Dr. Thomas Greif

Home Research Collaborators

Email:
tgreif[at]cfa.harvard.edu

Phone:
(+1) 617-384-7553

Postal Address:
Harvard-Smithsonian
Center for Astrophysics
60 Garden Street
Cambdrige, MA 02138, USA

Documents:
Publications (ADS link)
MA thesis (pdf)
PhD thesis (pdf)

Welcome to my personal homepage. I'm a theoretical astrophysicist at the Harvard-Smithsonian Center for Astrophysics. I use hydrodynamical simulations to investigate the formation of the first stars and galaxies, the influence of stellar radiation, and the chemical enrichment of the intergalactic medium. Currently, I employ the moving-mesh code Arepo, which I run on massively parallel supercomputers such as Stampede at the Texas Advanced Computing Center. I'm also interested in various aspects of computational astrophysics, including chemistry solvers, metal mixing, and radiative transfer algorithms. Most of my programs are written in C and Python for normal multi-core processors, but I have also begun to exploit coprocessors and graphics processing units.

Radiation hydrodynamics simulations

I recently performed the first three-dimensional radiation hydrodynamics simulation of line emission from molecular hydrogen in a primordial gas cloud. This study demonstrated that the results obtained with previous approximate methods did not reliably model the transport of the radiation. The image below shows the escape fraction of the radiation in the central 200 au of the cloud, color-coded from blue (lowest) to red (highest).

The second image shows the initial (grey lines) and final (colored lines) profiles of a number of lines after passing through the computational domain. The attenuation of the radiation is much stronger at the center of the lines than in the wings, due to the exponential frequency dependence of the cross section. Asymmetries in the profiles arise due to Doppler shifts via bulk velocities in the cloud (for example radial inflow and turbulence).



Chemothermal instability in primordial gas clouds

Once primordial gas clouds collapse to densities at which the formation of molecular hydrogen via three-body reactions sets in, a so-called chemothermal instability may be triggered. The rapidly increasing abundance of molecular hydrogen results in more efficient cooling, which in turn increases the collapse rate. This runaway process results in a substantial drop in temperature that may allow the gas to fragment. The figure below shows the gas density color-coded from blue to red in the central 200 au of the minihalo. The potential fragmentation of the cloud is evident from the formation of a second clump just above the primary.

A simulation movie may be found here (depending on browser, you may need to right-click and 'save link as...').

Formation and evolution of primordial protostars

This study investigated the formation of a protostellar disk following the initial collapse of the gas in a minihalo. The high accretion rate and efficient cooling of the gas facilitate the gravitational fragmentation of the disk. Due to strong gravitational torques between the protostars and the disk, the fragments migrate to the center of the cloud, where they merge with the primary protostar. The figure below shows the temperature in the central 10, color-coded from black to white. Here, a binary system with a few smaller protostars has formed.

The evolution of the radius, angular momentum, and gravitational and pressure gradient torques of the secondary protostars are shown below. A particularly interesting feature is the slingshot migration of one of the protostars following a close encounter with another protostar (denoted by the dotted line).

A simulation movie may be found here (depending on browser, you may need to right-click and 'save link as...').