Research

Photo Credit: Guang-Xing Li

Research

Star Formation

Giant Molecular Filament in the Milky Way

Molecular gas in the Milky Way is believed to be organized in molecular clouds. As as result, structures larger than the individual clouds are generally overlooked.

Using Galactic Ring Survey data we identified one of the biggest molecular structures in the Milky Way. The gas structure have a size of ~ 500 pc and consistes of several molecular clouds. This discovery suggests that molecular clouds are not isolated objects, but should be viewd as objects evolving together with the galactic disks. This agrees with proposals from earlier theoretical works 1, 2.

filament

Spitzer Infrared (upper panel) and FCRAO CO (lower panel) images of the gas filament. From A 500 pc filamentary gas wisp in the disk of the Milky Way .

Here is an article introducing our work.

Molecular Outflow from Protostars

Almost all forming starts emitted outflows during parts of their lifetimes. However it is unclear how the outflows form.

In our turbulent mixing entrainment model, the outflowing gas is a mixture of wind emitted by the forming star and the ambient envelope, and such a mixing process has been significantly enhanced by the turbulence of the ambient gas.

Such a natural possibility has been considered by earlier works where the authors conclude that the efficiency of such turbulent mixing is not enough to make molecular outflows. However, such arguments are not physically reasonable as the turbulent motion of the ambient gas has been neglected. In our work, we demonstrated that if such turbulent motion has been taken into account, the properties of the outflows can be naturally explained.

Below is a simulated channel map of an outflow.

filament

Channel map of a simulated outflow. The results can be found in Turbulent entrainment origin of protostellar outflows .

Gravity in Star Formation

Gravity is important in star formation. But how exactly gravity functions in star formation process remains obscure. It is unclear if gravity is important compared to other processes such as turbulence and magnetic field.

In a recent paper we developed a new G-virial method to estimate the importance of gravity in the Position-Position-Velocity (PPV) space. The method can be applied to many existing observations, and used to identify gravitationally-coherent regions in the position-position-velocity space and to quantify the properties of the regions.

More details about the method can be found here

filament

A visualisation of the Perseus molecular cloud. Blue color stands for the 13CO(1-0) emission and red color stands for the G-virial, which is a measure of the importance of gravity </a>.

Black Hole Accretion Disks

From 2007 to 2010 I worked on black hole accretion disks. Since the disks are consuming matter at their extremes, heat is trapped in the disks and as a result the disks become geometrically thick. However, the thickness of the disks is usually neglected in earlier works when the emission from the disks are calculated.

We found that observational signatures of these disks will change significantly if we take their thicknesses into account. We also pointed out that this self-shadowing behavior of the accretion disks has been captured in earlier X-ray observations, but has not been properly interpreted.

shadowing

Simulated images of a black hole accreting at Eddington luminosity. The image on the left is a image of a (usually-assumed) geometerically-thin accretion disk, and the image on the right is for a (realistic) geometerically-thick disk. Details can be found here .