My research centers on theoretical study on astrophysical fluid dynamics, MHD and radiative process in the context of accretion flows and other objects. A common thread in my research is to utilize physical theories and numerical experiments to understand how those astronomical objects form, sustain and evolve.

A few of my research interests are:

Turbulent flows driven by Gravitational Instability Top
Disks that are cold and/or massive may become unstable to gravitational instability. A few examples are protostellar system which have extended massive disk at their early stage, and the outskirts of very thin AGN disks. However, the unstable disks may still get away from the fatal fragmentation if the shock heating due to collapsing could not be radiated away rapidly. A self-regulated state exists when heating is balanced by cooling and is termed as "Gravitoturbulence" (Gammie 2001). To date, numerous work have been done with either 2D local/global or 3D global only simulations. And those 3D studies are mostly based on SPH simulations which are usually less accurate with shock waves and often have large numerical viscosities. On the other hand, 3D local simulation could be very useful to investigate the details of the turbulence given its higher resolution and better algorthim for handling shock heating.[Read More]
Density clip of a 3D disk to show both the midplane and the top of the box (cooling time ~ 10 dynamical time). Click vimeo link for the MOVIE

Gap Depth Top
Gap clearing by giant planets has been proposed to explain the optically thin cavities observed in many protoplanetary disks. How much material remains in the gap determines not only how detectable young planets are in their birth environments, but also how strong co-rotation torques are, which impacts how planets can survive fast orbital migration. We determine numerically how the average surface density inside the gap depends on planet-to-star mass ratio q, Shakura-Sunyaev viscosity parameter α, and disk aspect ratio h/r. We find the surface density contrast could easily go beyond three orders of magnitude as observed in transitional disk cavity. Collaborators: Jeffrey Fung (University of Toronto) and Eugene Chiang (UCB). ADS, astro-ph.
For Neptune mass planets(Duffell and MacFadyen 2013):
For Jupitor mass planets(this work):
For Brown Dwarf(this work):

Planetesimal Formation and Gravitational Instability Top
Self-gravity is an attractive means of forming the building blocks of planets, a.k.a. the first-generation planetesimals. For ensembles of dust particles to aggregate into self-gravitating, bound structures, they must first collect into regions of extraordinarily high density in circumstellar gas disks. One question to ask therefore is how dense is necessary to trigger the instability and form bound solid objects? To answer this, I modified the ATHENA code to simulate dusty, compressible, self-gravitating flows in a 3D shearing box configuration, working in the limit that dust particles are small enough to be perfectly entrained in gas. We find the density requirement is extremely high to form planetesimals from small particles via direct gravitational collapse due to the stabilized effects of gas pressure. More detailed description of this work is provided here: ADS, astro-ph. Some movies are available here.

Planetesimal and GI
Image shown two dense dust clumps form via gravitational collapse after ten dynamic timescales (CLICK the image or vimeo link for the MOVIE).

3D MHD Simulations of Circumbinary Accretion Disks Top
Circumbinary disks are commonly observed surrounding young binary stars, post-AGB stars, and protoplanetary systems. They are also believed to be present around massive binary black holes (MBBHs, a pair of massive BHs) during the galaxy merging process. They are dynamically rich system to study on, and more importantly, the EM signals emmitted from the disk might help identify the pre-mergers. We performed the first 3D MHD simulations of such a circumbinary disk surrounding an equal mass binary. We then did a thorough analysis on the disk structures (such as gap width, stream properties, disk eccentricity, etc.) and angular momentum transport (ADS astro-ph. Some movies are available here).

The big accretion rate we found in this work let us consider the accretion efficiency problem of a binary. Recently, we run parallell simulations for disk around single mass object and disks around binary objects to measure to what extent the accretion rates are affected by the binary torque with respect to single mass object. Surprisingly, we find the circumbinary disks have comparable or even higher rate of accretion than normal single mass disks. Currently, we are writing up a paper to report our findings. STAY TUNED!
Images shown surface density at initial (top panel) and the quasi-steady stage (bottom). CLICK the bottom panel for a MOVIE. This vimeo link contains a shorter but higher time resolution version.
circumbinary disk: initial
circumbinary disk: final + sigma movie

MHD Turbulence Driven by Magnetorotational Instability (MRI) Top
Recently, a couple of numerical simulations of MHD turbulence in unstratified shearing box find the turbulence strength decays toward zero as the experimental resolution increases. One of the hypotheses trying to reconcil this problem is that the unstratified box is lack of a real physical length scale, and the saturation level diminishes as the only length scale, the size of the grid cell, decreases. In order to testify it, we study the properties of the MHD turbulence in a stratified shearing box. By introducing the disk scaleheight as the physical length scale, we find good convergence with respect to the box resolution (see the image on the top). In addition, we find the averaged toroidal field flips signs periodically near the midplane before being expelled to higher altitude (see the bottom image, the "butterfly" diagram which manifests some dynamo mechanism). ADS astro-ph
The ratios of stress to pressure as a function of time under different resoutions:32x64x256, 64x64x256, 32x128x256, 32x64x512, and 64x128x512 from left to right and top to bottom.Red: the Maxwell stress; Green: Reynolds stress; Black: The sum of those two.
Space-time diagram of the azimuth component of magnetic field ("butterfly" diagram).
butterfly diagram

AGN Torus Top
The dynamics and structure of toroidal obscuration around active galactic nuclei remain uncertain and controversial. In this project, we extend earlier work on the dynamical role of infrared radiation pressure by adding the effects of two kinds of distributed heating: Compton heating due to hard X-rays from the nucleus and local starlight heating. We find numerical solutions to the axisymmetric hydrostatic equilibrium, energy balance, and photon diffusion equations including these effects. Within the regime of typical parameters, the two different sources of additional heating have very similar effects: the density profile within the torus becomes shallower both radially and vertically, but for plausible heating rates, there is only minor change (relative to the source-free case) in the distribution of column density with solid angle. The most interesting consequence of distributed heating is that it selects out a relatively narrow range of parameters permitting an equilibrium, particularly (L/L_E)/\tau_T. We discuss the implications of both the narrowness of the permitted range and its approximate coincidence with the range inferred from observations. (ADS astro-ph)

Radiation energy density(top panel) and matter density(bottom) for AGN torus with hard X-ray luminosity 15% of the total

Last updated Oct, 2013