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Research

Accretion disks are flattened astronomical objects made of rapidly rotating gas which slowly spirals onto a central gravitating body. The releasing of gravitational energy of the infalling mass in accretion disks is one of the most important mechanism for astronomical objects to get powered. For example, accretion is the energy source for stellar binaries, active galactic nuclei(AGN), protostellar disks and even some gamma-ray bursts.

My research activities are aim at investigating all kinds of disks and related physical phenomena by modeling them using the tehchinque of numerical simulations. With the help of numerical experiments, I study how the accretion disk accretes and evolves, try to explain the observed facts and predict new physics.

Current Projects

Gravitational Instability of Small Particles in Stratified Dusty Disks 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. We have 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. More detailed descreption of this work is provided here: ADS, astro-ph. Some movies are available here.
 Fig.1. A time sequence of nonlinear development of gravitational instability in 3D stratified dust disk. Shown are volume rendering of dust density for the bottom half of a disk at t=0, 5.0, 7.3 and 10.3 \Omega^{-1}.

Previous Projects

MHD Simulation of the Circumbinary Accretion Disk: We performed the first three-dimensional magnetohydrodynamic (MHD) simulations of a circumbinary disk surrounding an equal mass binary. The binary maintains a fixed circular orbit of separation $a$. As in previous hydrodynamical simulations, strong torques by the binary can maintain a gap of radius $\simeq 2a$. Streams curve inward from $r \simeq 2a$ toward the binary; some of their mass passes through the inner boundary, while the remainder swings back out to the disk. However, we also find that near its inner edge the disk develops both a strong $m=1$ asymmetry and growing orbital eccentricity. Because the MHD stresses introduce more matter into the gap, the total torque per unit disk mass is $\simeq 14$ times larger than found previously. The inner boundary accretion rate per unit disk mass is $\simeq 40$ times greater than found from previous hydrodynamical calculations. The implied binary shrinkage rate is determined by a balance between the rate at which the binary gains angular momentum by accretion and loses it by gravitational torque. The large accretion rate brings these two rates nearly into balance, but in net, we find that $\dot a/a < 0$, and its magnitude is about 2.7 times larger than predicted by the earlier hydrodynamic simulations. If the binary comprises two massive black holes, the accretion rate may be great enough for one or both to be AGN, with consequences for the physical state of the gas both in the disk body and in its inner gap. More detailed descreption of this work is provided here: ADS astro-ph. Some movies are available here.
 Fig.2. Snapshots of circumbinary disk slices.Left: density on midplane; Right: density at \phi=\pi/2 plane

Numerically Converged Amplitude of MHD Turbulence: Recently, a couple of numerical simulations of MHD turbulence in unstratified shearing box drew similar convergence problems: the saturated level of the magnetic stress decreases toward zero as the experimental resolution increases.  One of the hypotheses trying to explain the 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 turbulence driven by the magnetorotational instability in a stratified shearing box (see Fig.2 below) with outflow boundary conditions and an equation of state determined by self-consistent dissipation and radiation losses. A series of simulations with increasing resolution are performed within a fixed computational box. We achieve numerical convergence with respect to radial and azimuthal resolution. As vertical resolution is improved, the ratio of stress to pressure increases slowly, but the absolute levels of both the stress and the pressure increase noticeably. These results are in contrast with those of previous work on unstratified shearing boxes. We argue that the persistence of strong magnetic field at higher resolution found in the stratified case is due to buoyancy. In addition, we find that the time-averaged vertical correlation length of the magnetic field near the disk midplane is sime3 times larger than that found in previous unstratified simulations, decreasing slowly with improved vertical resolution. We further show that the undulatory Parker instability drives the magnetic field upwelling at several scale heights from the midplane that is characteristic of stratified magnetohydrodynamics-turbulent disks. (ADS astro-ph). Collaborators: Prof. Julian Krolik (JHU), Dr. Shigenobu Hirose (JAMSTEC).

 Fig.3. 3D map of azimuthal component of the B-field. Fig.4. 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.

Radiation Pressure-supported AGN Tori: 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)

Last updated September, 2010