Theoretical Studies in Star Formation

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Dr. McKee and his group investigate star formation and its history to study the processes governing the evolution of the universe. The star formation history determines the structure and evolution of galaxies, and the production of the chemical elements over time. This work addresses three central problems in the theory of star formation. (1) The role of magnetic fields in the formation of low-mass stars. Magnetic fields affect their formation as stars cannot from if the field is too strong. The group uses a numerical magneto-radiative hydrodynamic - adaptive mesh refinement code to simulate the effects of magnetic fields on star formation. Large scale star formation in giant molecular clouds is affected by the shape of the magnetic field in these clouds. Observations support both hypotheses that either the field is ordered or tangled. Detailed simulations will be compared to observations to resolve this issue. On intermediate scales, processes during star cluster formation influences the star formation rates and the stellar masses. Simulations that include the effects of radiation and outflows from protostars forming out of a magnetized medium will provide a more complete picture of star formation in clusters. One smaller scales, magnetic fields are effective at extracting the rotational energy of the gas in disks that have been observed around forming and recently formed stars. Most star formation simulations to date result in either no disks at all, or in disks that are much smaller than observed. Here high resolution simulations with all relevant physical processes that couple the gas to the magnetic field are carried out to address this issue.(2) The rate of star formation through cosmic time. The evolution of disk galaxies like the Milky Way depends on the rate at which gas accretes onto the galaxy and the rate at which this gas is transformed into stars. Gas accretion has been determined by large scale cosmological simulations. The PI has contributed to the development of two complementary theories of the star formation rate. These theories will be unified and, with simulations, extended to the case when heavy element abundances were low in the early universe. The results can serve as ingredients for more accurate theories of the evolution of galaxies.(3) The formation of the first stars. The very first stars formed out of gas that had essentially no elements heavier than Helium. When these stars exploded at the end of their lives, they seeded the universe with the first heavy elements. The amount of heavy elements produced depends sensitively on the mass of these stars. This mass was likely affected by of the energetic radiation emitted by the stars as they formed, which will be studied in the highest resolution simulations yet performed in order to determine the mass of the first stars.This award supports a postdoctoral researcher. The computational methodologies developed as part of this project will be published for use by the community.