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Dr. Thomas Greif
Harvard-Smithsonian Center for Astrophysics (CfA)
Institute for Theory and Computation (ITC)
60 Garden Street
Cambdrige, MA 02138, USA
tgreif(at)cfa.harvard.edu
Phone: +1 617 384 7553
CV (pdf)
Welcome to my personal homepage. I'm a theoretical astrophysicist at the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. I investigate the formation of the first stars and galaxies, the influence of stellar radiation, and the chemical enrichment of the intergalactic medium with three-dimensional hydrodynamics simulations. Two of my most recent studies are discussed in more detail below.


Recent Studies


Chemothermal instability in primordial gas clouds

As primordial gas clouds collapse to high densities, the so-called chemo-thermal instability may triggered. This is caused by the rapidly increasing molecular hydrogen (H2) abundance due to three-body reactions, and the resulting increase in the H2 line cooling rate. Since the H2 fraction and cooling rate are closely coupled, a cooling catastrophe may ensue – in the sense that a small decrease in temperature leads to even more efficient cooling. In practice, a number of other physical processes counteract this instability, such that the net effect is somewhat reduced. Nevertheless, the resulting drop in temperature is sufficient to allow the gas to fragment. From top left to bottom right, the figure below shows the number density of hydrogen nuclei, temperature, H2 fraction and free-fall time over sound-crossing time. The latter is a measure for the gravitational stability of the gas (logarithmic values below zero denote unstable regions), and shows that a second clump to the top right of the primary clump has condensed out of the parent cloud. The susceptibility of the gas to fragmentation has important implications for the initial mass function of the first stars.

The journal article may be found HERE, and 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 work focused on the formation and evolution of primordial protostars in dark matter minihalos at high redshift. Following the collapse of the gas over many orders of magnitude in density, a protostar forms at the center of the halo and begins to accrete material from the surrounding cloud. Due to the high accretion rate and the efficient cooling of the gas, a Keplerian disk forms that becomes gravitationally unstable and fragments into a handful of protostars. The protostars then begin to migrate towards the primary protostar at the center, exploiting the strong gravitational torques that are present in the disk. At the end of the simulations, the central protostar has grown to about five times the mass of the second most massive protostar. The figure below shows the evolution of the protostellar system in four different minihalos (each row corresponds to an individual minihalo). The density of the gas is color-coded from black (lowest density) to yellow (highest density).

The journal article may be found HERE, and simulation movies may be downloaded below (depending on browser, you may need to right-click and 'save link as...').

Minihalo #1 Minihalo #2 Minihalo #3 Minihalo #4
Density
Temperature
Density
Temperature
Density
Temperature
Density
Temperature