The Formation of Population III Protostars

The formation of Population III stars is a well-posed problem, in that
the initial conditions are specified by the cosmological standard
model and extraordinarily well probed by observations of the cosmic
microwave background radiation, galaxy surveys and many other
means. Using the adaptive mesh refinement code Enzo, we follow the
collapse of several 10^6 solar mass halos from cosmological initial
conditions through protostellar densities, achieving unprecedented
resolution of sub-solar radius, with over 30 levels of
refinement. We developed a high density chemistry and radiation
model that includes the effects of chemical heating and cooling,
collision induced emission as well as optical depth effects of the
molecular line and continuum transfer. Our calculations are able to
follow the gas from cosmological scales to the AU scales when the
protostar becomes optically thick to radiation and is able to contract
only adiabatically (densities of 10^19 cm^-3). We find that the
accretion rates are affected by the molecular hydrogen formation and
destruction rates, and we see variation of nearly an order of
magnitude on length scales of 10-200 AU, at which accretion disks are
seen to form. We suggest that quantum chemical calculations as
well as laboratory measurements of the molecular hydrogen three body
formation rate will be needed to decide details of the population III
protostar forming process. The mass of the protostar formed at one
percent of a solar mass is indeed what had been found previously in
one dimensional calculations. The accretion shock, however, is not
spherical, and a disk geometry leads to hot, atomic shocked material
in the polar regions. In summary, it is now possible to study the
formation of population III protostars starting from cosmological
initial conditions, covering 12 magnitudes in scale and 22 magnitudes
in density.