calculations as well as the most realistic (local) numerical simulations to date, a new paradigm on the global structure of PPDs is emerging: the inner disk largely laminar with wind-driven accretion, and the outer disk contains layered turbulence. In particular, the gas dynamics of the inner disk can be dramatically different depending on the polarity of the external magnetic field threading the disk as a result of the Hall effect. Turbuelnce in the midplane of the outer disk is strongly damped due to ambipolar diffusion. Future global simulations are essential to firmly construct the full picture of PPD structure and evolution.

The new results on PPD gas dynamics, especially low level of turbulence and wind-driven accretion, have definitive observable signatures to be tested with ALMA. They also have profound implications for many aspects of planet formation, including dust growth and transport, planetesimal formation, core accretion, and planet-disk interaction. They are among the topics of my current research.

Representative publications:

Bai, X.-N., Stone, J.M. 2013, Wind-driven Accretion in Protoplanetary Disks. I. Suppression of the Magneto-rotational Instability and Launching of the Magnetocentrifugal Wind, ApJ, 769, 76

Bai, X.-N. 2013, Wind-driven Accretion in Protoplanetary Disks. II. Radial Dependence and Global Picture, ApJ, 772, 96

Bai, X.-N., Ye, J., Goodman, J., Yuan, F. 2016, Magneto-thermal Disk Winds from Protoplanetary Disks, ApJ, 818, 152

Bai, X.-N. 2017, Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics, ApJ, 845, 75

Accretion Disks with External Magnetic Flux

Astrophysical accretion disks are very likely threaded by external vertical magnetic flux. This is almost certainly true for PPDs, and evidence is also accumulating for accretion disks sourrounding compact objects. The presence of external magnetic flux strongly enhances the level of turbulence, and hence the efficiency of gas accretion via the magneto-rotational instability (MRI). It also allows the disk to launch a strong wind/outflow (see the left Figure below). In this sense, global evolution of accretion disks is largely determined by the amount of magnetic flux they possess, which is in turn determined by how do accretion disks acquire and transport external magnetic flux.

The processes of magnetic flux acquisition and transport in accretion disks, and how they couple with disk evolution, can be very different in different astrophysical systems. In particular, in protoplanetary disks, disk evolution and magnetic flux transport can depend on the polarity of the large-scale poloidal field, thanks to the Hall effect (see the right Figure above). The outcome can also depend on the inner and outer boundary conditions in disks. They are among the topics of current investigation, applying to both protoplanetary disks and other accretion disks around compact objects.

Representative publications:

Bai, X.-N., Stone, J.M. 2013, Local Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow, ApJ, 767, 30

Bai, X.-N., Stone, J.M. 2014, Magnetic Flux Concentration and Zonal Flows in Magnetorotational Instability Turbulence, ApJ, 796, 31

Bai, X.-N., Stone, J.M. 2017, Hall Effect-Mediated Magnetic Flux Transport in Protoplanetary Disks, ApJ, 836, 46

Cosmic-ray Acceleration and Transport

The origin and transport of cosmic-rays (CRs) are among the forefront topics of astrophysics, and involve collieionless kinetic effects with complext non-linear interactions with magnetic field and background thermal plasma. The kinetic effects are convensionally studied using particle-in-cell (PIC) methods, which are fully self-consistent but demands tremendous computational cost due to the necessity to resolve microscopic scales. We developed an MHD-PIC approach, which bypasses the microscopic scale while fully retains the kinetic nature of the CRs. With this code, we are able to capture various CR-driven instabilities and study particle accleration in non-relativistic shocks with substantially reduced computational cost (see the Figure below), potentially allowing us to study long-term shock evolution and to explore much larger parameter spaces.
Time Evolution of a Shock The code also allows us to explore a variety of other problems associated with the microphysics of CR transport and CR-driven wind in the Galaxy. Of particular interest is the gyro-resonant CR streaming instability (CRSI, Kulsrud and Pearce, 1969). Our code has great advantage in overcoming the difficulty on the huge scale-separation involved, allowing us to accurately explore, for the first time, the development of CRSI that goes beyond the analytical linear and quasi-linear theories. It is the outcome of such instability that determines the level of coupling between the gas and CRs, and hence CR feedback on macroscopic scales.

Representative publication:

Bai, X.-N., Caprioli, D., Sironi, L., Spitkovsky, A. 2015, Magnetohydrodynamic-Particle-in-Cell Method for Coupling Cosmic Rays with a Thermal Plasma: Application to Non-relativistic Shocks, ApJ, 809, 55

Bai, X.-N., Ostriker, E.C., Plotnikov, I., Stone, J.M. 2019, MHD-PIC Simulations of the Cosmic-Ray Streaming Instability: Linear Growth and Quasi-linear Evolution, ApJ, 876, 60

Code Development

I was involved in the Athena MHD code project, where I have developed the module for solid (dust) particles interacting with gas aerodynamically with feedback. I further developed the module for CR particles that interact with gas electromagnetically with feedback. I have also developed the non-ideal MHD module (ambipolar diffusion and the Hall term) for Athena, and implemented super time-stepping. The code (except for the CR particle module) is publicly available for downloading at the Athena site. I am also involved in the develoment of a new hybrid-kinetic PIC code Pegsus. In addition, I have written a chemistry code which is capable of evolving a complex chemical reaction network including thousands of reactions for millions of years.

More recently, I have been working with the Athena++ MHD code, the successor of the Athena MHD code that incorporates much more flexible coordinate system and grid spacing, as well as static and adpative mesh refinement. The code also contains a suite of additional phsyics, some are currently under active development. There are oppotunities available for students/postdocs to get involved in code development.

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